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{{About|evolution in biology}}
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{{See introduction}}
{{Evolutionary biology}}

<!--NOTE: Please do not change the lead sentence(s) without consulting the discussion page first. This lead has been discussed and there is general consensus that this is the best one for now. Thanks.-->
'''Evolution''' is the change in the [[Heredity|inherited]] [[Phenotypic trait|characteristics]] of [[biology|biological]] [[population]]s over successive [[generation]]s. Evolutionary processes give rise to diversity at every [[biological organisation|level of biological organisation]], including [[biodiversity|species]], [[organism|individual organisms]] and [[molecular evolution|molecules]] such as [[DNA]] and [[protein]]s.<ref name="Hall08">{{cite book | editor1-last=Hall | editor1-first=B. K. | editor2-last=Hallgrímsson | editor2-first=B. |title = Strickberger's Evolution |year = 2008 |edition = 4th |publisher = Jones & Bartlett |isbn = 0-7637-0066-5 |url = http://www.jblearning.com/catalog/9780763700669/ |pages = 762}}</ref>

Life on Earth [[abiogenesis|originated]] and then evolved from a [[last universal ancestor|universal common ancestor]] approximately 3.8 billion years ago. Repeated [[speciation]] and the [[anagenesis|divergence]] of life can be [[inference|inferred]] from shared sets of biochemical and morphological traits, or by shared [[Nucleic acid sequence|DNA sequences]].<ref name="The Cell by Panno">{{cite book|last=Panno|first=Joseph|title=The Cell: Evolution of the First Organism|year=2005|publisher=Facts on File INC|isbn=0-8160-4946-7|pages=159}}</ref> These [[Homology (biology)|homologous]] traits and sequences are more similar among species that share a more recent common ancestor, and can be used to [[phylogenetics|reconstruct]] [[Tree of life (biology)|evolutionary histories]], using both existing species and the [[fossil record]]. Existing patterns of [[biodiversity]] have been shaped both by [[speciation]] and by [[extinction]].<ref name="Cracraft05">{{cite book | editor1-last=Cracraft | editor1-first=J. | editor2-last=Donoghue | editor2-first=M. J. |title = Assembling the tree of life |publisher = Oxford University Press |year = 2005 |page = |isbn = 0-19-517234-5 |url = http://books.google.ca/books?id=6lXTP0YU6_kC&printsec=frontcover&dq=Assembling+the+tree+of+life#v=onepage&q&f=false | pages=576}}</ref>

[[Charles Darwin]] and [[Alfred Wallace]] were the first to formulate [[On the Origin of Species|a scientific argument]] for the [[scientific theory|theory]] of evolution by means of [[natural selection]]. Evolution by natural selection is a process that is inferred from three [[fact]]s about populations: 1) more offspring are produced than can possibly survive, 2) traits vary among individuals, leading to different rates of survival and reproduction, and 3) trait differences are [[Heritability|heritable]].<ref name="Lewontin70">{{cite journal |last1 = Lewontin |first1 = R. C. |title = The units of selection |journal = Annual Review of Ecology and Systematics |year = 1970 |volume = 1 |pages = 1–18 |jstor = 2096764 |doi = 10.1146/annurev.es.01.110170.000245 }}</ref> Thus, when members of a population die they are replaced by the [[offspring|progeny]] of parents that were better [[adaptation|adapted]] to survive and reproduce in the [[Environment (biophysical)|environment]] in which natural selection took place. This process creates and preserves traits that are [[teleonomy|seemingly fitted]] for the [[function (biology)|functional]] roles they perform.<ref name="On The Origin of Species">{{cite book |last1 = Darwin |first1 = Charles |title = On The Origin of Species |chapter = XIV |year = 1859 |page = 503 |url = http://en.wikisource.org/wiki/On_the_Origin_of_Species_(1859)/Chapter_XIV |isbn = 0-8014-1319-2 }}</ref> Natural selection is the only known cause of [[adaptation]], but not the only known cause of evolution. Other, nonadaptive causes of [[microevolution|evolution]] include [[mutation]] and [[genetic drift]].<ref name="Kimura M 1991 367–86">{{cite journal |author = Kimura M |title = The neutral theory of molecular evolution: a review of recent evidence |url = http://www.jstage.jst.go.jp/article/jjg/66/4/66_367/_article |journal = Jpn. J. Genet. |volume = 66 |issue = 4 |pages = 367–86 |year = 1991 |pmid = 1954033 |doi = 10.1266/jjg.66.367 |ref = harv }}</ref>

In the early 20th century, [[Classical genetics|genetics]] was [[Modern evolutionary synthesis|integrated]] with Darwin's theory of evolution by natural selection through the discipline of [[population genetics]]. The importance of natural selection as a cause of evolution was accepted into other branches of [[biology]]. Moreover, previously held notions about evolution, such as [[orthogenesis]] and [[Largest-scale trends in evolution|"progress"]] became [[Obsolete scientific theory|obsolete]].<ref>{{cite book |last = Provine |first = W. B. |year = 1988 |title = Evolutionary progress |chapter = Progress in evolution and meaning in life |pages = 49–79 |publisher = University of Chicago Press }}</ref> Scientists continue to [[current research in evolutionary biology|study various aspects of evolution]] by forming and testing hypotheses, constructing [[Mathematical and theoretical biology|scientific theories]], using [[observational study|observational data]], and performing [[experiments]] in both the [[habitat|field]] and the laboratory. Biologists [[Scientific consensus|agree]] that descent with modification is one of the most reliably established [[evolution as fact and theory|facts]] in science.<ref name="NAS">{{cite book |author = National Academy of Science Institute of Medicine |title = Science, Evolution, and Creationism |publisher = National Academy Press |year = 2008|isbn=0-309-10586-2 |url = http://www.nap.edu/catalog.php?record_id=11876 }}</ref> Discoveries in evolutionary biology have made a significant impact not just within the traditional branches of biology, but also in other academic disciplines (e.g., [[biological anthropology|anthropology]] and [[evolutionary psychology|psychology]]) and on society at large.<ref name="Moore09">{{cite book |last1 = Moore |first1 = R. |last2 = Decker |first2 = M. |last3 = Cotner |first3 = S. |title = Chronology of the Evolution-Creationism Controversy |publisher = Greenwood |year = 2009 |page = 454 |isbn = 0-313-36287-4 |url = http://books.google.ca/books?id=4KwJRNgscdEC&printsec=frontcover&dq=Chronology+of+the+Evolution-Creationism+Controversy#v=onepage&q&f=false }}</ref><ref name="Futuyma99">{{cite web | editor1-last=Futuyma | editor1-first=Douglas J. |title = Evolution, Science, and Society: Evolutionary Biology and the National Research Agenda |publisher = Office of University Publications, Rutgers, The State University of New Jersey |year = 1999 |url = http://www.rci.rutgers.edu/~ecolevol/fulldoc.pdf }} {{dead link|date=September 2012}}</ref>

== History of evolutionary thought ==

{{Further|History of evolutionary thought}}
The proposal that one type of animal could descend from an animal of another type goes back to some of the first [[pre-Socratic philosophy|pre-Socratic]] Greek philosophers, such as [[Anaximander#Origin of humankind|Anaximander]] and [[Empedocles#Cosmogony|Empedocles]].<ref name="Kirk1">{{cite book |last1 = Kirk |first1 = Geoffrey |last2 = Raven |first2 = John |last3 = Schofield |first3 = John |title = The Presocratic Philosophers: A Critical History with a Selection of Texts |edition=3rd |publisher = The University of Chicago Press |location = Chicago |year = 1984a |isbn = 0-521-27455-9 |pages=100–142}}</ref><ref name="Kirk2">{{cite book |last1 = Kirk |first1 = Geoffrey |last2 = Raven |first2 = John |last3 = Schofield |first3 = John |title = The Presocratic Philosophers: A Critical History with a Selection of Texts |edition=3rd |publisher = The University of Chicago Press |location = Chicago |year = 1984b |isbn = 0-521-27455-9 |pages=280–321}}</ref> In contrast to these [[Materialism|materialistic]] views, Aristotle understood all natural things, not only [[life|living]] things, as being imperfect [[potentiality and actuality|actualisations]] of different fixed natural possibilities, known as "[[Theory of forms|forms]]", "[[idealism|ideas]]", or (in Latin translations) "species".<ref name="Torrey37">{{cite journal | last1 = Torrey | first1 = Harry Beal | last2 = Felin | first2 = Frances | title = Was Aristotle an evolutionist? | journal = The Quarterly Review of Biology | year = 1937 | month = March | volume = 12 | issue = 1 | pages = 1–18 | jstor = 2808399 | doi = 10.1086/394520 }}</ref><ref name="Hull67">{{cite journal | last1=Hull | first1=D. L. | year=1967 | title=The metaphysics of evolution | journal=The British Journal for the History of Science | volume=3 | issue=4 | pages=309–337 | jstor=4024958 | doi=10.1017/S0007087400002892}}</ref> This was part of his [[teleology|teleological]] understanding of [[Nature (philosophy)|nature]] in which all things have an intended role to play in a [[divinity|divine]] [[cosmos|cosmic]] order. The Roman [[poet]] and [[philosopher]] [[Lucretius|Titus Lucretius Carus]] proposed the possibility of evolutionary changes of organisms.<ref name=Carus2011>Carus TL (2011) ''De Rerum Natura''. New York, NY: Nabu Press.</ref> Variations of this idea became the standard understanding of the [[Middle Ages]], and were integrated into Christian learning, but Aristotle did not demand that real types of animals corresponded one-for-one with exact metaphysical forms, and specifically gave examples of how new types of living things could come to be.<ref>Mason, ''A History of the Sciences'' pp 43–44</ref>

In the 17th century the new [[scientific method|method]] of [[modern science]] rejected Aristotle's approach, and sought explanations of natural phenomena in terms of [[laws of nature]] which were the same for all visible things, and did not need to assume any fixed natural categories, nor any divine cosmic order. But this new approach was slow to take root in the biological sciences, which became the last bastion of the concept of fixed natural types. [[John Ray]] used one of the previously more general terms for fixed natural types, "species", to apply to animal and plant types, but unlike Aristotle he strictly identified each type of living thing as a species, and proposed that each species can be defined by the features that perpetuate themselves each generation.<ref>Mayr ''Growth of biological thought'' p256; original was Ray, ''History of Plants''. 1686, trans E. Silk.</ref> These species were designed by God, but showing differences caused by local conditions. The biological classification introduced by [[Carolus Linnaeus]] in 1735 also viewed species as fixed according to a divine plan.<ref>{{cite web|url=http://www.ucmp.berkeley.edu/history/linnaeus.html|title=Carl Linnaeus - berkeley.edu|accessdate=February 11, 2012}}</ref>

[[File:Charles Darwin aged 51.jpg|thumb|left|In 1842 [[Charles Darwin]] penned his first sketch of what became ''[[On the Origin of Species]]''.<ref name="Darwin09">{{cite book |last1 = Darwin |first1 = F. |title= The foundations of the origin of species, a sketch written in 1942 by Charles Darwin |year = 1909 |publisher = Cambridge University Press |page = 53 |url = http://darwin-online.org.uk/pdf/1909_Foundations_F1555.pdf }}</ref>]]
Other naturalists of this time speculated on evolutionary change of species over time according to natural laws. [[Maupertuis]] wrote in 1751 of natural modifications occurring during reproduction and accumulating over many generations to produce new species.<ref>Bowler, Peter J. 2003. ''Evolution: the history of an idea''. Berkeley, CA. p73–75</ref> [[Georges-Louis Leclerc, Comte de Buffon|Buffon]] suggested that species could degenerate into different organisms, and [[Erasmus Darwin]] proposed that all warm-blooded animals could have descended from a single micro-organism (or "filament").<ref>{{cite web|url=http://www.ucmp.berkeley.edu/history/Edarwin.html|title=Erasmus Darwin - berkeley.edu|accessdate=February 11, 2012}}</ref> The first full-fledged evolutionary scheme was [[Lamarck]]'s "transmutation" theory of 1809<ref name=Lamarck1809>Lamarck (1809) Philosophie Zoologique</ref> which envisaged spontaneous generation continually producing simple forms of life developed greater complexity in parallel lineages with an inherent progressive tendency, and that on a local level these lineages adapted to the environment by inheriting changes caused by use or disuse in parents.<ref name="Margulis91" /><ref name="Gould02">{{cite book | last = Gould | first = S.J. | authorlink = Stephen Jay Gould | title = [[The Structure of Evolutionary Theory]] | publisher = Belknap Press ([[Harvard University Press]]) | location = Cambridge | year = 2002 | isbn = 978-0-674-00613-3 | ref = harv }}</ref> (The latter process was later called Lamarckism.)<ref name="Margulis91">{{cite book | last1 = Margulis | first1 = Lynn | last2 = Fester | first2 = René | title = Symbiosis as a source of evolutionary innovation: Speciation and morphogenesis | publisher = The MIT Press | year = 1991 | page = 470 | isbn = 0-262-13269-9 | url = http://books.google.ca/books?id=3sKzeiHUIUQC&pg=PA162&dq=inauthor:%22Lynn+Margulis%22+lamarck#v=onepage&q=inauthor%3A%22Lynn%20Margulis%22%20lamarck&f=false }}</ref><ref name="ImaginaryLamarck">{{cite book |last = Ghiselin |first = Michael T. |authorlink = Michael Ghiselin|publication-date = September/October 1994 |contribution = Nonsense in schoolbooks: 'The Imaginary Lamarck'|contribution-url =http://www.textbookleague.org/54marck.htm |title = The Textbook Letter |publisher = The Textbook League |url = http://www.textbookleague.org/ |accessdate = January 23, 2008 }}</ref><ref>{{cite book |last = Magner |first = Lois N. |title = A History of the Life Sciences |edition = Third |publisher = [[Marcel Dekker]], [[CRC Press]] |year = 2002 |isbn = 978-0-203-91100-6 |url = http://books.google.com/?id=YKJ6gVYbrGwC&printsec=frontcover#v=onepage&q }}</ref><ref name="Jablonka07">{{cite journal |last1 = Jablonka |first1 = E. |last2 = Lamb |first2 = M. J. |year = 2007 |title = Précis of evolution in four dimensions |journal = Behavioural and Brain Sciences |volume = 30 |pages = 353–392 |doi = 10.1017/S0140525X07002221 |url = http://journals.cambridge.org/download.php?file=%2FBBS%2FBBS30_04%2FS0140525X07002361a.pdf&code=eb63ecba4b606e8e388169c5ae3c5095 |issue = 4 }}</ref> These ideas were condemned by established naturalists as speculation lacking empirical support. In particular [[Georges Cuvier]] insisted that species were unrelated and fixed, their similarities reflecting divine design for functional needs. In the meantime, Ray's ideas of benevolent design had been developed by [[William Paley]] into a [[natural theology]] which proposed complex adaptations as evidence of divine design, and was admired by Charles Darwin.<ref name="Darwin91">{{cite book | editor1-last=Burkhardt | editor1-first=F. | editor2-last=Smith | editor2-first=S. |year = 1991 |title = The correspondence of Charles Darwin |volume = 7 |pages = 1858–1859 |publisher = Cambridge University Press |place = Cambridge }}</ref><ref name="Sulloway09">{{cite journal |last1 = Sulloway |first1 = F. J. |year = 2009 |title = Why Darwin rejected intelligent design |journal = Journal of Biosciences |volume = 34 |issue = 2 |pages = 173–183 |doi = 10.1007/s12038-009-0020-8 |pmid = 19550032 }}</ref><ref name="Dawkins90">{{cite book |last1 = Dawkins |first1 = R. |title = Blind Watchmaker |year = 1990 |publisher = Penguin Books |isbn = 0-14-014481-1 |page = 368 }}</ref>

The critical break from the concept of fixed species in biology began with the theory of evolution by natural selection, which was formulated by Charles Darwin. Partly influenced by ''[[An Essay on the Principle of Population]]'' by [[Thomas Robert Malthus]], Darwin noted that population growth would lead to a "struggle for existence" where favorable variations could prevail as others perished. Each generation, many offspring fail to survive to an age of reproduction because of limited resources. This could explain the diversity of animals and plants from a common ancestry through the working of natural laws working the same for all types of thing.<ref name="Sober09">{{cite journal |last1 = Sober |first1 = E. |year = 2009 |title = Did Darwin write the origin backwards? |journal = Proceedings of the National Academy of Sciences |volume = 106 |issue = S1 |pages = 10048–10055 |doi = 10.1073/pnas.0901109106 |url = http://www.pnas.org/content/106/suppl.1/10048.full.pdf+html |bibcode = 2009PNAS..10610048S }}</ref><ref>Mayr, Ernst (2001) ''What evolution is''. Weidenfeld & Nicolson, London. p165</ref><ref>{{cite book |author = Bowler, Peter J. |title = Evolution: the history of an idea |publisher = University of California Press |location = Berkeley |year = 2003 |pages = 145–146 |isbn = 0-520-23693-9 |oclc = |doi = }} page 147"</ref><ref>{{cite journal |doi = 10.1086/282646 |author = Sokal RR, Crovello TJ |title = The biological species concept: A critical evaluation |journal = The American Naturalist |volume = 104 |issue = 936 |pages = 127–153 |year = 1970 |pmid = |url = http://hymenoptera.tamu.edu/courses/ento601/pdf/Sokal_Crovello_1970.pdf |format = PDF |jstor = 2459191 }}</ref> Darwin was developing his theory of "[[natural selection]]" from 1838 onwards until [[Alfred Russel Wallace]] sent him a similar theory in 1858. Both men presented their [[On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection|separate papers]] to the [[Linnean Society of London]].<ref>{{cite journal | last1 = Darwin | first1 = Charles | last2 = Wallace | first2 = Alfred | url = http://darwin-online.org.uk/content/frameset?itemID=F350&viewtype=text&pageseq=1 | title = On the Tendency of Species to form Varieties and on the Perpetuation of Varieties and Species by Natural Means of Selection | journal = Journal of the Proceedings of the Linnean Society of London. Zoology | volume = 3 | issue = 2 | year = 1858 | month = August | pages = 45–62 | accessdate = May 13, 2007 | doi = 10.1111/j.1096-3642.1858.tb02500.x | ref = harv }}</ref> At the end of 1859, Darwin's publication of ''[[On the Origin of Species]]'' explained natural selection in detail and in a way that led to an increasingly wide acceptance of [[Darwinism|Darwinian evolution]]. [[Thomas Henry Huxley]] applied Darwin's ideas to humans, using [[paleontology]] and [[comparative anatomy]] to provide strong evidence that humans and [[apes]] shared a common ancestry. Some were disturbed by this since it implied that humans did not have a special place in the universe.<ref>{{cite web |url = http://www.britannica.com/EBchecked/topic/277746/T-H-Huxley |title = Encyclopædia Britannica Online |publisher = Britannica.com |date = |accessdate = January 11, 2012 }}</ref>

Precise mechanisms of reproductive heritability and the origin of new traits remained a mystery. Towards this end, Darwin developed his provisional theory of [[pangenesis]].<ref name="Liu09">{{cite journal |last1 = Liu |first1 = Y. S. |last2 = Zhou |first2 = X. M. |last3 = Zhi |first3 = M. X. |last4 = Li |first4 = X. J. |last5 = Wan |first5 = Q. L. |title = Darwin's contributions to genetics |journal = J Appl Genet |volume = 50 |issue = 3 |pages = 177–184 |year = 2009 |url = http://jay.up.poznan.pl/JAG/pdfy/2009_Volume_50/2009_Volume_50_3-177-184.pdf |doi = 10.1007/BF03195671 |pmid = 19638672 }}</ref> In 1865 [[Gregor Mendel]] reported that traits were inherited in a predictable manner through the independent assortment and segregation of elements (later known as [[genes]]). Mendel's laws of inheritance eventually supplanted most of Darwin's pangenesis theory.<ref name=Weiling>{{cite journal |author = Weiling F |title = Historical study: Johann Gregor Mendel 1822–1884 |journal = Am. J. Med. Genet. |volume = 40 |issue = 1 |pages = 1–25; discussion 26 |year = 1991 |pmid = 1887835 |doi = 10.1002/ajmg.1320400103 |ref = harv }}</ref> [[August Weismann]] made the important distinction between [[Germline|germ cells (sperm and eggs)]] and [[somatic cells]] of the body, demonstrating that heredity passes through the germ line only. [[Hugo de Vries]] connected Darwin's pangenesis theory to Wiesman's germ/soma cell distinction and proposed that Darwin's pangenes were concentrated in the [[cell nucleus]] and when expressed they could move into the [[cytoplasm]] to change the cells structure. De Vries was also one of the researchers who made Mendel's work well-known, believing that Mendelian traits corresponded to the transfer of heritable variations along the germline.<ref name="Wright84">{{cite book |last1 = Wright |first1 = S. |title = Evolution and the Genetics of Populations, Volume 1: Genetic and Biometric Foundations |page = 480 |publisher = University of Chicago Press |isbn = 0-226-91038-5 |url = http://books.google.ca/books?id=4pTdTWi83ecC&printsec=frontcover&dq=sewall+wright+volume+1#v=onepage&q&f=false |date = 1984-06-15 }}</ref> To explain how new variants originate, De Vries developed a [[mutation]] theory that led to a temporary rift between those who accepted Darwinian evolution and biometricians who allied with de Vries.<ref name="Gould02" /><ref>{{cite book |author = [[Will Provine]] |title = The Origins of Theoretical Population Genetics |year = 1971 |isbn = 0-226-68464-4 |publisher = University of Chicago Press }}</ref><ref>Stamhuis, Meijer and Zevenhuizen. [http://www.ncbi.nlm.nih.gov/pubmed/10439561?dopt=Citation ''Hugo de Vries on heredity, 1889–1903. Statistics, Mendelian laws, pangenes, mutations.''], Isis. 1999 Jun;90(2):238-67.</ref> At the turn of the 20th century, pioneers in the field of population genetics, such as [[J.B.S. Haldane]], [[Sewall Wright]], and [[Ronald Fisher]], set the foundations of evolution onto a robust statistical philosophy. The false contradiction between Darwin's theory, genetic mutations, and Mendelian inheritance was thus reconciled.<ref>Quammen, D. (2006). [http://www.nytimes.com/2006/08/27/books/review/Desmond.t.html?n=Top/Reference/Times%20Topics/People/D/Darwin,%20Charles%20Robert ''The reluctant Mr. Darwin: An intimate portrait of Charles Darwin and the making of his theory of evolution.''] New York, NY: W.W. Norton & Company.</ref>

In the 1920s and 1930s a [[modern evolutionary synthesis]] connected natural selection, mutation theory, and Mendelian inheritance into a unified theory that applied generally to any branch of biology. The modern synthesis was able to explain patterns observed across species in populations, through [[transitional fossils|fossil transitions]] in palaeontology, and even complex cellular mechanisms in [[developmental biology]].<ref name="Gould02" /><ref>{{cite book |last = Bowler |first = Peter J. |authorlink = Peter J. Bowler |year = 1989 |title = The Mendelian Revolution: The Emergence of Hereditarian Concepts in Modern Science and Society |publisher = Johns Hopkins University Press |location = Baltimore |isbn = 978-0-8018-3888-0 }}</ref> The publication of the structure of DNA by [[James D. Watson|James Watson]] and [[Francis Crick]] in 1953 demonstrated a physical basis for inheritance.<ref name="Watson53">{{cite journal |last1 = Watson |first1 = J. D. |last2 = Crick |first2 = F. H. C. |title = Molecular structure of nucleic acids: A structure for deoxyribose nucleic acid |journal = Nature |volume = 171 |pages = 737–738 |doi = 10.1038/171737a0 |url = http://profiles.nlm.nih.gov/SC/B/B/Y/W/_/scbbyw.pdf |bibcode = 1953Natur.171..737W |issue = 4356 |pmid = 13054692 |year = 1953 }}</ref> [[Molecular biology]] improved our understanding of the relationship between genotype and phenotype. Advancements were also made in [[phylogenetic]] [[systematics]], mapping the transition of traits into a comparative and testable framework through the publication and use of [[Phylogenetic tree|evolutionary trees]].<ref name="Hennig99">{{cite book |last1 = Hennig |first1 = W. |last2 = Lieberman |first2 = B. S. |title = Phylogenetic systematics |page = 280 |publisher = University of Illinois Press |edition = New edition (Mar 1, 1999) |isbn = 0-252-06814-9 |year = 1999 |url = http://books.google.ca/books?id=xsi6QcQPJGkC&printsec=frontcover&dq=phylogenetic+systematics#v=onepage&q&f=false }}</ref><ref name="Wiley11">{{cite book |title = Phylogenetics: Theory and practice of phylogenetic systematics |year = 2011 |edition = 2nd |publisher = Wiley-Blackwell |page = 390 |doi = 10.1002/9781118017883.fmatter }}</ref> In 1973, evolutionary biologist [[Theodosius Dobzhansky]] penned that "nothing in biology makes sense except in the light of evolution", because it has brought to light the relations of what first seemed disjointed facts in natural history into a coherent [[explanatory]] body of knowledge that describes and predicts many observable facts about life on this planet.<ref name="Dobzhansky73">{{cite journal |last1 = Dobzhansky |first1 = T. |year = 1973 |title = Nothing in biology makes sense except in the light of evolution |journal = The American Biology Teacher |volume = 35 |issue = 3 |pages = 125–129 |url = http://img.signaly.cz/upload/1/0/9a462eb6be1ed7828f57a184cde3c0/Dobzhansky.pdf |doi = 10.2307/4444260 }}</ref>

Since then, the modern synthesis has been further extended to explain biological phenomena across the full and integrative scale of the [[Biological organisation|biological hierarchy]], from genes to species. This extension has been dubbed "[[Evolutionary developmental biology|eco-evo-devo]]".<ref name=Kutschera>{{cite journal |author = Kutschera U, Niklas K |title = The modern theory of biological evolution: an expanded synthesis |journal = Naturwissenschaften |volume = 91 |issue = 6 |pages = 255–76 |year = 2004 |pmid = 15241603 |doi = 10.1007/s00114-004-0515-y |ref = harv |bibcode = 2004NW.....91..255K }}</ref><ref name=Kutschera/><ref name="Cracraft04">{{cite book | editor1-last=Cracraft | editor1-first=J. | editor2-last=Bybee | editor2-first=R. W. |title = Evolutionary science and society: Educating a new generation |year = 2004 |place = Chicago, IL |series = Revised Proceedings of the BSCS, AIBS Symposium |url = http://www.bscs.org/curriculumdevelopment/highschool/evolution/pdf.html }}</ref><ref name="Avise10">{{cite journal |last1 = Avise |first1 = J. C. |last2 = Ayala |first2 = F. J. |title = In the Light of Evolution IV. The Human Condition (introduction) |year = 2010 |journal = Proceedings of the National Academy of Sciences USA |volume = 107 |issue = S2 |pages = 8897–8901 |url = http://faculty.sites.uci.edu/johncavise/files/2011/03/311-intro-to-ILE-IV.pdf |doi = 10.1073/pnas.100321410 }}</ref>

== Heredity ==

{{Further|Introduction to genetics|Genetics|Heredity|Norms of reaction}}
[[File:ADN static.png|thumb|upright|[[DNA]] structure. [[nucleobase|Bases]] are in the centre, surrounded by phosphate–sugar chains in a [[double helix]].]]
Evolution in organisms occurs through changes in heritable [[trait (biology)|traits]]&nbsp;– particular characteristics of an organism. In humans, for example, [[eye colour]] is an inherited characteristic and an individual might inherit the "brown-eye trait" from one of their parents.<ref>{{cite journal |author = Sturm RA, Frudakis TN |title = Eye colour: portals into pigmentation genes and ancestry |journal = Trends Genet. |volume = 20 |issue = 8 |pages = 327–32 |year = 2004 |pmid = 15262401 |doi = 10.1016/j.tig.2004.06.010 |ref = harv }}</ref> Inherited traits are controlled by [[gene]]s and the complete set of genes within an organism's [[genome]] is called its [[genotype]].<ref name=Pearson_2006>{{cite journal |author = Pearson H |title = Genetics: what is a gene? |journal = Nature |volume = 441 |issue = 7092 |pages = 398–401 |year = 2006 |pmid = 16724031 |doi = 10.1038/441398a |ref = harv |bibcode = 2006Natur.441..398P }}</ref>

The complete set of observable traits that make up the structure and behaviour of an organism is called its [[phenotype]]. These traits come from the interaction of its genotype with the [[Environment (biophysical)|environment]].<ref>{{cite journal |author = Visscher PM, Hill WG, Wray NR |title = Heritability in the genomics era—concepts and misconceptions |journal = Nat. Rev. Genet. |volume = 9 |issue = 4 |pages = 255–66 |year = 2008 |pmid = 18319743 |doi = 10.1038/nrg2322 |ref = harv }}</ref> As a result, many aspects of an organism's phenotype are not inherited. For example, [[sun tanning|suntanned]] skin comes from the interaction between a person's genotype and sunlight; thus, suntans are not passed on to people's children. However, some people tan more easily than others, due to differences in their genotype; a striking example are people with the inherited trait of [[albinism]], who do not tan at all and are very sensitive to [[sunburn]].<ref>{{cite journal |author = Oetting WS, Brilliant MH, King RA |title = The clinical spectrum of albinism in humans |journal = Molecular medicine today |volume = 2 |issue = 8 |pages = 330–5 |year = 1996 |pmid = 8796918 |doi = 10.1016/1357-4310(96)81798-9 |ref = harv }}</ref>

Heritable traits are passed from one generation to the next via [[DNA]], a [[molecule]] that encodes genetic information.<ref name=Pearson_2006/> DNA is a long [[polymer]] composed of four types of bases. The sequence of bases along a particular DNA molecule specify the genetic information, in a manner similar to a sequence of letters spelling out a sentence. Before a cell divides, the DNA is copied, so that each of the resulting two cells will inherit the DNA sequence. Portions of a DNA molecule that specify a single functional unit are called [[gene]]s; different genes have different sequences of bases. Within [[cell (biology)|cells]], the long strands of DNA form condensed structures called [[chromosome]]s. The specific location of a DNA sequence within a chromosome is known as a [[locus (genetics)|locus]]. If the DNA sequence at a locus varies between individuals, the different forms of this sequence are called [[allele]]s. DNA sequences can change through [[mutation]]s, producing new alleles. If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism.<ref name=Futuyma>{{cite book |last = Futuyma |first = Douglas J. |authorlink = Douglas J. Futuyma |year = 2005 |title = Evolution |publisher = Sinauer Associates, Inc |location = Sunderland, Massachusetts |isbn = 0-87893-187-2 }}</ref> However, while this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by [[quantitative trait locus|multiple interacting genes]].<ref>{{cite journal |author = Phillips PC |title = Epistasis—the essential role of gene interactions in the structure and evolution of genetic systems |journal = Nat. Rev. Genet. |volume = 9 |issue = 11 |pages = 855–67 |year = 2008 |pmid = 18852697 |doi = 10.1038/nrg2452 |pmc = 2689140 |ref = harv }}</ref><ref name=Lin>{{cite journal |author = Wu R, Lin M |title = Functional mapping&nbsp;– how to map and study the genetic architecture of dynamic complex traits |journal = Nat. Rev. Genet. |volume = 7 |issue = 3 |pages = 229–37 |year = 2006 |pmid = 16485021 |doi = 10.1038/nrg1804 |ref = harv }}</ref>

Recent findings have confirmed important examples of heritable changes that cannot be explained by changes to the sequence of nucleotides in the DNA. These phenomena are classed as [[epigenetic]] inheritance systems.<ref name="Jablonk09">{{cite journal |last1 = Jablonka |first1 = E. |last2 = Raz |first2 = G. |title = Transgenerational epigenetic inheritance: Prevalence, mechanisms and implications for the study of heredity and evolution |journal = The Quarterly Review of Biology |volume = 84 |issue = 2 |pages = 131–176 |year = 2009 |url = http://compgen.unc.edu/wiki/images/d/df/JablonkaQtrRevBio2009.pdf |pmid = 19606595 |doi = 10.1086/598822 }}</ref> [[DNA methylation]] marking [[chromatin]], self-sustaining metabolic loops, gene silencing by [[RNA interference]] and the three dimensional [[Protein structure|conformation]] of proteins (such as [[prions]]) are areas where epigenetic inheritance systems have been discovered at the organismic level.<ref name="Bossdorf10">{{cite journal |last1 = Bossdorf |first1 = O. |last2 = Arcuri |first2 = D. |last3 = Richards |first3 = C. L. |last4 = Pigliucci |first4 = M. |title = Experimental alteration of DNA methylation affects the phenotypic plasticity of ecologically relevant traits in ''Arabidopsis thaliana'' |journal = Evolutionary Ecology |volume = 24 |issue = 3 |pages = 541–553 |year = 2010 |url = http://www.springerlink.com/content/c847255ur67w2487/ |doi = 10.1007/s10682-010-9372-7 }}</ref><ref name="Jablonka05">{{cite book |last1 = Jablonka |first1 = E. |last2 = Lamb |first2 = M. |title = Evolution in four dimensions: Genetic, epigenetic, behavioural and symbolic |year = 2005 |publisher = MIT Press |url = http://books.google.ca/books?id=EaCiHFq3MWsC&printsec=frontcover |isbn = 0-262-10107-6 }}</ref> Developmental biologists suggest that complex interactions in [[gene regulatory network|genetic networks]] and communication among cells can lead to heritable variations that may underlay some of the mechanics in [[developmental plasticity]] and [[Canalisation (genetics)|canalization]].<ref name="Jablonka02">{{cite journal |last1 = Jablonka |first1 = E. |last2 = Lamb |first2 = M. J. |title = The changing concept of epigenetics |journal = Annals of the New York Academy of Sciences |volume = 981 |issue = 1 |pages = 82–96 |year = 2002 |url = http://a-c-elitzur.co.il/uploads/articlesdocs/Jablonka.pdf |doi = 10.1111/j.1749-6632.2002.tb04913.x |bibcode = 2002NYASA.981...82J |pmid=12547675}}</ref> Heritability may also occur at even larger scales. For example, ecological inheritance through the process of [[niche construction]] is defined by the regular and repeated activities of organisms in their environment. This generates a legacy of effects that modify and feed back into the selection regime of subsequent generations. Descendants inherit genes plus environmental characteristics generated by the ecological actions of ancestors.<ref name="Laland06">{{cite journal |title = Perspective: Seven reasons (not) to neglect niche construction |last1 = Laland |first1 = K. N. |last2 = Sterelny |first2 = K. |journal = Evolution |volume = 60 |issue = 8 |pages = 1751–1762 |year = 2006 |url = http://lalandlab.st-andrews.ac.uk/pdf/laland_Evolution_2006.pdf |doi = 10.1111/j.0014-3820.2006.tb00520.x }}</ref> Other examples of heritability in evolution that are not under the direct control of genes include the inheritance of [[Dual inheritance theory|cultural traits]] and [[symbiogenesis]].<ref name="Chapman98">{{cite journal |title = Morphogenesis by symbiogenesis |last1 = Chapman |first1 = M. J. |last2 = Margulis |first2 = L. |journal = International Microbiology |volume = 1 |issue = 4 |pages = 319–326 |year = 1998 |url = http://www.im.microbios.org/04december98/14%20Chapman.pdf |pmid = 10943381 }}</ref><ref name="Wilson07">{{cite journal |last1 = Wilson |first1 = D. S. |last2 = Wilson |first2 = E. O. |title = Rethinking the theoretical foundation of sociobiology |journal = The Quarterly Review of Biology |volume = 82 |issue = 4 |year = 2007 |url = http://evolution.binghamton.edu/dswilson/wp-content/uploads/2010/01/Rethinking-sociobiology.pdf |doi = 10.1086/522809 |pages = 327–348 |pmid = 18217526 }}</ref>

== Variation ==

{{Multiple image|direction=vertical|align=right|image1=Biston.betularia.7200.jpg |image2=Biston.betularia.f.carbonaria.7209.jpg|width=200| caption1=White [[peppered moth]] |caption2=Black morph in [[peppered moth evolution]]}}
{{Further|Genetic diversity|Population genetics}}
An individual organism's [[phenotype]] results from both its [[genotype]] and the influence from the [[Environment (biophysical)|environment]] it has lived in. A substantial part of the variation in phenotypes in a population is caused by the differences between their genotypes.<ref name=Lin/> The [[modern evolutionary synthesis]] defines evolution as the change over time in this genetic variation. The frequency of one particular allele will become more or less prevalent relative to other forms of that gene. Variation disappears when a new allele reaches the point of [[fixation (population genetics)|fixation]]&nbsp;— when it either disappears from the population or replaces the ancestral allele entirely.<ref name=Amos>{{cite journal |author = Harwood AJ |title = Factors affecting levels of genetic diversity in natural populations |journal = Philos. Trans. R. Soc. Lond., B, Biol. Sci. |volume = 353 |issue = 1366 |pages = 177–86 |year = 1998 |pmid = 9533122 |pmc = 1692205 |doi = 10.1098/rstb.1998.0200 |last2 = Harwood |first2 = J |ref = harv }}</ref>

Natural selection will only cause evolution if there is enough [[genetic variation]] in a population. Before the discovery of [[Mendelian genetics]], one common hypothesis was [[blending inheritance]]. But with blending inheritance, genetic variance would be rapidly lost, making evolution by natural selection implausible. The ''[[Hardy-Weinberg principle]]'' provides the solution to how variation is maintained in a population with [[Mendelian inheritance]]. The frequencies of alleles (variations in a gene) will remain constant in the absence of selection, mutation, migration and genetic drift.<ref name="Ewens W.J. 2004">{{cite book |author = Ewens W.J. |year = 2004 |title = Mathematical Population Genetics (2nd Edition) |publisher = Springer-Verlag, New York |isbn = 0-387-20191-2 }}</ref>

Variation comes from [[mutation]]s in [[genetic material]], reshuffling of genes through [[sexual reproduction]] and migration between populations ([[gene flow]]). Despite the constant introduction of new variation through mutation and gene flow, most of the [[genome]] of a species is identical in all individuals of that species.<ref>{{cite journal |author = Butlin RK, Tregenza T |title = Levels of genetic polymorphism: marker loci versus quantitative traits |journal = Philos. Trans. R. Soc. Lond., B, Biol. Sci. |volume = 353 |issue = 1366 |pages = 187–98 |year = 1998 |pmid = 9533123 |pmc = 1692210 |doi = 10.1098/rstb.1998.0201 |ref = harv }}</ref> However, even relatively small differences in genotype can lead to dramatic differences in phenotype: for example, chimpanzees and humans differ in only about 5% of their genomes.<ref>{{cite journal |author = Wetterbom A, Sevov M, Cavelier L, Bergström TF |title = Comparative genomic analysis of human and chimpanzee indicates a key role for indels in primate evolution |journal = J. Mol. Evol. |volume = 63 |issue = 5 |pages = 682–90 |year = 2006 |pmid = 17075697 |doi = 10.1007/s00239-006-0045-7 |ref = harv }}</ref>

=== Mutation ===

{{Further|Mutation}}
[[File:Gene-duplication.svg|thumb|100px|Duplication of part of a [[chromosome]].]]
Mutations are changes in the DNA sequence of a cell's genome. When mutations occur, they can either have no effect, alter the [[gene product|product of a gene]], or prevent the gene from functioning. Based on studies in the fly ''[[Drosophila melanogaster]]'', it has been suggested that if a mutation changes a protein produced by a gene, this will probably be harmful, with about 70% of these mutations having damaging effects, and the remainder being either neutral or weakly beneficial.<ref>{{cite journal |author = Sawyer SA, Parsch J, Zhang Z, Hartl DL |title = Prevalence of positive selection among nearly neutral amino acid replacements in Drosophila |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 104 |issue = 16 |pages = 6504–10 |year = 2007 |pmid = 17409186 |doi = 10.1073/pnas.0701572104 |pmc = 1871816 |ref = harv |bibcode = 2007PNAS..104.6504S }}</ref>

Mutations can involve large sections of a chromosome becoming [[gene duplication|duplicated]] (usually by [[genetic recombination]]), which can introduce extra copies of a gene into a genome.<ref>{{cite journal |doi = 10.1038/nrg2593 |pmid = 19597530 |volume = 10 |issue = 8 |pages = 551–564 |last = Hastings |first = P J |title = Mechanisms of change in gene copy number |journal = Nature Reviews. Genetics |year = 2009 |last2 = Lupski |first2 = JR |last3 = Rosenberg |first3 = SM |last4 = Ira |first4 = G |pmc = 2864001 |ref = harv }}</ref> Extra copies of genes are a major source of the raw material needed for new genes to evolve.<ref>{{cite book |last = Carroll SB, Grenier J, Weatherbee SD |title = From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design. Second Edition |publisher = Blackwell Publishing |year = 2005 |location = Oxford |isbn = 1-4051-1950-0 |author = Sean B. Carroll; Jennifer K. Grenier; Scott D. Weatherbee. }}</ref> This is important because most new genes evolve within [[gene family|gene families]] from pre-existing genes that share common ancestors.<ref>{{cite journal |author = Harrison P, Gerstein M |title = Studying genomes through the aeons: protein families, pseudogenes and proteome evolution |journal = J Mol Biol |volume = 318 |issue = 5 |pages = 1155–74 |year = 2002 |pmid = 12083509 |doi = 10.1016/S0022-2836(02)00109-2 |ref = harv }}</ref> For example, the human eye uses four genes to make structures that sense light: three for [[Cone cell|colour vision]] and one for [[Rod cell|night vision]]; all four are descended from a single ancestral gene.<ref>{{cite journal |author = Bowmaker JK |title = Evolution of colour vision in vertebrates |journal = Eye (London, England) |volume = 12 |issue = Pt 3b |pages = 541–7 |year = 1998 |pmid = 9775215 |ref = harv |doi = 10.1038/eye.1998.143 }}</ref>

New genes can be generated from an ancestral gene when a duplicate copy mutates and acquires a new function. This process is easier once a gene has been duplicated because it increases the [[Redundancy (engineering)|redundancy]] of the system; one gene in the pair can acquire a new function while the other copy continues to perform its original function.<ref>{{cite journal |doi = 10.1101/gr.9.4.317 |author = Gregory TR, Hebert PD |title = The modulation of DNA content: proximate causes and ultimate consequences |url = http://genome.cshlp.org/content/9/4/317.full |journal = Genome Res. |volume = 9 |issue = 4 |pages = 317–24 |year = 1999 |pmid = 10207154 |ref = harv }}</ref><ref>{{cite journal |author = Hurles M |title = Gene duplication: the genomic trade in spare parts |journal = PLoS Biol. |volume = 2 |issue = 7 |pages = E206 |year = 2004 |pmid = 15252449 |pmc = 449868 |doi = 10.1371/journal.pbio.0020206 |ref = harv }}</ref> Other types of mutations can even generate entirely new genes from previously noncoding DNA.<ref>{{cite journal |title = The evolution and functional diversification of animal microRNA genes |author = Liu N, Okamura K, Tyler DM |journal = Cell Res. |year = 2008 |volume = 18 |pages = 985–96 |doi = 10.1038/cr.2008.278 |url = http://www.nature.com/cr/journal/v18/n10/full/cr2008278a.html |pmid = 18711447 |issue = 10 |pmc = 2712117 |ref = harv }}</ref><ref>{{cite journal |author = Siepel A |title = Darwinian alchemy: Human genes from noncoding DNA |journal = Genome Res. |volume = 19 |issue = 10 |pages = 1693–5 |year = 2009 |pmid = 19797681 |doi = 10.1101/gr.098376.109 |url = http://genome.cshlp.org/content/19/10/1693.full |pmc = 2765273 |ref = harv }}</ref>

The generation of new genes can also involve small parts of several genes being duplicated, with these fragments then recombining to form new combinations with new functions.<ref>{{cite journal |author = Orengo CA, Thornton JM |title = Protein families and their evolution-a structural perspective |journal = Annu. Rev. Biochem. |volume = 74 |issue = 1 |pages = 867–900 |year = 2005 |pmid = 15954844 |doi = 10.1146/annurev.biochem.74.082803.133029 |ref = harv }}</ref><ref>{{cite journal |author = Long M, Betrán E, Thornton K, Wang W |title = The origin of new genes: glimpses from the young and old |journal = Nat. Rev. Genet. |volume = 4 |issue = 11 |pages = 865–75 |year = 2003 |pmid = 14634634 |doi = 10.1038/nrg1204 |ref = harv }}</ref> When new genes are assembled from shuffling pre-existing parts, [[protein domain|domains]] act as modules with simple independent functions, which can be mixed together to produce new combinations with new and complex functions.<ref>{{cite journal |author = Wang M, Caetano-Anollés G |title = The evolutionary mechanics of domain organization in proteomes and the rise of modularity in the protein world |journal = Structure |volume = 17 |issue = 1 |pages = 66–78 |year = 2009 |doi = 10.1016/j.str.2008.11.008 |pmid = 19141283 |ref = harv }}</ref> For example, [[polyketide synthase]]s are large enzymes that make antibiotics; they contain up to one hundred independent domains that each catalyze one step in the overall process, like a step in an assembly line.<ref>{{cite journal |author = Weissman KJ, Müller R |title = Protein-protein interactions in multienzyme megasynthetases |journal = Chembiochem |volume = 9 |issue = 6 |pages = 826–48 |year = 2008 |pmid = 18357594 |doi = 10.1002/cbic.200700751 |ref = harv }}</ref>

=== Sex and recombination ===

{{Further|Sexual reproduction|Genetic recombination|Evolution of sexual reproduction}}

In asexual organisms, genes are inherited together, or ''linked'', as they cannot mix with genes of other organisms during reproduction. In contrast, the offspring of [[sex]]ual organisms contain random mixtures of their parents' chromosomes that are produced through [[independent assortment]]. In a related process called [[homologous recombination]], sexual organisms exchange DNA between two matching chromosomes.<ref>{{cite journal |author = Radding C |title = Homologous pairing and strand exchange in genetic recombination |journal = Annu. Rev. Genet. |volume = 16 |issue = 1 |pages = 405–37 |year = 1982 |pmid = 6297377 |doi = 10.1146/annurev.ge.16.120182.002201 |ref = harv }}</ref> Recombination and reassortment do not alter allele frequencies, but instead change which alleles are associated with each other, producing offspring with new combinations of alleles.<ref name=Agrawal>{{cite journal |author = Agrawal AF |title = Evolution of sex: why do organisms shuffle their genotypes? |journal = Curr. Biol. |volume = 16 |issue = 17 |page = R696 |year = 2006 |pmid = 16950096 |doi = 10.1016/j.cub.2006.07.063 |ref = harv |pages = R696–704 }}</ref> Sex usually increases genetic variation and may increase the rate of evolution.<ref>{{cite journal |author = Peters AD, Otto SP |title = Liberating genetic variance through sex |journal = BioEssays |volume = 25 |issue = 6 |pages = 533–7 |year = 2003 |pmid = 12766942 |doi = 10.1002/bies.10291 |ref = harv }}</ref><ref>{{cite journal |author = Goddard MR, Godfray HC, Burt A |title = Sex increases the efficacy of natural selection in experimental yeast populations |journal = Nature |volume = 434 |issue = 7033 |pages = 636–40 |year = 2005 |pmid = 15800622 |doi = 10.1038/nature03405 |ref = harv |bibcode = 2005Natur.434..636G }}</ref>

=== Gene flow ===

{{Further|Gene flow}}

[[Gene flow]] is the exchange of genes between populations and between species.<ref name="Morjan C, Rieseberg L 2004 1341–56">{{cite journal |author = Morjan C, Rieseberg L |title = How species evolve collectively: implications of gene flow and selection for the spread of advantageous alleles |journal = Mol. Ecol. |volume = 13 |issue = 6 |pages = 1341–56 |year = 2004 |pmid = 15140081 |doi = 10.1111/j.1365-294X.2004.02164.x |pmc = 2600545 |ref = harv }}</ref> It can therefore be a source of variation that is new to a population or to a species. [[Gene flow]] can be caused by the movement of individuals between separate populations of organisms, as might be caused by the movement of mice between inland and coastal populations, or the movement of [[pollen]] between heavy metal tolerant and heavy metal sensitive populations of grasses.

Gene transfer between species includes the formation of [[Hybrid (biology)|hybrid]] organisms and [[horizontal gene transfer]]. [[Horizontal gene transfer]] is the transfer of genetic material from one organism to another organism that is not its offspring; this is most common among [[bacteria]].<ref>{{cite journal |author = Boucher Y, Douady CJ, Papke RT, Walsh DA, Boudreau ME, Nesbo CL, Case RJ, Doolittle WF |title = Lateral gene transfer and the origins of prokaryotic groups |doi = 10.1146/annurev.genet.37.050503.084247 |journal = Annu Rev Genet |volume = 37 |issue = 1 |pages = 283–328 |year = 2003 |pmid = 14616063 |ref = harv }}</ref> In medicine, this contributes to the spread of [[antibiotic resistance]], as when one bacteria acquires resistance genes it can rapidly transfer them to other species.<ref name=GeneticEvolution>{{cite journal |author = Walsh T |title = Combinatorial genetic evolution of multiresistance |journal = Curr. Opin. Microbiol. |volume = 9 |issue = 5 |pages = 476–82 |year = 2006 |pmid = 16942901 |doi = 10.1016/j.mib.2006.08.009 |ref = harv }}</ref> Horizontal transfer of genes from bacteria to eukaryotes such as the yeast ''[[Saccharomyces cerevisiae]]'' and the adzuki bean beetle ''Callosobruchus chinensis'' has occurred.<ref>{{cite journal |author = Kondo N, Nikoh N, Ijichi N, Shimada M, Fukatsu T |title = Genome fragment of Wolbachia endosymbiont transferred to X chromosome of host insect |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 99 |issue = 22 |pages = 14280–5 |year = 2002 |pmid = 12386340 |doi = 10.1073/pnas.222228199 |pmc = 137875 |ref = harv |bibcode = 2002PNAS...9914280K }}</ref><ref>{{cite journal |author = Sprague G |title = Genetic exchange between kingdoms |journal = Curr. Opin. Genet. Dev. |volume = 1 |issue = 4 |pages = 530–3 |year = 1991 |pmid = 1822285 |doi = 10.1016/S0959-437X(05)80203-5 |ref = harv }}</ref> An example of larger-scale transfers are the eukaryotic [[Bdelloidea|bdelloid rotifers]], which have received a range of genes from bacteria, fungi and plants.<ref>{{cite journal |author = Gladyshev EA, Meselson M, Arkhipova IR |title = Massive horizontal gene transfer in bdelloid rotifers |journal = Science |volume = 320 |issue = 5880 |pages = 1210–3 |year = 2008 |pmid = 18511688 |doi = 10.1126/science.1156407 |ref = harv |bibcode = 2008Sci...320.1210G }}</ref> [[Virus]]es can also carry DNA between organisms, allowing transfer of genes even across [[domain (biology)|biological domains]].<ref>{{cite journal |author = Baldo A, McClure M |title = Evolution and horizontal transfer of dUTPase-encoding genes in viruses and their hosts |journal = J. Virol. |volume = 73 |issue = 9 |pages = 7710–21 |date = September 1, 1999 |pmid = 10438861 |pmc = 104298 |ref = harv }}</ref>

Large-scale gene transfer has also occurred between the ancestors of [[Eukaryote|eukaryotic cells]] and [[bacteria]], during the acquisition of [[chloroplast]]s and [[Mitochondrion|mitochondria]]. It is possible that eukaryotes themselves originated from horizontal gene transfers between [[bacteria]] and [[archaea]].<ref>{{cite journal |author = River, M. C. and Lake, J. A. |title = The ring of life provides evidence for a genome fusion origin of eukaryotes |journal = Nature |volume = 431 |issue = 9 |pages = 152–5 |year = 2004 |pmid = 15356622 |doi = 10.1038/nature02848 |ref = harv |bibcode = 2004Natur.431..152R }}</ref>

== Mechanisms ==

[[File:Mutation and selection diagram.svg|thumb|300px|[[Mutation]] followed by [[natural selection]], results in a population with darker colouration.]]
From a [[Neo-Darwinian]] perspective, evolution occurs when there are changes in the frequencies of alleles within a population of interbreeding organisms.<ref name="Ewens W.J. 2004" /> For example, the allele for black colour in a population of moths becoming more common. Mechanisms that can lead to changes in allele frequencies include [[natural selection]], [[genetic drift]], [[genetic hitchhiking]], [[mutation]] and [[gene flow]].

=== Natural selection ===

{{Further|Natural selection|Fitness (biology)}}
Evolution by means of [[natural selection]] is the process by which genetic mutations that enhance reproduction become and remain, more common in successive generations of a population. It has often been called a "self-evident" mechanism because it necessarily follows from three simple facts:
* Heritable variation exists within populations of organisms.
* Organisms produce more progeny than can survive.
* These offspring vary in their ability to survive and reproduce.

These conditions produce competition between organisms for survival and reproduction. Consequently, organisms with traits that give them an advantage over their competitors pass these advantageous traits on, while traits that do not confer an advantage are not passed on to the next generation.<ref name=Hurst>{{cite journal |author = Hurst LD |title = Fundamental concepts in genetics: genetics and the understanding of selection |journal = Nat. Rev. Genet. |volume = 10 |issue = 2 |pages = 83–93 |year = 2009 |pmid = 19119264 |doi = 10.1038/nrg2506 |ref = harv }}</ref>

The central concept of natural selection is the [[fitness (biology)|evolutionary fitness]] of an organism.<ref name=Orr>{{cite journal |author = Orr HA |title = Fitness and its role in evolutionary genetics |journal = Nat. Rev. Genet. |volume = 10 |issue = 8 |pages = 531–9 |year = 2009 |pmid = 19546856 |doi = 10.1038/nrg2603 |pmc = 2753274 |ref = harv }}</ref> Fitness is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation.<ref name=Orr/> However, fitness is not the same as the total number of offspring: instead fitness is indicated by the proportion of subsequent generations that carry an organism's genes.<ref name=Haldane>{{cite journal |author = Haldane J |title = The theory of natural selection today |journal = Nature |volume = 183 |issue = 4663 |pages = 710–3 |year = 1959 |pmid = 13644170 |doi = 10.1038/183710a0 |ref = harv |bibcode = 1959Natur.183..710H }}</ref> For example, if an organism could survive well and reproduce rapidly, but its offspring were all too small and weak to survive, this organism would make little genetic contribution to future generations and would thus have low fitness.<ref name=Orr/>

If an allele increases fitness more than the other alleles of that gene, then with each generation this allele will become more common within the population. These traits are said to be "selected ''for''". Examples of traits that can increase fitness are enhanced survival and increased [[fecundity]]. Conversely, the lower fitness caused by having a less beneficial or deleterious allele results in this allele becoming rarer&nbsp;— they are "selected ''against''".<ref name="Lande">{{cite journal |author = Lande R, Arnold SJ |year = 1983 |title = The measurement of selection on correlated characters |journal = Evolution |volume = 37 |pages = 1210–26 |doi = 10.2307/2408842 |issue = 6 |ref = harv |jstor = 2408842 }}</ref> Importantly, the fitness of an allele is not a fixed characteristic; if the environment changes, previously neutral or harmful traits may become beneficial and previously beneficial traits become harmful.<ref name="Futuyma" /> However, even if the direction of selection does reverse in this way, traits that were lost in the past may not re-evolve in an identical form (see [[Dollo's law]]).<ref>{{cite journal |doi = 10.1111/j.1558-5646.2008.00505.x |pmid = 18764918 |volume = 62 |issue = 11 |pages = 2727–2741 |last = Goldberg |first = Emma E |title = On phylogenetic tests of irreversible evolution |journal = Evolution |year = 2008 |last2 = Igić |first2 = B |ref = harv }}</ref><ref>{{cite journal |doi = 10.1016/j.tree.2008.06.013 |pmid = 18814933 |volume = 23 |issue = 11 |pages = 602–609 |last = Collin |first = Rachel |title = Reversing opinions on Dollo's Law |journal = Trends in Ecology & Evolution |year = 2008 |last2 = Miglietta |first2 = MP |ref = harv }}</ref>

[[File:Selection Types Chart.png|thumb|left|A chart showing three types of selection.
1. [[Disruptive selection]]
2. [[Stabilizing selection]]
3. [[Directional selection]]]]

Natural selection within a population for a trait that can vary across a range of values, such as height, can be categorised into three different types. The first is [[directional selection]], which is a shift in the average value of a trait over time&nbsp;— for example, organisms slowly getting taller.<ref>{{cite journal |author = Hoekstra H, Hoekstra J, Berrigan D, Vignieri S, Hoang A, Hill C, Beerli P, Kingsolver J |title = Strength and tempo of directional selection in the wild |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 98 |issue = 16 |pages = 9157–60 |year = 2001 |pmid = 11470913 |pmc = 55389 |doi = 10.1073/pnas.161281098 |ref = harv |bibcode = 2001PNAS...98.9157H }}</ref> Secondly, [[disruptive selection]] is selection for extreme trait values and often results in [[bimodal distribution|two different values]] becoming most common, with selection against the average value. This would be when either short or tall organisms had an advantage, but not those of medium height. Finally, in [[stabilizing selection]] there is selection against extreme trait values on both ends, which causes a decrease in [[variance]] around the average value and less diversity.<ref name=Hurst/><ref>{{cite journal |author = Felsenstein |title = Excursions along the Interface between Disruptive and Stabilizing Selection |journal = Genetics |volume = 93 |issue = 3 |pages = 773–95 |date = November 1, 1979 |pmid = 17248980 |pmc = 1214112 |ref = harv }}</ref> This would, for example, cause organisms to slowly become all the same height.

A special case of natural selection is [[sexual selection]], which is selection for any trait that increases mating success by increasing the attractiveness of an organism to potential mates.<ref>{{cite journal |author = Andersson M, Simmons L |title = Sexual selection and mate choice |journal = Trends Ecol. Evol. (Amst.) |volume = 21 |issue = 6 |pages = 296–302 |year = 2006 |pmid = 16769428 |doi = 10.1016/j.tree.2006.03.015 |ref = harv }}</ref> Traits that evolved through sexual selection are particularly prominent in males of some animal species, despite traits such as cumbersome antlers, mating calls or bright colours that attract predators, decreasing the survival of individual males.<ref>{{cite journal |author = Kokko H, Brooks R, McNamara J, Houston A |title = The sexual selection continuum |pmc = 1691039 |journal = Proc. Biol. Sci. |volume = 269 |issue = 1498 |pages = 1331–40 |year = 2002 |pmid = 12079655 |doi = 10.1098/rspb.2002.2020 |ref = harv }}</ref> This survival disadvantage is balanced by higher reproductive success in males that show these [[Handicap principle|hard to fake]], sexually selected traits.<ref>{{cite journal |author = Hunt J, Brooks R, Jennions M, Smith M, Bentsen C, Bussière L |title = High-quality male field crickets invest heavily in sexual display but die young |journal = Nature |volume = 432 |issue = 7020 |pages = 1024–7 |year = 2004 |pmid = 15616562 |doi = 10.1038/nature03084 |ref = harv |bibcode = 2004Natur.432.1024H }}</ref>

Natural selection most generally makes nature the measure against which individuals and individual traits, are more or less likely to survive. "Nature" in this sense refers to an [[ecosystem]], that is, a system in which organisms interact with every other element, [[abiotic|physical]] as well as [[biotic component|biological]], in their local [[environment (biophysical)|environment]]. Eugene Odum, a founder of ecology, defined an ecosystem as: "Any unit that includes all of the organisms...in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity and material cycles (ie: exchange of materials between living and nonliving parts) within the system."<ref name="Odum1971">Odum, EP (1971) Fundamentals of ecology, third edition, Saunders New York</ref> Each population within an ecosystem occupies a distinct [[Ecological niche|niche]], or position, with distinct relationships to other parts of the system. These relationships involve the life history of the organism, its position in the [[food chain]] and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection.

Natural selection can act at [[unit of selection|different levels of organisation]], such as genes, cells, individual organisms, groups of organisms and species.<ref name="Okasha07">{{cite book |last1 = Okasha |first1 = S. |year = 2007 |title = Evolution and the Levels of Selection |publisher = Oxford University Press |isbn = 0-19-926797-9 }}</ref><ref name=Gould>{{cite journal |author = Gould SJ |title = Gulliver's further travels: the necessity and difficulty of a hierarchical theory of selection |journal = Philos. Trans. R. Soc. Lond., B, Biol. Sci. |volume = 353 |issue = 1366 |pages = 307–14 |year = 1998 |pmid = 9533127 |pmc = 1692213 |doi = 10.1098/rstb.1998.0211 |ref = harv }}</ref><ref name=Mayr1997>{{cite journal |author = Mayr E |title = The objects of selection |doi = 10.1073/pnas.94.6.2091 |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 94 |issue = 6 |pages = 2091–4 |year = 1997 |pmid = 9122151 |pmc = 33654 |ref = harv |bibcode = 1997PNAS...94.2091M }}</ref> Selection can act at multiple levels simultaneously.<ref>{{cite journal |author = Maynard Smith J |title = The units of selection |journal = Novartis Found. Symp. |volume = 213 |pages = 203–11; discussion 211–7 |year = 1998 |pmid = 9653725 |ref = harv }}</ref> An example of selection occurring below the level of the individual organism are genes called [[transposon]]s, which can replicate and spread throughout a [[genome]].<ref>{{cite journal |author = Hickey DA |title = Evolutionary dynamics of transposable elements in prokaryotes and eukaryotes |journal = Genetica |volume = 86 |issue = 1–3 |pages = 269–74 |year = 1992 |pmid = 1334911 |doi = 10.1007/BF00133725 |ref = harv }}</ref> Selection at a level above the individual, such as [[group selection]], may allow the evolution of co-operation, as discussed below.<ref>{{cite journal |author = Gould SJ, Lloyd EA |title = Individuality and adaptation across levels of selection: how shall we name and generalise the unit of Darwinism? |doi = 10.1073/pnas.96.21.11904 |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 96 |issue = 21 |pages = 11904–9 |year = 1999 |pmid = 10518549 |pmc = 18385 |ref = harv |bibcode = 1999PNAS...9611904G }}</ref>

=== Biased mutation ===

In addition to being a major source of variation, mutation may also function as a mechanism of evolution when there are different probabilities at the molecular level for different mutations to occur, a process known as mutation bias.<ref>{{cite journal |author = Lynch, M. |year = 2007 |title = The frailty of adaptive hypotheses for the origins of organismal complexity |journal = PNAS |volume = 104 |pages = 8597–8604 |doi = 10.1073/pnas.0702207104 |bibcode = 2007PNAS..104.8597L |pmid = 17494740 |pmc = 1876435 |issue = suppl. 1 }}</ref> If two genotypes, for example one with the nucleotide G and another with the nucleotide A in the same position, have the same fitness, but mutation from G to A happens more often than mutation from A to G, then genotypes with A will tend to evolve.<ref>{{cite journal |title = Deterministic Mutation Rate Variation in the Human Genome |journal = Genome Research |author = Smith N.G.C., Webster M.T., Ellegren, H. |year = 2002 |volume = 12 |pages = 1350–1356 |doi = 10.1101/gr.220502 |url = http://genome.cshlp.org/content/12/9/1350.abstract |issue = 9 |pmid = 12213772 |pmc = 186654 }}</ref> Different insertion vs. deletion mutation biases in different taxa can lead to the evolution of different genome sizes.<ref>{{cite journal |author = Petrov DA, Sangster TA, Johnston JS, Hartl DL, Shaw KL |year = 2000 |title = Evidence for DNA loss as a determinant of genome size |journal = Science |volume = 287 |pages = 1060–1062 |doi = 10.1126/science.287.5455.1060 |issue = 5455 |bibcode = 2000Sci...287.1060P |pmid = 10669421 }}</ref><ref>{{cite journal |author = Petrov DA |year = 2002 |title = DNA loss and evolution of genome size in Drosophila |journal = Genetica |volume = 115 |pages = 81–91 |doi = 10.1023/A:1016076215168 |issue = 1 |pmid = 12188050 }}</ref> Developmental or mutational biases have also been observed in [[Morphology (biology)|morphological]] evolution.<ref>{{cite journal |author = Kiontke K, Barriere A , Kolotuev I, Podbilewicz B , Sommer R, Fitch DHA , Felix MA |year = 2007 |title = Trends, stasis, and drift in the evolution of nematode vulva development |journal = Current Biology |volume = 17 |pages = 1925–1937 |doi = 10.1016/j.cub.2007.10.061 |issue = 22 |pmid = 18024125 }}</ref><ref>{{cite journal |author = Braendle C, Baer CF, Felix MA |year = 2010 |title = Bias and Evolution of the Mutationally Accessible Phenotypic Space in a Developmental System |journal = PLoS Genetics |volume = 6 | id = e1000877 | doi = 10.1371/journal.pgen.1000877 |issue = 3 |editor1-last = Barsh |editor1-first = Gregory S |pages = e1000877 |pmid = 20300655 |pmc = 2837400 }}</ref> For example, according to the [[Baldwin effect|phenotype-first theory of evolution]], mutations can eventually cause the [[genetic assimilation]] of traits that were previously [[phenotypic plasticity|induced by the environment]].<ref name="Palmer 2004">{{cite journal |last = Palmer |first = RA |title = Symmetry breaking and the evolution of development |journal = [[Science (journal)|Science]] |year = 2004 |pages = 828–833 |volume = 306 |doi = 10.1126/science.1103707 |pmid = 15514148 |issue = 5697 |bibcode = 2004Sci...306..828P }}</ref><ref name="West-Eberhard 2003">{{cite book |last = West-Eberhard |first = M-J. |year = 2003 |title = Developmental plasticity and evolution |publisher = Oxford University Press |location = New York |isbn = 978-0-19-512235-0 }}</ref>

Mutation bias effects are superimposed on other processes. If selection would favor either one out of two mutations, but there is no extra advantage to having both, then the mutation that occurs the most frequently is the one that is most likely to become fixed in a population.<ref>{{cite journal |author = Stoltzfus, A and Yampolsky, L.Y. |year = 2009 |title = Climbing Mount Probable: Mutation as a Cause of Nonrandomness in Evolution |journal = J Hered |volume = 100 |pages = 637–647 |doi = 10.1093/jhered/esp048 |pmid = 19625453 |issue = 5 }}</ref><ref>{{cite journal |author = Yampolsky, L.Y. and Stoltzfus, A |year = 2001 |title = Bias in the introduction of variation as an orienting factor in evolution |journal = Evol Dev |volume = 3 |pages = 73–83 |doi = 10.1046/j.1525-142x.2001.003002073.x |pmid = 11341676 |issue = 2 }}</ref> Mutations leading to the loss of function of a gene are much more common than mutations that produce a new, fully functional gene. Most loss of function mutations are selected against. But when selection is weak, mutation bias towards loss of function can affect evolution.<ref>{{cite journal |author = Haldane, JBS |year = 1933 |title = The Part Played by Recurrent Mutation in Evolution |journal = American Naturalist |volume = 67 |pages = 5–19 |jstor = 2457127 |doi = 10.1086/280465 |issue = 708 }}</ref> For example, [[pigment]]s are no longer useful when animals live in the darkness of caves, and tend to be lost.<ref>{{cite journal |doi = 10.1016/j.cub.2007.01.051 |pmid = 17306543 |volume = 17 |issue = 5 |pages = 452–454 |last = Protas |first = Meredith |title = Regressive evolution in the Mexican cave tetra, Astyanax mexicanus |journal = Current Biology |year = 2007 |last2 = Conrad |first2 = M |last3 = Gross |first3 = JB |last4 = Tabin |first4 = C |last5 = Borowsky |first5 = R |pmc = 2570642 |ref = harv }}</ref> This kind of loss of function can occur because of mutation bias, and/or because the function had a cost, and once the benefit of the function disappeared, natural selection leads to the loss. Loss of [[endospore|sporulation]] ability in a [[Bacillus subtilis|bacterium]] during laboratory evolution appears to have been caused by mutation bias, rather than natural selection against the cost of maintaining sporulation ability.<ref>{{cite journal |author = Maughan H, Masel J, Birky WC, Nicholson WL |title = The roles of mutation accumulation and selection in loss of sporulation in experimental populations of Bacillus subtilis |doi = 10.1534/genetics.107.075663 |journal = Genetics |volume = 177 |pages = 937–948 |year = 2007 |issue = 2 |pmid = 17720926 |pmc = 2034656 }}</ref> When there is no selection for loss of function, the speed at which loss evolves depends more on the mutation rate than it does on the [[effective population size]],<ref>{{cite journal |author = Masel J, King OD, Maughan H |title = The loss of adaptive plasticity during long periods of environmental stasis |doi = 10.1086/510212 |journal = American Naturalist |volume = 169 |issue = 1 |pages = 38–46 |year = 2007 |pmid = 17206583 |pmc = 1766558 }}</ref> indicating that it is driven more by mutation bias than by genetic drift.

=== Genetic drift ===

{{Further|Genetic drift|Effective population size}}
[[File:Allele-frequency.png|thumb|Simulation of [[genetic drift]] of 20 unlinked alleles in populations of 10 (top) and 100 (bottom). Drift to [[Fixation (population genetics)|fixation]] is more rapid in the smaller population.]]
Genetic drift is the change in [[allele frequency]] from one generation to the next that occurs because alleles are subject to [[sampling error]].<ref name="Masel 2011">{{cite journal |volume = 21 |pages = R837–R838 |author = Masel J |title = Genetic drift |journal = Current Biology |year = 2011 |doi = 10.1016/j.cub.2011.08.007 |url = http://www.sciencedirect.com/science/article/pii/S0960982211008827 |issue = 20 |pmid = 22032182 }}</ref> As a result, when selective forces are absent or relatively weak, allele frequencies tend to "drift" upward or downward randomly (in a [[random walk]]). This drift halts when an allele eventually becomes [[Fixation (population genetics)|fixed]], either by disappearing from the population, or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone. Even in the absence of selective forces, genetic drift can cause two separate populations that began with the same genetic structure to drift apart into two divergent populations with different sets of alleles.<ref>{{cite journal |author = Lande R |title = Fisherian and Wrightian theories of speciation |journal = Genome |volume = 31 |issue = 1 |pages = 221–7 |year = 1989 |pmid = 2687093 |ref = harv |doi = 10.1139/g89-037 }}</ref>

It is usually difficult to measure the relative importance of selection and neutral processes, including drift.<ref>{{cite journal |doi = 10.1038/nrg2207 |pmid = 17943192 |volume = 8 |issue = 11 |pages = 845–856 |last = Mitchell-Olds |first = Thomas |title = Which evolutionary processes influence natural genetic variation for phenotypic traits? |journal = Nature Reviews Genetics |year = 2007 |last2 = Willis |first2 = JH |last3 = Goldstein |first3 = DB |ref = harv }}</ref> The comparative importance of adaptive and non-adaptive forces in driving evolutionary change is an area of [[current research in evolutionary biology|current research]].<ref>{{cite journal |author = Nei M |title = Selectionism and neutralism in molecular evolution |doi = 10.1093/molbev/msi242 |journal = Mol. Biol. Evol. |volume = 22 |issue = 12 |pages = 2318–42 |year = 2005 |pmid = 16120807 |pmc = 1513187 |ref = harv }}</ref>

The [[neutral theory of molecular evolution]] proposed that most evolutionary changes are the result of the fixation of [[neutral mutation]]s by genetic drift.<ref name="Kimura M 1991 367–86" /> Hence, in this model, most genetic changes in a population are the result of constant mutation pressure and genetic drift.<ref>{{cite journal |author = Kimura M |title = The neutral theory of molecular evolution and the world view of the neutralists |journal = Genome |volume = 31 |issue = 1 |pages = 24–31 |year = 1989 |pmid = 2687096 |ref = harv |doi = 10.1139/g89-009 }}</ref> This form of the neutral theory is now largely abandoned, since it does not seem to fit the genetic variation seen in nature.<ref>{{cite journal |author = Kreitman M |title = The neutral theory is dead. Long live the neutral theory |journal = BioEssays |volume = 18 |issue = 8 |pages = 678–83; discussion 683 |year = 1996 |pmid = 8760341 |doi = 10.1002/bies.950180812 |ref = harv }}</ref><ref>{{cite journal |author = Leigh E.G. (Jr) |year = 2007 |title = Neutral theory: a historical perspective |journal = [[Journal of Evolutionary Biology]] |pmid = 17956380 |volume = 20 |issue = 6 |pages = 2075–91 |doi = 10.1111/j.1420-9101.2007.01410.x |ref = harv }}</ref> However, a more recent and better-supported version of this model is the [[nearly neutral theory of molecular evolution|nearly neutral theory]], where a mutation that would be neutral in a small population is not necessarily neutral in a large population.<ref name=Hurst/> Other alternative theories propose that genetic drift is dwarfed by other stochastic forces in evolution, such as [[genetic hitchhiking]], also known as genetic draft.<ref name="Masel 2011" /><ref name="gillespie 2001">{{cite journal |volume = 55 |issue = 11 |pages = 2161–2169 |last = Gillespie |first = John H. |title = Is the population size of a species relevant to its evolution? |journal = Evolution |year = 2001 |pmid = 11794777 }}</ref><ref>{{cite journal |volume = 188 |pages = 975–996 |author = R.A. Neher and B.I. Shraiman |title = Genetic Draft and Quasi-Neutrality in Large Facultatively Sexual Populations |journal = Genetics |year = 2011 |doi = 10.1534/genetics.111.128876 |pmid = 21625002 |pmc = 3176096 |issue = 4 }}</ref>

The time for a neutral allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations.<ref>{{cite journal |author = Otto S, Whitlock M |title = The probability of fixation in populations of changing size |journal = Genetics |volume = 146 |issue = 2 |pages = 723–33 |date = June 1, 1997 |pmid = 9178020 |pmc = 1208011 |ref = harv }}</ref> The number of individuals in a population is not critical, but instead a measure known as the [[effective population size]].<ref name=Charlesworth>{{cite journal |author = Charlesworth B |title = Fundamental concepts in genetics: Effective population size and patterns of molecular evolution and variation |journal = Nat. Rev. Genet. |volume = 10 |pages = 195–205 |year = 2009 |pmid = 19204717 |doi = 10.1038/nrg2526 |issue = 3 |ref = harv }}</ref> The effective population is usually smaller than the total population since it takes into account factors such as the level of inbreeding and the stage of the lifecycle in which the population is the smallest.<ref name="Charlesworth" /> The effective population size may not be the same for every gene in the same population.<ref>{{cite journal |author = Asher D. Cutter and Jae Young Choi |title = Natural selection shapes nucleotide polymorphism across the genome of the nematode Caenorhabditis briggsae |journal = Genome Research |volume = 20 |pages = 1103–1111 |year = 2010 |doi=10.1101/gr.104331.109 |pmid=20508143 |pmc=2909573 |issue = 8}}</ref>

=== Genetic hitchhiking ===

{{Further|Genetic hitchhiking|Hill-Robertson effect|Selective sweep|Genetic drift}}

Recombination allows alleles on the same strand of DNA to become separated. However, the rate of recombination is low (approximately two events per chromosome per generation). As a result, genes close together on a chromosome may not always be shuffled away from each other and genes that are close together tend to be inherited together, a phenomenon known as [[genetic linkage|linkage]].<ref>{{cite journal |author = Lien S, Szyda J, Schechinger B, Rappold G, Arnheim N |title = Evidence for heterogeneity in recombination in the human pseudoautosomal region: high resolution analysis by sperm typing and radiation-hybrid mapping |journal = Am. J. Hum. Genet. |volume = 66 |issue = 2 |pages = 557–66 |year = 2000 |pmid = 10677316 |pmc = 1288109 |doi = 10.1086/302754 |ref = harv }}</ref> This tendency is measured by finding how often two alleles occur together on a single chromosome compared to [[independence (probability theory)|expectations]], which is called their [[linkage disequilibrium]]. A set of alleles that is usually inherited in a group is called a [[haplotype]]. This can be important when one allele in a particular haplotype is strongly beneficial: natural selection can drive a [[selective sweep]] that will also cause the other alleles in the haplotype to become more common in the population; this effect is called [[genetic hitchhiking]] or genetic draft.<ref>{{cite journal |doi = 10.1098/rstb.2000.0716 |pmid = 11127900 |volume = 355 |issue = 1403 |pages = 1553–1562 |last = Barton |first = N H |title = Genetic hitchhiking |journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |year = 2000 |pmc = 1692896 |ref = harv }}</ref> Genetic draft caused by the fact that some neutral genes are genetically linked to others that are under selection can be partially captured by an appropriate effective population size.<ref name="gillespie 2001" />

=== Gene flow ===

{{Further|Gene flow|Hybrid (biology)|Horizontal gene transfer}}
[[Gene flow]] is the exchange of genes between populations and between species.<ref name="Morjan C, Rieseberg L 2004 1341–56" /> The presence or absence of gene flow fundamentally changes the course of evolution. Due to the complexity of organisms, any two completely isolated populations will eventually evolve genetic incompatibilities through neutral processes, as in the [[Bateson-Dobzhansky-Muller Model|Bateson-Dobzhansky-Muller model]], even if both populations remain essentially identical in terms of their adaptation to the environment.

If genetic differentiation between populations develops, gene flow between populations can introduce traits or alleles which are disadvantageous in the local population and this may lead to organism within these populations to evolve mechanisms that prevent mating with genetically distant populations, eventually resulting in the appearance of new species. Thus, exchange of genetic information between individuals is fundamentally important for the development of the biological species concept (BSC).

During the development of the modern synthesis, [[Sewall Wright]]'s developed his [[shifting balance theory]] that gene flow between partially isolated populations was an important aspect of adaptive evolution.<ref>{{cite journal |last1 = Wright |first1 = Sewall |year = 1932 |title = The roles of mutation, inbreeding, crossbreeding and selection in evolution |url = http://www.blackwellpublishing.com/ridley/classictexts/wright.asp |journal = Proc. 6th Int. Cong. Genet |volume = 1 |issue = |pages = 356–366 }}</ref> However, recently there has been substantial criticism of the importance of the [[shifting balance theory]].<ref name="Coyne 1997">{{cite journal | last1 = Coyne | first1 = Jerry A. | coauthors = Barton, Turelli | title = Perspective: A Critique of Sewall Wright's Shifting Balance Theory of Evolution | journal = Evolution | year = 1997 | volume = 51 | issue = 3 | series = 3 | pages = 643–671 | doi = 10.2307/2411143 }}</ref>

== Outcomes ==

Evolution influences every aspect of the form and behaviour of organisms. Most prominent are the specific behavioural and physical [[adaptation]]s that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by [[Co-operation (evolution)|co-operating]] with each other, usually by aiding their relatives or engaging in mutually beneficial [[symbiosis]]. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed.

These outcomes of evolution are sometimes divided into [[macroevolution]], which is evolution that occurs at or above the level of species, such as [[extinction]] and [[speciation]] and [[microevolution]], which is smaller evolutionary changes, such as adaptations, within a species or population.<ref name=ScottEC>{{cite journal |author = Scott EC, Matzke NJ |title = Biological design in science classrooms |volume = 104 |journal = Proc. Natl. Acad. Sci. U.S.A. |issue = suppl_1 |pages = 8669–76 |year = 2007 |pmid = 17494747 |pmc = 1876445 |doi = 10.1073/pnas.0701505104 |ref = harv |bibcode = 2007PNAS..104.8669S }}</ref> In general, macroevolution is regarded as the outcome of long periods of microevolution.<ref>{{cite journal |author = Hendry AP, Kinnison MT |title = An introduction to microevolution: rate, pattern, process |journal = Genetica |volume = 112–113 |pages = 1–8 |year = 2001 |pmid = 11838760 |doi = 10.1023/A:1013368628607 |ref = harv }}</ref> Thus, the distinction between micro- and macroevolution is not a fundamental one&nbsp;– the difference is simply the time involved.<ref>{{cite journal |author = Leroi AM |title = The scale independence of evolution |journal = Evol. Dev. |volume = 2 |issue = 2 |pages = 67–77 |year = 2000 |pmid = 11258392 |doi = 10.1046/j.1525-142x.2000.00044.x |ref = harv }}</ref> However, in macroevolution, the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution might involve selection at different levels&nbsp;– with microevolution acting on genes and organisms, versus macroevolutionary processes such as [[species selection]] acting on entire species and affecting their rates of speciation and extinction.{{sfn|Gould|2002|pp=657–658}}<ref>{{cite journal |author = Gould SJ |title = Tempo and mode in the macroevolutionary reconstruction of Darwinism |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 91 |issue = 15 |pages = 6764–71 |year = 1994 |pmid = 8041695 |pmc = 44281 |doi = 10.1073/pnas.91.15.6764 |ref = harv |bibcode = 1994PNAS...91.6764G }}</ref><ref name=Jablonski2000>{{cite journal |author = Jablonski, D. |year = 2000 |title = Micro- and macroevolution: scale and hierarchy in evolutionary biology and paleobiology |journal = Paleobiology |volume = 26 |issue = sp4 |pages = 15–52 |doi = 10.1666/0094-8373(2000)26[15:MAMSAH]2.0.CO;2 |ref = harv }}</ref>

A common misconception is that evolution has goals or long-term plans; realistically however, evolution has no long-term goal and does not necessarily produce greater complexity.<ref name=sciam>Michael J. Dougherty. [http://www.sciam.com/article.cfm?id=is-the-human-race-evolvin Is the human race evolving or devolving?] ''[[Scientific American]]'' July 20, 1998.</ref><ref>[[TalkOrigins Archive]] response to [[Creationist]] claims&nbsp;– [http://www.talkorigins.org/indexcc/CB/CB932.html Claim CB932: Evolution of degenerate forms]</ref> Although [[evolution of complexity|complex species]] have evolved, they occur as a side effect of the overall number of organisms increasing and simple forms of life still remain more common in the biosphere.<ref name=Carroll>{{cite journal |author = Carroll SB |title = Chance and necessity: the evolution of morphological complexity and diversity |journal = Nature |volume = 409 |issue = 6823 |pages = 1102–9 |year = 2001 |pmid = 11234024 |doi = 10.1038/35059227 |ref = harv }}</ref> For example, the overwhelming majority of species are microscopic [[prokaryote]]s, which form about half the world's [[biomass]] despite their small size,<ref>{{cite journal |author = Whitman W, Coleman D, Wiebe W |title = Prokaryotes: the unseen majority |doi = 10.1073/pnas.95.12.6578 |journal = Proc Natl Acad Sci U S A |volume = 95 |issue = 12 |pages = 6578–83 |year = 1998 |pmid = 9618454 |pmc = 33863 |ref = harv |bibcode = 1998PNAS...95.6578W }}</ref> and constitute the vast majority of Earth's biodiversity.<ref name=Schloss>{{cite journal |author = Schloss P, Handelsman J |title = Status of the microbial census |journal = Microbiol Mol Biol Rev |volume = 68 |issue = 4 |pages = 686–91 |year = 2004 |pmid = 15590780 |pmc = 539005 |doi = 10.1128/MMBR.68.4.686-691.2004 |ref = harv }}</ref> Simple organisms have therefore been the dominant form of life on Earth throughout its history and continue to be the main form of life up to the present day, with complex life only appearing more diverse because it is [[biased sample|more noticeable]].<ref>{{cite journal |author = Nealson K |title = Post-Viking microbiology: new approaches, new data, new insights |journal = Orig Life Evol Biosph |volume = 29 |issue = 1 |pages = 73–93 |year = 1999 |pmid = 11536899 |doi = 10.1023/A:1006515817767 |ref = harv }}</ref> Indeed, the evolution of [[microorganism]]s is particularly important to [[current research in evolutionary biology|modern evolutionary research]], since their rapid reproduction allows the study of [[experimental evolution]] and the observation of evolution and adaptation in real time.<ref name=Buckling>{{cite journal |author = Buckling A, Craig Maclean R, Brockhurst MA, Colegrave N |title = The Beagle in a bottle |journal = Nature |volume = 457 |issue = 7231 |pages = 824–9 |year = 2009 |pmid = 19212400 |doi = 10.1038/nature07892 |ref = harv |bibcode = 2009Natur.457..824B }}</ref><ref>{{cite journal |author = Elena SF, Lenski RE |title = Evolution experiments with microorganisms: the dynamics and genetic bases of adaptation |journal = Nat. Rev. Genet. |volume = 4 |issue = 6 |pages = 457–69 |year = 2003 |pmid = 12776215 |doi = 10.1038/nrg1088 |ref = harv }}</ref>

=== Adaptation ===
{{details|Adaptation}}
[[File:Homology vertebrates.svg|thumb|300px|[[Homology (biology)|Homologous]] bones in the limbs of [[tetrapods]]. The bones of these animals have the same basic structure, but have been adapted for specific uses.]]
Adaptation is the process that makes organisms better suited to their [[habitat]].<ref>Mayr, Ernst 1982. ''The growth of biological thought''. Harvard. p483: "Adaptation... could no longer be considered a static condition, a product of a creative past and became instead a continuing dynamic process."</ref><ref>The ''Oxford Dictionary of Science'' defines ''adaptation'' as "Any change in the structure or functioning of an organism that makes it better suited to its environment".</ref> Also, the term adaptation may refer to a [[Trait (biology)|trait]] that is important for an organism's survival. For example, the adaptation of horses' teeth to the grinding of grass. By using the term ''adaptation'' for the evolutionary process and ''adaptive trait'' for the product (the bodily part or function), the two senses of the word may be distinguished. Adaptations are produced by [[natural selection]].<ref>{{cite journal |author = Orr H |title = The genetic theory of adaptation: a brief history |journal = Nat. Rev. Genet. |volume = 6 |issue = 2 |pages = 119–27 |year = 2005 |pmid = 15716908 |doi = 10.1038/nrg1523 |ref = harv }}</ref> The following definitions are due to [[Theodosius Dobzhansky]].
# ''Adaptation'' is the evolutionary process whereby an organism becomes better able to live in its [[habitat]] or habitats.<ref>{{cite book |last1 = Dobzhansky |first1 = T. |last2 = Hecht |first2 = MK |last3 = Steere |first3 = WC |year = 1968 |chapter = On some fundamental concepts of evolutionary biology |title = Evolutionary biology volume 2 |pages = 1–34 |publisher = Appleton-Century-Crofts |location = New York |edition = 1st }}</ref>
# ''Adaptedness'' is the state of being adapted: the degree to which an organism is able to live and reproduce in a given set of habitats.<ref>{{cite book |last1 = Dobzhansky |first1 = T. |year = 1970 |title = Genetics of the evolutionary process |publisher = Columbia |location = N.Y. |pages = 4–6, 79–82, 84–87 |isbn = 0-231-02837-7 }}</ref>
# An ''adaptive trait'' is an aspect of the developmental pattern of the organism which enables or enhances the probability of that organism surviving and reproducing.<ref>{{cite journal |doi = 10.2307/2406099 |last1 = Dobzhansky |first1 = T. |year = 1956 |title = Genetics of natural populations XXV. Genetic changes in populations of ''Drosophila pseudoobscura'' and ''Drosphila persimilis'' in some locations in California |journal = Evolution |volume = 10 |issue = 1 |pages = 82–92 |jstor = 2406099 }}</ref>

Adaptation may cause either the gain of a new feature, or the loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to [[antibiotic]] selection, with genetic changes causing [[antibiotic resistance]] by both modifying the target of the drug, or increasing the activity of transporters that pump the drug out of the cell.<ref>{{cite journal |author = Nakajima A, Sugimoto Y, Yoneyama H, Nakae T |title = High-level fluoroquinolone resistance in Pseudomonas aeruginosa due to interplay of the MexAB-OprM efflux pump and the DNA gyrase mutation |journal = Microbiol. Immunol. |volume = 46 |issue = 6 |pages = 391–5 |year = 2002 |pmid = 12153116 |ref = harv }}</ref> Other striking examples are the bacteria ''[[Escherichia coli]]'' evolving the ability to use [[citric acid]] as a nutrient in a [[E. coli long-term evolution experiment|long-term laboratory experiment]],<ref>{{cite journal |author = Blount ZD, Borland CZ, Lenski RE |title = Inaugural Article: Historical contingency and the evolution of a key innovation in an experimental population of Escherichia coli |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 105 |issue = 23 |pages = 7899–906 |year = 2008 |pmid = 18524956 |doi = 10.1073/pnas.0803151105 |pmc = 2430337 |ref = harv |bibcode = 2008PNAS..105.7899B }}</ref> ''[[Flavobacterium]]'' evolving a novel enzyme that allows these bacteria to grow on the by-products of [[nylon]] manufacturing,<ref>{{cite journal |author = Okada H, Negoro S, Kimura H, Nakamura S |title = Evolutionary adaptation of plasmid-encoded enzymes for degrading nylon oligomers |journal = Nature |volume = 306 |issue = 5939 |pages = 203–6 |year = 1983 |pmid = 6646204 |doi = 10.1038/306203a0 |ref = harv |bibcode = 1983Natur.306..203O }}</ref><ref>{{cite journal |author = Ohno S |title = Birth of a unique enzyme from an alternative reading frame of the preexisted, internally repetitious coding sequence |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 81 |issue = 8 |pages = 2421–5 |year = 1984 |pmid = 6585807 |pmc = 345072 |doi = 10.1073/pnas.81.8.2421 |ref = harv |bibcode = 1984PNAS...81.2421O }}</ref> and the soil bacterium ''[[Sphingobium]]'' evolving an entirely new [[metabolic pathway]] that degrades the synthetic [[pesticide]] [[pentachlorophenol]].<ref>{{cite journal |author = Copley SD |title = Evolution of a metabolic pathway for degradation of a toxic xenobiotic: the patchwork approach |journal = Trends Biochem. Sci. |volume = 25 |issue = 6 |pages = 261–5 |year = 2000 |pmid = 10838562 |doi = 10.1016/S0968-0004(00)01562-0 |ref = harv }}</ref><ref>{{cite journal |author = Crawford RL, Jung CM, Strap JL |title = The recent evolution of pentachlorophenol (PCP)-4-monooxygenase (PcpB) and associated pathways for bacterial degradation of PCP |journal = Biodegradation |volume = 18 |issue = 5 |pages = 525–39 |year = 2007 |pmid = 17123025 |doi = 10.1007/s10532-006-9090-6 |ref = harv }}</ref> An interesting but still controversial idea is that some adaptations might increase the ability of organisms to generate genetic diversity and adapt by natural selection (increasing organisms' [[evolvability]]).<ref>{{cite journal |doi = 10.2307/3212376 |author = Eshel I |title = Clone-selection and optimal rates of mutation |journal = Journal of Applied Probability |volume = 10 |issue = 4 |pages = 728–738 |year = 1973 |jstor = 3212376 }}</ref><ref>{{cite journal |author = Masel J, Bergman A, |title = The evolution of the evolvability properties of the yeast prion [PSI+] |journal = Evolution |volume = 57 |issue = 7 |pages = 1498–1512 |year = 2003 |pmid = 12940355 }}</ref><ref>{{cite journal |author = Lancaster AK, Bardill JP, True HL, Masel J |year = 2010 |title = The Spontaneous Appearance Rate of the Yeast Prion [PSI+] and Its Implications for the Evolution of the Evolvability Properties of the [PSI+] System |journal = Genetics |pmid = 19917766 |volume = 184 |issue = 2 |pmc = 2828720 |pages = 393–400 |doi = 10.1534/genetics.109.110213 }}</ref><ref>{{cite journal |author = Draghi J, Wagner G |year = 2008 |title = Evolution of evolvability in a developmental model |journal = Theoretical Population Biology |volume = 62 |pages = 301–315 }}</ref>

[[File:Whale skeleton.png|300px|thumb|left|A [[baleen whale]] skeleton, ''a'' and ''b'' label [[flipper (anatomy)|flipper]] bones, which were [[adaptation|adapted]] from front [[leg]] bones: while ''c'' indicates [[Vestigiality|vestigial]] leg bones, suggesting an adaptation from land to sea.<ref name="transformation445">{{cite journal |author = Bejder L, Hall BK |title = Limbs in whales and limblessness in other vertebrates: mechanisms of evolutionary and developmental transformation and loss |journal = Evol. Dev. |volume = 4 |issue = 6 |pages = 445–58 |year = 2002 |pmid = 12492145 |doi = 10.1046/j.1525-142X.2002.02033.x |ref = harv }}</ref>]]
Adaptation occurs through the gradual modification of existing structures. Consequently, structures with similar internal organisation may have different functions in related organisms. This is the result of a single [[homology (biology)|ancestral structure]] being adapted to function in different ways. The bones within [[bat]] wings, for example, are very similar to those in [[mouse|mice]] feet and [[primate]] hands, due to the descent of all these structures from a common mammalian ancestor.<ref>{{cite journal |doi = 10.1554/05-233.1 |pmid = 16526515 |volume = 59 |issue = 12 |pages = 2691–704 |last = Young |first = Nathan M. |title = Serial homology and the evolution of mammalian limb covariation structure |journal = Evolution |accessdate = September 24, 2009 |year = 2005 |url = http://www.bioone.org/doi/abs/10.1554/05-233.1 |last2 = Hallgrímsson |first2 = B |ref = harv }}</ref> However, since all living organisms are related to some extent,<ref name=Penny1999/> even organs that appear to have little or no structural similarity, such as [[arthropod]], squid and vertebrate eyes, or the limbs and wings of arthropods and vertebrates, can depend on a common set of homologous genes that control their assembly and function; this is called [[deep homology]].<ref>{{cite journal |doi = 10.1017/S1464793102006097 |pmid = 14558591 |volume = 78 |issue = 3 |pages = 409–433 |last = Hall |first = Brian K |title = Descent with modification: the unity underlying homology and homoplasy as seen through an analysis of development and evolution |journal = Biological Reviews of the Cambridge Philosophical Society |year = 2003 |ref = harv }}</ref><ref>{{cite journal |doi = 10.1038/nature07891 |pmid = 19212399 |volume = 457 |issue = 7231 |pages = 818–823 |last = Shubin |first = Neil |title = Deep homology and the origins of evolutionary novelty |journal = Nature |year = 2009 |last2 = Tabin |first2 = C |last3 = Carroll |first3 = S |ref = harv |bibcode = 2009Natur.457..818S }}</ref>

During evolution, some structures may lose their original function and become [[vestigial structure]]s.<ref name=Fong>{{cite journal |author = Fong D, Kane T, Culver D |title = Vestigialization and Loss of Nonfunctional Characters |journal = Ann. Rev. Ecol. Syst. |volume = 26 |issue = 4 |pages = 249–68 |year = 1995 |doi = 10.1146/annurev.es.26.110195.001341 |ref = harv |pmid = }}</ref> Such structures may have little or no function in a current species, yet have a clear function in ancestral species, or other closely related species. Examples include [[pseudogene]]s,<ref>{{cite journal |author = Zhang Z, Gerstein M |title = Large-scale analysis of pseudogenes in the human genome |journal = Curr. Opin. Genet. Dev. |volume = 14 |issue = 4 |pages = 328–35 |year = 2004 |pmid = 15261647 |doi = 10.1016/j.gde.2004.06.003 |ref = harv }}</ref> the non-functional remains of eyes in blind cave-dwelling fish,<ref>{{cite journal |author = Jeffery WR |title = Adaptive evolution of eye degeneration in the Mexican blind cavefish |doi = 10.1093/jhered/esi028 |journal = J. Hered. |volume = 96 |issue = 3 |pages = 185–96 |year = 2005 |pmid = 15653557 |ref = harv }}</ref> wings in flightless birds,<ref>{{cite journal |author = Maxwell EE, Larsson HC |title = Osteology and myology of the wing of the Emu (Dromaius novaehollandiae) and its bearing on the evolution of vestigial structures |journal = J. Morphol. |volume = 268 |issue = 5 |pages = 423–41 |year = 2007 |pmid = 17390336 |doi = 10.1002/jmor.10527 |ref = harv }}</ref> and the presence of hip bones in whales and snakes.<ref name="transformation445" /> Examples of [[Human vestigiality|vestigial structures in humans]] include [[wisdom teeth]],<ref>{{cite journal |author = Silvestri AR, Singh I |title = The unresolved problem of the third molar: would people be better off without it? |url = http://jada.ada.org/cgi/content/full/134/4/450 |journal = Journal of the American Dental Association (1939) |volume = 134 |issue = 4 |pages = 450–5 |year = 2003 |pmid = 12733778 |doi = |ref = harv }}</ref> the [[coccyx]],<ref name=Fong/> the [[vermiform appendix]],<ref name=Fong/> and other behavioural vestiges such as [[goose bumps]]<ref>{{cite book |last = Coyne |first = Jerry A. |authorlink = Jerry A. Coyne |title = Why Evolution is True |publisher = Penguin Group |year = 2009 |isbn = 978-0-670-02053-9 |page = 62 }}</ref><ref>Darwin, Charles. (1872) ''[[The Expression of the Emotions in Man and Animals]]'' John Murray, London.</ref> and [[primitive reflexes]].<ref>{{cite book |title = Psychology |edition = fifth |author = Peter Gray |year = 2007 |page = 66 |publisher = Worth Publishers |isbn = 0-7167-0617-2 }}</ref><ref>{{cite book |title = Why Evolution Is True |last = Coyne |first = Jerry A. |year = 2009 |pages = 85–86 |publisher = Penguin Group |isbn = 978-0-670-02053-9 }}</ref><ref>{{cite book |title = Archetype: A Natural History of the Self |author = Anthony Stevens |year = 1982 |page = 87 |publisher = Routledge & Kegan Paul |isbn = 0-7100-0980-1 }}</ref>

However, many traits that appear to be simple adaptations are in fact [[exaptation]]s: structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in the process.{{sfn|Gould|2002|pp=1235–1236}} One example is the African lizard ''Holaspis guentheri'', which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to tree—an [[exaptation]].{{sfn|Gould|2002|pp=1235–1236}} Within cells, [[molecular machine]]s such as the bacterial [[flagella]]<ref>{{cite journal |doi = 10.1038/nrmicro1493 |pmid = 16953248 |volume = 4 |issue = 10 |pages = 784–790 |last = Pallen |first = Mark J. |title = From The Origin of Species to the origin of bacterial flagella |journal = Nat Rev Micro |accessdate = September 18, 2009 |date = 2006-10 |url = http://home.planet.nl/~gkorthof/pdf/Pallen_Matzke.pdf |last2 = Matzke |first2 = NJ |ref = harv }}</ref> and [[translocase of the inner membrane|protein sorting machinery]]<ref>{{cite journal |doi = 10.1073/pnas.0908264106 |pmid = 19717453 |volume = 106 |issue = 37 |pages = 15791–15795 |last = Clements |first = Abigail |title = The reducible complexity of a mitochondrial molecular machine |journal = Proceedings of the National Academy of Sciences |year = 2009 |last2 = Bursac |first2 = D |last3 = Gatsos |first3 = X |last4 = Perry |first4 = AJ |last5 = Civciristov |first5 = S |last6 = Celik |first6 = N |last7 = Likic |first7 = VA |last8 = Poggio |first8 = S |last9 = Jacobs-Wagner |first9 = C |pmc = 2747197 |ref = harv |bibcode = 2009PNAS..10615791C }}</ref> evolved by the recruitment of several pre-existing proteins that previously had different functions.<ref name=ScottEC/> Another example is the recruitment of enzymes from [[glycolysis]] and [[xenobiotic metabolism]] to serve as structural proteins called [[crystallin]]s within the lenses of organisms' [[eye]]s.<ref>{{cite journal |author = Piatigorsky J, Kantorow M, Gopal-Srivastava R, Tomarev SI |title = Recruitment of enzymes and stress proteins as lens crystallins |journal = EXS |volume = 71 |pages = 241–50 |year = 1994 |pmid = 8032155 |ref = harv }}</ref><ref>{{cite journal |author = Wistow G |title = Lens crystallins: gene recruitment and evolutionary dynamism |journal = Trends Biochem. Sci. |volume = 18 |issue = 8 |pages = 301–6 |year = 1993 |pmid = 8236445 |doi = 10.1016/0968-0004(93)90041-K |ref = harv }}</ref>

A critical principle of [[ecology]] is that of [[competitive exclusion principle|competitive exclusion]]: no two species can occupy the same niche in the same environment for a long time.<ref>{{cite journal |author = Hardin G |authorlink = Garrett Hardin |title = The competitive exclusion principle |journal = Science |volume = 131 |issue = 3409 |pages = 1292–7 |year = 1960 |pmid = 14399717 |doi = 10.1126/science.131.3409.1292 |ref = harv |bibcode = 1960Sci...131.1292H }}</ref> Consequently, natural selection will tend to force species to adapt to different [[ecological niche]]s. This may mean that, for example, two species of [[cichlid]] fish adapt to live in different [[habitat]]s, which will minimise the competition between them for food.<ref>{{cite journal |author = Kocher TD |title = Adaptive evolution and explosive speciation: the cichlid fish model |journal = Nat. Rev. Genet. |volume = 5 |issue = 4 |pages = 288–98 |year = 2004 |pmid = 15131652 |doi = 10.1038/nrg1316 |url = http://hcgs.unh.edu/staff/kocher/pdfs/Kocher2004.pdf |ref = harv }}</ref>

An area of current investigation in [[evolutionary developmental biology]] is the [[Developmental biology|developmental]] basis of adaptations and exaptations.<ref>{{cite journal |author = Johnson NA, Porter AH |title = Toward a new synthesis: population genetics and evolutionary developmental biology |journal = Genetica |volume = 112–113 |pages = 45–58 |year = 2001 |pmid = 11838782 |doi = 10.1023/A:1013371201773 |ref = harv }}</ref> This research addresses the origin and evolution of [[Embryogenesis|embryonic development]] and how modifications of development and developmental processes produce novel features.<ref>{{cite journal |author = Baguñà J, Garcia-Fernàndez J |title = Evo-Devo: the long and winding road |url = http://www.ijdb.ehu.es/web/paper.php?doi=14756346 |journal = Int. J. Dev. Biol. |volume = 47 |issue = 7–8 |pages = 705–13 |year = 2003 |pmid = 14756346 |ref = harv }}<br />*{{cite journal |author = Love AC. |year = 2003 |title = Evolutionary Morphology, Innovation and the Synthesis of Evolutionary and Developmental Biology |journal = Biology and Philosophy |volume = 18 |issue = 2 |pages = 309–345 |doi = 10.1023/A:1023940220348 |url = http://www.springerlink.com/content/k7745m8871l3m360/ |ref = harv }}</ref> These studies have shown that evolution can alter development to produce new structures, such as embryonic bone structures that develop into the jaw in other animals instead forming part of the middle ear in mammals.<ref>{{cite journal |author = Allin EF |title = Evolution of the mammalian middle ear |journal = J. Morphol. |volume = 147 |issue = 4 |pages = 403–37 |year = 1975 |pmid = 1202224 |doi = 10.1002/jmor.1051470404 |ref = harv }}</ref> It is also possible for structures that have been lost in evolution to reappear due to changes in developmental genes, such as a mutation in [[chicken]]s causing embryos to grow teeth similar to those of [[crocodile]]s.<ref>{{cite journal |author = Harris MP, Hasso SM, Ferguson MW, Fallon JF |title = The development of archosaurian first-generation teeth in a chicken mutant |journal = Curr. Biol. |volume = 16 |issue = 4 |pages = 371–7 |year = 2006 |pmid = 16488870 |doi = 10.1016/j.cub.2005.12.047 |ref = harv }}</ref> It is now becoming clear that most alterations in the form of organisms are due to changes in a small set of conserved genes.<ref>{{cite journal |author = Carroll SB |title = Evo-devo and an expanding evolutionary synthesis: a genetic theory of morphological evolution |journal = Cell |volume = 134 |issue = 1 |pages = 25–36 |year = 2008 |pmid = 18614008 |doi = 10.1016/j.cell.2008.06.030 |ref = harv }}</ref>

=== Co-evolution ===

[[File:Thamnophis sirtalis sirtalis Wooster.jpg|thumb|[[Common Garter Snake]] (''Thamnophis sirtalis sirtalis'') which has evolved resistance to [[tetrodotoxin]] in its amphibian prey.]]

{{Further|Co-evolution}}
Interactions between organisms can produce both conflict and co-operation. When the interaction is between pairs of species, such as a [[pathogen]] and a [[host (biology)|host]], or a [[Predation|predator]] and its prey, these species can develop matched sets of adaptations. Here, the evolution of one species causes adaptations in a second species. These changes in the second species then, in turn, cause new adaptations in the first species. This cycle of selection and response is called [[co-evolution]].<ref>{{cite journal |author = Wade MJ |title = The co-evolutionary genetics of ecological communities |journal = Nat. Rev. Genet. |volume = 8 |issue = 3 |pages = 185–95 |year = 2007 |pmid = 17279094 |doi = 10.1038/nrg2031 |ref = harv }}</ref> An example is the production of [[tetrodotoxin]] in the [[rough-skinned newt]] and the evolution of tetrodotoxin resistance in its predator, the [[Common Garter Snake|common garter snake]]. In this predator-prey pair, an [[evolutionary arms race]] has produced high levels of toxin in the newt and correspondingly high levels of toxin resistance in the snake.<ref>{{cite journal |author = Geffeney S, Brodie ED, Ruben PC, Brodie ED |title = Mechanisms of adaptation in a predator-prey arms race: TTX-resistant sodium channels |journal = Science |volume = 297 |issue = 5585 |pages = 1336–9 |year = 2002 |pmid = 12193784 |doi = 10.1126/science.1074310 |ref = harv |bibcode = 2002Sci...297.1336G }}<br />*{{cite journal |author = Brodie ED, Ridenhour BJ, Brodie ED |title = The evolutionary response of predators to dangerous prey: hotspots and coldspots in the geographic mosaic of coevolution between garter snakes and newts |journal = Evolution |volume = 56 |issue = 10 |pages = 2067–82 |year = 2002 |pmid = 12449493 |ref = harv }}<br />*{{cite news |url = http://www.nytimes.com/2009/12/22/science/22creature.html?hpw |title = Remarkable Creatures&nbsp;– Clues to Toxins in Deadly Delicacies of the Animal Kingdom |publisher = New York Times |author = Sean B. Carroll |date = December 21, 2009 }}</ref>

=== Co-operation ===

{{Further|Co-operation (evolution)}}
Not all co-evolved interactions between species involve conflict.<ref>{{cite journal |author = Sachs J |title = Cooperation within and among species |journal = J. Evol. Biol. |volume = 19 |issue = 5 |pages = 1415–8; discussion 1426–36 |year = 2006 |pmid = 16910971 |doi = 10.1111/j.1420-9101.2006.01152.x |ref = harv }}<br />*{{cite journal |author = Nowak M |title = Five rules for the evolution of cooperation |journal = Science |volume = 314 |issue = 5805 |pages = 1560–3 |year = 2006 |pmid = 17158317 |doi = 10.1126/science.1133755 |ref = harv |bibcode = 2006Sci...314.1560N |pmc=3279745}}</ref> Many cases of mutually beneficial interactions have evolved. For instance, an extreme cooperation exists between plants and the [[Mycorrhiza|mycorrhizal fungi]] that grow on their roots and aid the plant in absorbing nutrients from the soil.<ref>{{cite journal |author = Paszkowski U |title = Mutualism and parasitism: the yin and yang of plant symbioses |journal = Curr. Opin. Plant Biol. |volume = 9 |issue = 4 |pages = 364–70 |year = 2006 |pmid = 16713732 |doi = 10.1016/j.pbi.2006.05.008 |ref = harv }}</ref> This is a [[Reciprocity (evolution)|reciprocal]] relationship as the plants provide the fungi with sugars from photosynthesis. Here, the fungi actually grow inside plant cells, allowing them to exchange nutrients with their hosts, while sending [[signal transduction|signals]] that suppress the plant [[immune system]].<ref>{{cite journal |author = Hause B, Fester T |title = Molecular and cell biology of arbuscular mycorrhizal symbiosis |journal = Planta |volume = 221 |issue = 2 |pages = 184–96 |year = 2005 |pmid = 15871030 |doi = 10.1007/s00425-004-1436-x |ref = harv }}</ref>

Coalitions between organisms of the same species have also evolved. An extreme case is the [[eusociality]] found in [[Eusociality|social insects]], such as [[bee]]s, [[termite]]s and [[ant]]s, where sterile insects feed and guard the small number of organisms in a [[Colony (biology)|colony]] that are able to reproduce. On an even smaller scale, the [[somatic cell]]s that make up the body of an animal limit their reproduction so they can maintain a stable organism, which then supports a small number of the animal's [[germ cell]]s to produce offspring. Here, somatic cells respond to specific signals that instruct them whether to grow, remain as they are, or die. If cells ignore these signals and multiply inappropriately, their uncontrolled growth [[carcinogenesis|causes cancer]].<ref name=Bertram>{{cite journal |author = Bertram J |title = The molecular biology of cancer |journal = Mol. Aspects Med. |volume = 21 |issue = 6 |pages = 167–223 |year = 2000 |pmid = 11173079 |doi = 10.1016/S0098-2997(00)00007-8 |ref = harv }}</ref>

Such cooperation within species may have evolved through the process of [[kin selection]], which is where one organism acts to help raise a relative's offspring.<ref>{{cite journal |author = Reeve HK, Hölldobler B |title = The emergence of a superorganism through intergroup competition |doi = 10.1073/pnas.0703466104 |journal = Proc Natl Acad Sci U S A. |volume = 104 |issue = 23 |pages = 9736–40 |year = 2007 |pmid = 17517608 |pmc = 1887545 |ref = harv |bibcode = 2007PNAS..104.9736R }}</ref> This activity is selected for because if the ''helping'' individual contains alleles which promote the helping activity, it is likely that its kin will ''also'' contain these alleles and thus those alleles will be passed on.<ref>{{cite journal |author = Axelrod R, Hamilton W |title = The evolution of cooperation |journal = Science |volume = 211 |issue = 4489 |pages = 1390–6 |year = 2005 |pmid = 7466396 |doi = 10.1126/science.7466396 |ref = harv |bibcode = 1981Sci...211.1390A }}</ref> Other processes that may promote cooperation include [[group selection]], where cooperation provides benefits to a group of organisms.<ref>{{cite journal |author = Wilson EO, Hölldobler B |title = Eusociality: origin and consequences |doi = 10.1073/pnas.0505858102 |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 102 |issue = 38 |pages = 13367–71 |year = 2005 |pmid = 16157878 |pmc = 1224642 |ref = harv |bibcode = 2005PNAS..10213367W }}</ref>

=== Speciation ===

{{Further|Speciation}}
[[File:Speciation modes edit.svg|left|thumb|350px|The four mechanisms of speciation.]]
[[Speciation]] is the process where a species diverges into two or more descendant species.<ref name=Gavrilets>{{cite journal |author = Gavrilets S |title = Perspective: models of speciation: what have we learned in 40 years? |journal = Evolution |volume = 57 |issue = 10 |pages = 2197–215 |year = 2003 |pmid = 14628909 |doi = 10.1554/02-727 |ref = harv }}</ref>

There are multiple ways to define the concept of "species". The choice of definition is dependent on the particularities of the species concerned.<ref name=Queiroz>{{cite journal |author = de Queiroz K |title = Ernst Mayr and the modern concept of species |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 102 |issue = Suppl 1 |pages = 6600–7 |year = 2005 |pmid = 15851674 |pmc = 1131873 |doi = 10.1073/pnas.0502030102 |ref = harv |bibcode = 2005PNAS..102.6600D }}</ref> For example, some species concepts apply more readily toward sexually reproducing organisms while others lend themselves better toward asexual organisms. Despite the diversity of various species concepts, these various concepts can be placed into one of three broad philosophical approaches: interbreeding, ecological and phylogenetic.<ref name="Ereshsefsky92">{{cite journal |doi = 10.1086/289701 |last = Ereshefsky |first = M. |title = Eliminative pluralism |journal = Philosophy of Science |volume = 59 |issue = 4 |pages = 671–690 |year = 1992 |jstor = 188136 }}</ref> The biological species concept (BSC) is a classic example of the interbreeding approach. Defined by Ernst Mayr in 1942, the BSC states that "species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups".<ref name="Mayr42">{{cite book |author = Mayr, E. |title = Systematics and the Origin of Species |year = 1942 |publisher = Columbia Univ. Press |place = New York |isbn = 978-0-231-05449-2 | page = 120 }}</ref> Despite its wide and long-term use, the BSC like others is not without controversy, for example because these concepts cannot be applied to prokaryotes,<ref>{{cite journal |author = Fraser C, Alm EJ, Polz MF, Spratt BG, Hanage WP |title = The bacterial species challenge: making sense of genetic and ecological diversity |journal = Science |volume = 323 |issue = 5915 |pages = 741–6 |year = 2009 |pmid = 19197054 |doi = 10.1126/science.1159388 |ref = harv |bibcode = 2009Sci...323..741F }}</ref> and this is called the [[species problem]].<ref name=Queiroz /> Some researchers have attempted a unifying monistic definition of species, while others adopt a pluralistic approach and suggest that there may be different ways to logically interpret the definition of a species.<ref name=Queiroz /><ref name="Ereshsefsky92" /> "

[[reproductive isolation|Barriers to reproduction]] between two diverging sexual populations are required for the populations to [[speciation|become new species]]. Gene flow may slow this process by spreading the new genetic variants also to the other populations. Depending on how far two species have diverged since their [[most recent common ancestor]], it may still be possible for them to produce offspring, as with [[horse]]s and [[donkey]]s mating to produce [[mule]]s.<ref>{{cite journal |author = Short RV |title = The contribution of the mule to scientific thought |journal = J. Reprod. Fertil. Suppl. |issue = 23 |pages = 359–64 |year = 1975 |pmid = 1107543 |ref = harv }}</ref> Such [[Hybrid (biology)|hybrids]] are generally [[infertility|infertile]]. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype.<ref>{{cite journal |author = Gross B, Rieseberg L |title = The ecological genetics of homoploid hybrid speciation |doi = 10.1093/jhered/esi026 |journal = J. Hered. |volume = 96 |issue = 3 |pages = 241–52 |year = 2005 |pmid = 15618301 |pmc = 2517139 |ref = harv }}</ref> The importance of hybridisation in producing [[hybrid speciation|new species]] of animals is unclear, although cases have been seen in many types of animals,<ref>{{cite journal |author = Burke JM, Arnold ML |title = Genetics and the fitness of hybrids |journal = Annu. Rev. Genet. |volume = 35 |issue = 1 |pages = 31–52 |year = 2001 |pmid = 11700276 |doi = 10.1146/annurev.genet.35.102401.085719 |ref = harv }}</ref> with the [[gray tree frog]] being a particularly well-studied example.<ref>{{cite journal |author = Vrijenhoek RC |title = Polyploid hybrids: multiple origins of a treefrog species |journal = Curr. Biol. |volume = 16 |issue = 7 |page = R245 |year = 2006 |pmid = 16581499 |doi = 10.1016/j.cub.2006.03.005 |ref = harv |pages = R245–7 }}</ref>

Speciation has been observed multiple times under both controlled laboratory conditions and in nature.<ref>{{cite journal |author = Rice, W.R. |year = 1993 |title = Laboratory experiments on speciation: what have we learned in 40 years |journal = Evolution |volume = 47 |issue = 6 |pages = 1637–1653 |doi = 10.2307/2410209 |author2 = Hostert |ref = harv }}<br />*{{cite journal |author = Jiggins CD, Bridle JR |title = Speciation in the apple maggot fly: a blend of vintages? |journal = Trends Ecol. Evol. (Amst.) |volume = 19 |issue = 3 |pages = 111–4 |year = 2004 |pmid = 16701238 |doi = 10.1016/j.tree.2003.12.008 |ref = harv }}<br />*{{cite web |author = Boxhorn, J |year = 1995 |url = http://www.talkorigins.org/faqs/faq-speciation.html |title = Observed Instances of Speciation |publisher = [[TalkOrigins Archive]] |accessdate = December 26, 2008 }}<br />*{{cite journal |author = Weinberg JR, Starczak VR, Jorg, D |title = Evidence for Rapid Speciation Following a Founder Event in the Laboratory |journal = Evolution |volume = 46 |issue = 4 |pages = 1214–20 |year = 1992 |doi = 10.2307/2409766 |ref = harv |jstor = 2409766 }}</ref> In sexually reproducing organisms, speciation results from reproductive isolation followed by genealogical divergence. There are four mechanisms for speciation. The most common in animals is [[allopatric speciation]], which occurs in populations initially isolated geographically, such as by [[habitat fragmentation]] or migration. Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms.<ref>{{cite journal |year = 2008 |title = Rapid large-scale evolutionary divergence in morphology and performance associated with exploitation of a different dietary resource |journal = Proceedings of the National Academy of Sciences |volume = 105 |issue = 12 |pages = 4792–5 |pmid = 18344323 |doi = 10.1073/pnas.0711998105 |author = Herrel, A.; Huyghe, K.; Vanhooydonck, B.; Backeljau, T.; Breugelmans, K.; Grbac, I.; Van Damme, R.; Irschick, D.J. |pmc = 2290806 |ref = harv |bibcode = 2008PNAS..105.4792H }}</ref><ref name=Losos1997>{{cite journal |year = 1997 |title = Adaptive differentiation following experimental island colonization in Anolis lizards |journal = Nature |volume = 387 |issue = 6628 |pages = 70–3 |doi = 10.1038/387070a0 |author = Losos, J.B. Warhelt, K.I. Schoener, T.W. |ref = harv |bibcode = 1997Natur.387...70L }}</ref> As selection and drift act independently on populations isolated from the rest of their species, separation may eventually produce organisms that cannot interbreed.<ref>{{cite journal |author = Hoskin CJ, Higgle M, McDonald KR, Moritz C |year = 2005 |title = Reinforcement drives rapid allopatric speciation |journal = Nature |pmid = 16251964 |volume = 437 |issue = 7063 |pages = 1353–356 |doi = 10.1038/nature04004 |ref = harv |bibcode = 2005Natur.437.1353H }}</ref>

The second mechanism of speciation is [[peripatric speciation]], which occurs when small populations of organisms become isolated in a new environment. This differs from allopatric speciation in that the isolated populations are numerically much smaller than the parental population. Here, the [[founder effect]] causes rapid speciation after an increase in [[inbreeding]] increases selection on homozygotes, leading to rapid genetic change.<ref>{{cite journal |author = Templeton AR |title = The theory of speciation via the founder principle |url = http://www.genetics.org/cgi/reprint/94/4/1011 |journal = Genetics |volume = 94 |issue = 4 |pages = 1011–38 |date = April 1, 1980 |pmid = 6777243 |pmc = 1214177 |ref = harv }}</ref>

The third mechanism of speciation is [[parapatric speciation]]. This is similar to peripatric speciation in that a small population enters a new habitat, but differs in that there is no physical separation between these two populations. Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations.<ref name=Gavrilets/> Generally this occurs when there has been a drastic change in the environment within the parental species' habitat. One example is the grass ''[[Anthoxanthum|Anthoxanthum odoratum]]'', which can undergo parapatric speciation in response to localised metal pollution from mines.<ref>{{cite journal |author = Antonovics J |title = Evolution in closely adjacent plant populations X: long-term persistence of prereproductive isolation at a mine boundary |journal = Heredity |volume = 97 |issue = 1 |pages = 33–7 |year = 2006 |pmid = 16639420 |url = http://www.nature.com/hdy/journal/v97/n1/full/6800835a.html |doi = 10.1038/sj.hdy.6800835 |ref = harv }}</ref> Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produced a gradual change in the flowering time of the metal-resistant plants, which eventually produced complete reproductive isolation. Selection against hybrids between the two populations may cause ''reinforcement'', which is the evolution of traits that promote mating within a species, as well as [[character displacement]], which is when two species become more distinct in appearance.<ref>{{cite journal |author = Nosil P, Crespi B, Gries R, Gries G |title = Natural selection and divergence in mate preference during speciation |journal = Genetica |volume = 129 |issue = 3 |pages = 309–27 |year = 2007 |pmid = 16900317 |doi = 10.1007/s10709-006-0013-6 |ref = harv }}</ref>
[[File:Darwin's finches.jpeg|frame|[[Geographical isolation]] of [[Darwin's finches|finches]] on the [[Galápagos Islands]] produced over a dozen new species.]]

Finally, in [[sympatric speciation]] species diverge without geographic isolation or changes in habitat. This form is rare since even a small amount of [[gene flow]] may remove genetic differences between parts of a population.<ref>{{cite journal |author = Savolainen V, Anstett M-C, Lexer C, Hutton I, Clarkson JJ, Norup MV, Powell MP, Springate D, Salamin N, Baker WJr |year = 2006 |title = Sympatric speciation in palms on an oceanic island |journal = Nature |volume = 441 |pages = 210–3 |pmid = 16467788 |doi = 10.1038/nature04566 |issue = 7090 |ref = harv |bibcode = 2006Natur.441..210S }}<br />*{{cite journal |author = Barluenga M, Stölting KN, Salzburger W, Muschick M, Meyer A |year = 2006 |title = Sympatric speciation in Nicaraguan crater lake cichlid fish |journal = Nature |volume = 439 |pages = 719–23 |pmid = 16467837 |doi = 10.1038/nature04325 |issue = 7077 |ref = harv |bibcode = 2006Natur.439..719B }}</ref> Generally, sympatric speciation in animals requires the evolution of both [[Polymorphism (biology)|genetic differences]] and [[assortative mating|non-random mating]], to allow reproductive isolation to evolve.<ref>{{cite journal |author = Gavrilets S |title = The Maynard Smith model of sympatric speciation |journal = J. Theor. Biol. |volume = 239 |issue = 2 |pages = 172–82 |year = 2006 |pmid = 16242727 |doi = 10.1016/j.jtbi.2005.08.041 |ref = harv }}</ref>

One type of sympatric speciation involves cross-breeding of two related species to produce a new [[Hybrid (biology)|hybrid]] species. This is not common in animals as animal hybrids are usually sterile. This is because during [[meiosis]] the [[homologous chromosome]]s from each parent are from different species and cannot successfully pair. However, it is more common in plants because plants often double their number of chromosomes, to form [[polyploidy|polyploids]].<ref>{{cite journal |author = Wood TE, Takebayashi N, Barker MS, Mayrose I, Greenspoon PB, Rieseberg LH |title = The frequency of polyploid speciation in vascular plants |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 106 |issue = 33 |pages = 13875–9 |year = 2009 |pmid = 19667210 |doi = 10.1073/pnas.0811575106 |pmc = 2728988 |ref = harv |bibcode = 2009PNAS..10613875W }}</ref> This allows the chromosomes from each parental species to form matching pairs during meiosis, since each parent's chromosomes are represented by a pair already.<ref>{{cite journal |author = Hegarty Mf, Hiscock SJ |title = Genomic clues to the evolutionary success of polyploid plants |journal = Current Biology |volume = 18 |issue = 10 |pages = 435–44 |year = 2008 |pmid = 18492478 |doi = 10.1016/j.cub.2008.03.043 |ref = harv }}</ref> An example of such a speciation event is when the plant species ''[[Arabidopsis thaliana]]'' and ''Arabidopsis arenosa'' cross-bred to give the new species ''Arabidopsis suecica''.<ref>{{cite journal |author = Jakobsson M, Hagenblad J, Tavaré S |title = A unique recent origin of the allotetraploid species Arabidopsis suecica: Evidence from nuclear DNA markers |journal = Mol. Biol. Evol. |volume = 23 |issue = 6 |pages = 1217–31 |year = 2006 |pmid = 16549398 |doi = 10.1093/molbev/msk006 |ref = harv }}</ref> This happened about 20,000 years ago,<ref>{{cite journal |author = Säll T, Jakobsson M, Lind-Halldén C, Halldén C |title = Chloroplast DNA indicates a single origin of the allotetraploid Arabidopsis suecica |journal = J. Evol. Biol. |volume = 16 |issue = 5 |pages = 1019–29 |year = 2003 |pmid = 14635917 |doi = 10.1046/j.1420-9101.2003.00554.x |ref = harv }}</ref> and the speciation process has been repeated in the laboratory, which allows the study of the genetic mechanisms involved in this process.<ref>{{cite journal |author = Bomblies K, Weigel D |title = Arabidopsis-a model genus for speciation |journal = Curr Opin Genet Dev |volume = 17 |issue = 6 |pages = 500–4 |year = 2007 |pmid = 18006296 |doi = 10.1016/j.gde.2007.09.006 |ref = harv }}</ref> Indeed, chromosome doubling within a species may be a common cause of reproductive isolation, as half the doubled chromosomes will be unmatched when breeding with undoubled organisms.<ref name=Semon>{{cite journal |author = Sémon M, Wolfe KH |title = Consequences of genome duplication |journal = Curr Opin Genet Dev |volume = 17 |issue = 6 |pages = 505–12 |year = 2007 |pmid = 18006297 |doi = 10.1016/j.gde.2007.09.007 |ref = harv }}</ref>

Speciation events are important in the theory of [[punctuated equilibrium]], which accounts for the pattern in the fossil record of short "bursts" of evolution interspersed with relatively long periods of stasis, where species remain relatively unchanged.<ref name=pe1972>Niles Eldredge and Stephen Jay Gould, 1972. [http://www.blackwellpublishing.com/ridley/classictexts/eldredge.asp "Punctuated equilibria: an alternative to phyletic gradualism"] In T.J.M. Schopf, ed., ''Models in Paleobiology''. San Francisco: Freeman Cooper. pp. 82–115. Reprinted in N. Eldredge ''Time frames''. Princeton: Princeton Univ. Press. 1985</ref> In this theory, speciation and rapid evolution are linked, with natural selection and genetic drift acting most strongly on organisms undergoing speciation in novel habitats or small populations. As a result, the periods of stasis in the fossil record correspond to the parental population and the organisms undergoing speciation and rapid evolution are found in small populations or geographically restricted habitats and therefore rarely being preserved as fossils.<ref>{{cite journal |author = Gould SJ |title = Tempo and mode in the macroevolutionary reconstruction of Darwinism |doi = 10.1073/pnas.91.15.6764 |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 91 |issue = 15 |pages = 6764–71 |year = 1994 |pmid = 8041695 |pmc = 44281 |ref = harv |bibcode = 1994PNAS...91.6764G }}</ref>

=== Extinction ===

{{Further|Extinction}}
[[File:Palais de la Decouverte Tyrannosaurus rex p1050042.jpg|thumb|left|''[[Tyrannosaurus rex]]''. Non-[[bird|avian]] [[dinosaur]]s died out in the [[Cretaceous–Paleogene extinction event]] at the end of the [[Cretaceous]] period.]]
[[Extinction]] is the disappearance of an entire species. Extinction is not an unusual event, as species regularly appear through speciation and disappear through extinction.<ref>{{cite journal |author = Benton MJ |title = Diversification and extinction in the history of life |journal = Science |volume = 268 |issue = 5207 |pages = 52–8 |year = 1995 |pmid = 7701342 |doi = 10.1126/science.7701342 |ref = harv |bibcode = 1995Sci...268...52B }}</ref> Nearly all animal and plant species that have lived on Earth are now extinct,<ref>{{cite journal |author = Raup DM |title = Biological extinction in Earth history |journal = Science |volume = 231 |issue = 4745 |pages = 1528–33 |year = 1986 |pmid = 11542058 |doi = 10.1126/science.11542058 |ref = harv |bibcode = 1986Sci...231.1528R }}</ref> and extinction appears to be the ultimate fate of all species.<ref>{{cite journal |author = Avise JC, Hubbell SP, Ayala FJ. |title = In the light of evolution II: Biodiversity and extinction |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 105 |issue = Suppl 1 |pages = 11453–7 |year = 2008 |pmid = 18695213 |pmc = 2556414 |doi = 10.1073/pnas.0802504105 |url = http://www.pnas.org/content/105/suppl.1/11453.full |ref = harv |bibcode = 2008PNAS..10511453A }}</ref> These extinctions have happened continuously throughout the history of life, although the rate of extinction spikes in occasional mass [[extinction event]]s.<ref name=Raup>{{cite journal |author = Raup DM |title = The role of extinction in evolution |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 91 |issue = 15 |pages = 6758–63 |year = 1994 |pmid = 8041694 |doi = 10.1073/pnas.91.15.6758 |pmc = 44280 |ref = harv |bibcode = 1994PNAS...91.6758R }}</ref> The [[Cretaceous–Paleogene extinction event]], during which the non-avian dinosaurs went extinct, is the most well-known, but the earlier [[Permian–Triassic extinction event]] was even more severe, with approximately 96% of species driven to extinction.<ref name=Raup/> The [[Holocene extinction event]] is an ongoing mass extinction associated with humanity's expansion across the globe over the past few thousand years. Present-day extinction rates are 100–1000 times greater than the background rate and up to 30% of current species may be extinct by the mid 21st century.<ref>{{cite journal |author = Novacek MJ, Cleland EE |title = The current biodiversity extinction event: scenarios for mitigation and recovery |doi = 10.1073/pnas.091093698 |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 98 |issue = 10 |pages = 5466–70 |year = 2001 |pmid = 11344295 |pmc = 33235 |ref = harv |bibcode = 2001PNAS...98.5466N }}</ref> Human activities are now the primary cause of the ongoing extinction event;<ref>{{cite journal |author = Pimm S, Raven P, Peterson A, Sekercioglu CH, Ehrlich PR |title = Human impacts on the rates of recent, present and future bird extinctions |doi = 10.1073/pnas.0604181103 |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 103 |issue = 29 |pages = 10941–6 |year = 2006 |pmid = 16829570 |pmc = 1544153 |ref = harv |bibcode = 2006PNAS..10310941P }}<br />*{{cite journal |author = Barnosky AD, Koch PL, Feranec RS, Wing SL, Shabel AB |title = Assessing the causes of late Pleistocene extinctions on the continents |journal = Science |volume = 306 |issue = 5693 |pages = 70–5 |year = 2004 |pmid = 15459379 |doi = 10.1126/science.1101476 |ref = harv |bibcode = 2004Sci...306...70B }}</ref> [[global warming]] may further accelerate it in the future.<ref>{{cite journal |author = Lewis OT |title = Climate change, species-area curves and the extinction crisis |journal = Philos. Trans. R. Soc. Lond., B, Biol. Sci. |volume = 361 |issue = 1465 |pages = 163–71 |year = 2006 |pmid = 16553315 |doi = 10.1098/rstb.2005.1712 |pmc = 1831839 |ref = harv }}</ref>

The role of extinction in evolution is not very well understood and may depend on which type of extinction is considered.<ref name=Raup/> The causes of the continuous "low-level" extinction events, which form the majority of extinctions, may be the result of competition between species for limited resources ([[competitive exclusion]]).<ref name=Kutschera/> If one species can out-compete another, this could produce [[species selection]], with the fitter species surviving and the other species being driven to extinction.<ref name=Gould/> The intermittent mass extinctions are also important, but instead of acting as a selective force, they drastically reduce diversity in a nonspecific manner and promote bursts of [[Adaptive radiation|rapid evolution]] and speciation in survivors.<ref>{{cite journal |author = Jablonski D |title = Lessons from the past: evolutionary impacts of mass extinctions |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 98 |issue = 10 |pages = 5393–8 |year = 2001 |pmid = 11344284 |pmc = 33224 |doi = 10.1073/pnas.101092598 |ref = harv |bibcode = 2001PNAS...98.5393J }}</ref>
{{-}}

== Evolutionary history of life ==

{{Main|Evolutionary history of life}}
{{See also|Timeline of evolution|Timeline of human evolution}}

=== Origin of life ===

{{Further|Abiogenesis|RNA world hypothesis}}
Highly energetic chemistry is thought to have produced a self-replicating molecule around {{nowrap|4 billion years}} ago, and half a billion years later the [[last universal common ancestor|last common ancestor of all life]] existed.<ref name=sa282_6_90>{{cite journal |last = Doolittle |first = W. Ford |title = Uprooting the tree of life |journal = Scientific American |month = February |year = 2000 |volume = 282 |issue = 6 |pages = 90–95 |doi = 10.1038/scientificamerican0200-90 |pmid = 10710791 }}</ref> The current [[scientific consensus]] is that the complex [[biochemistry]] that makes up life came from simpler chemical reactions.<ref>{{cite journal |author = Peretó J |title = Controversies on the origin of life |url = http://www.im.microbios.org/0801/0801023.pdf |journal = Int. Microbiol. |volume = 8 |issue = 1 |pages = 23–31 |year = 2005 |pmid = 15906258 |ref = harv }}</ref> The beginning of life may have included self-replicating molecules such as [[RNA]]<ref>{{cite journal |author = Joyce GF |title = The antiquity of RNA-based evolution |journal = Nature |volume = 418 |issue = 6894 |pages = 214–21 |year = 2002 |pmid = 12110897 |doi = 10.1038/418214a |ref = harv }}</ref> and the assembly of simple cells.<ref>{{cite journal |author = Trevors JT, Psenner R |title = From self-assembly of life to present-day bacteria: a possible role for nanocells |journal = FEMS Microbiol. Rev. |volume = 25 |issue = 5 |pages = 573–82 |year = 2001 |pmid = 11742692 |doi = 10.1111/j.1574-6976.2001.tb00592.x |ref = harv }}</ref>

=== Common descent ===

{{Further|Common descent|Evidence of common descent}}
[[File:Ape skeletons.png|320px|thumbnail|The [[Ape|hominoids]] are descendants of a [[common descent|common ancestor]].]]
All [[organism]]s on [[Earth]] are descended from a common ancestor or ancestral gene pool.<ref name=Penny1999>{{cite journal |author = Penny D, Poole A |title = The nature of the last universal common ancestor |journal = Curr. Opin. Genet. Dev. |volume = 9 |issue = 6 |pages = 672–77 |year = 1999 |pmid = 10607605 |doi = 10.1016/S0959-437X(99)00020-9 |ref = harv }}</ref><ref>{{cite journal |author = Theobald, DL. |title = A formal test of the theory of universal common ancestry |journal = Nature |volume = 465 |pages = 219–22 |year = 2010 |pmid = 20463738 |doi = 10.1038/nature09014 |issue = 7295 |ref = harv |bibcode = 2010Natur.465..219T }}</ref> Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events.<ref>{{cite journal |author = Bapteste E, Walsh DA |title = Does the 'Ring of Life' ring true? |journal = Trends Microbiol. |volume = 13 |issue = 6 |pages = 256–61 |year = 2005 |pmid = 15936656 |doi = 10.1016/j.tim.2005.03.012 |ref = harv }}</ref> The [[common descent]] of organisms was first deduced from four simple facts about organisms: First, they have geographic distributions that cannot be explained by local adaptation. Second, the diversity of life is not a set of completely unique organisms, but organisms that share [[homology (biology)|morphological similarities]]. Third, vestigial traits with no clear purpose resemble functional ancestral traits and finally, that organisms can be classified using these similarities into a hierarchy of nested groups&nbsp;– similar to a family tree.<ref name=Darwin>{{cite book |last = Darwin |first = Charles |authorlink = Charles Darwin |year = 1859 |title = On the Origin of Species |place = London |publisher = John Murray |edition = 1st |page = 1 |url = http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=16 |isbn = 0-8014-1319-2 }}</ref> However, modern research has suggested that, due to horizontal gene transfer, this "[[Tree of life (science)|tree of life]]" may be more complicated than a simple branching tree since some genes have spread independently between distantly related species.<ref>{{cite journal |author = Doolittle WF, Bapteste E |title = Pattern pluralism and the Tree of Life hypothesis |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 104 |issue = 7 |pages = 2043–9 |year = 2007 |pmid = 17261804 |pmc = 1892968 |doi = 10.1073/pnas.0610699104 |ref = harv |bibcode = 2007PNAS..104.2043D }}</ref><ref>{{cite journal |author = Kunin V, Goldovsky L, Darzentas N, Ouzounis CA |title = The net of life: reconstructing the microbial phylogenetic network |journal = Genome Res. |volume = 15 |issue = 7 |pages = 954–9 |year = 2005 |pmid = 15965028 |doi = 10.1101/gr.3666505 |pmc = 1172039 |ref = harv }}</ref>

Past species have also left records of their evolutionary history. [[Fossil]]s, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record.<ref name=Jablonski>{{cite journal |author = Jablonski D |title = The future of the fossil record |journal = Science |volume = 284 |issue = 5423 |pages = 2114–16 |year = 1999 |pmid = 10381868 |doi = 10.1126/science.284.5423.2114 |ref = harv }}</ref> By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Further, as prokaryotes such as [[bacteria]] and [[archaea]] share a limited set of common morphologies, their fossils do not provide information on their ancestry.

More recently, evidence for common descent has come from the study of [[biochemistry|biochemical]] similarities between organisms. For example, all living cells use the same basic set of [[nucleotide]]s and [[amino acid]]s.<ref>{{cite journal |author = Mason SF |title = Origins of biomolecular handedness |journal = Nature |volume = 311 |issue = 5981 |pages = 19–23 |year = 1984 |pmid = 6472461 |doi = 10.1038/311019a0 |ref = harv |bibcode = 1984Natur.311...19M }}</ref> The development of [[molecular genetics]] has revealed the record of evolution left in organisms' [[genome]]s: dating when species diverged through the [[molecular clock]] produced by mutations.<ref>{{cite journal |author = Wolf YI, Rogozin IB, Grishin NV, Koonin EV |title = Genome trees and the tree of life |journal = Trends Genet. |volume = 18 |issue = 9 |pages = 472–79 |year = 2002 |pmid = 12175808 |doi = 10.1016/S0168-9525(02)02744-0 |ref = harv }}</ref> For example, these DNA sequence comparisons have revealed that humans and chimpanzees share 98% of their genomes and analyzing the few areas where they differ helps shed light on when the common ancestor of these species existed.<ref>{{cite journal |author = Varki A, Altheide TK |title = Comparing the human and chimpanzee genomes: searching for needles in a haystack |journal = Genome Res. |volume = 15 |issue = 12 |pages = 1746–58 |year = 2005 |pmid = 16339373 |doi = 10.1101/gr.3737405 |ref = harv }}</ref>

=== Evolution of life ===

{{Main|Evolutionary history of life|Timeline of evolution}}
{{PhylomapA|size=320px|align=right|caption=[[Phylogenetic tree|Evolutionary tree]] showing the divergence of modern species from their common ancestor in the centre.<ref name=Ciccarelli>{{cite journal |author = Ciccarelli FD, Doerks T, von Mering C, Creevey CJ, Snel B, Bork P |title = Toward automatic reconstruction of a highly resolved tree of life |journal = Science |volume = 311 |issue = 5765 |pages = 1283–87 |year = 2006 |pmid = 16513982 |doi = 10.1126/science.1123061 |ref = harv |bibcode = 2006Sci...311.1283C }}</ref> The three [[Domain (biology)|domains]] are coloured, with [[bacteria]] blue, [[archaea]] green and [[eukaryote]]s red.}}
[[Prokaryote]]s inhabited the Earth from approximately 3–4 [[1000000000|billion]] years ago.<ref name=Cavalier-Smith>{{cite journal |author = Cavalier-Smith T |title = Cell evolution and Earth history: stasis and revolution |journal = Philos Trans R Soc Lond B Biol Sci |volume = 361 |issue = 1470 |pages = 969–1006 |year = 2006 |pmid = 16754610 |doi = 10.1098/rstb.2006.1842 |pmc = 1578732 |ref = harv }}</ref><ref>{{cite journal |author = Schopf J |title = Fossil evidence of Archaean life |journal = Philos Trans R Soc Lond B Biol Sci |volume = 361 |issue = 1470 |pages = 869–85 |year = 2006 |pmid = 16754604 |doi = 10.1098/rstb.2006.1834 |pmc = 1578735 |ref = harv }}<br />*{{cite journal |author = Altermann W, Kazmierczak J |title = Archean microfossils: a reappraisal of early life on Earth |journal = Res Microbiol |volume = 154 |issue = 9 |pages = 611–17 |year = 2003 |pmid = 14596897 |doi = 10.1016/j.resmic.2003.08.006 |ref = harv }}</ref> No obvious changes in [[morphology (biology)|morphology]] or cellular organisation occurred in these organisms over the next few billion years.<ref>{{cite journal |author = Schopf J |title = Disparate rates, differing fates: tempo and mode of evolution changed from the Precambrian to the Phanerozoic |doi = 10.1073/pnas.91.15.6735 |journal = Proc Natl Acad Sci U S A |volume = 91 |issue = 15 |pages = 6735–42 |year = 1994 |pmid = 8041691 |pmc = 44277 |ref = harv |bibcode = 1994PNAS...91.6735S }}</ref> The [[eukaryote|eukaryotic cells]] emerged between 1.6&nbsp;– 2.7 billion years ago. The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association called [[endosymbiont|endosymbiosis]].<ref name = "rgruqh">{{cite journal |author = Poole A, Penny D |title = Evaluating hypotheses for the origin of eukaryotes |journal = BioEssays |volume = 29 |issue = 1 |pages = 74–84 |year = 2007 |pmid = 17187354 |doi = 10.1002/bies.20516 |ref = harv }}</ref><ref name=Dyall>{{cite journal |author = Dyall S, Brown M, Johnson P |title = Ancient invasions: from endosymbionts to organelles |journal = Science |volume = 304 |issue = 5668 |pages = 253–57 |year = 2004 |pmid = 15073369 |doi = 10.1126/science.1094884 |ref = harv |bibcode = 2004Sci...304..253D }}</ref> The engulfed bacteria and the host cell then underwent co-evolution, with the bacteria evolving into either [[mitochondrion|mitochondria]] or [[hydrogenosome]]s.<ref>{{cite journal |author = Martin W |title = The missing link between hydrogenosomes and mitochondria |journal = Trends Microbiol. |volume = 13 |issue = 10 |pages = 457–59 |year = 2005 |pmid = 16109488 |doi = 10.1016/j.tim.2005.08.005 |ref = harv }}</ref> Another engulfment of [[cyanobacteria]]l-like organisms led to the formation of [[chloroplast]]s in algae and plants.<ref>{{cite journal |author = Lang B, Gray M, Burger G |title = Mitochondrial genome evolution and the origin of eukaryotes |journal = Annu Rev Genet |volume = 33 |issue = 1 |pages = 351–97 |year = 1999 |pmid = 10690412 |doi = 10.1146/annurev.genet.33.1.351 |ref = harv }}<br />*{{cite journal |author = McFadden G |title = Endosymbiosis and evolution of the plant cell |journal = Curr Opin Plant Biol |volume = 2 |issue = 6 |pages = 513–19 |year = 1999 |pmid = 10607659 |doi = 10.1016/S1369-5266(99)00025-4 |ref = harv }}</ref>

The history of life was that of the unicellular eukaryotes, prokaryotes and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the [[Ediacara biota|Ediacaran]] period.<ref name=Cavalier-Smith/><ref>{{cite journal |author = DeLong E, Pace N |title = Environmental diversity of bacteria and archaea |journal = Syst Biol |volume = 50 |issue = 4 |pages = 470–8 |year = 2001 |pmid = 12116647 |doi = 10.1080/106351501750435040 |ref = harv }}</ref> The [[evolution of multicellularity]] occurred in multiple independent events, in organisms as diverse as [[sponge]]s, [[brown algae]], [[cyanobacteria]], [[slime mold|slime moulds]] and [[myxobacteria]].<ref>{{cite journal |author = Kaiser D |title = Building a multicellular organism |journal = Annu. Rev. Genet. |volume = 35 |issue = 1 |pages = 103–23 |year = 2001 |pmid = 11700279 |doi = 10.1146/annurev.genet.35.102401.090145 |ref = harv }}</ref>

Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over approximately 10 million years, in an event called the [[Cambrian explosion]]. Here, the majority of [[Phylum|types]] of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct.<ref name=Valentine>{{cite journal |author = Valentine JW, Jablonski D, Erwin DH |title = Fossils, molecules and embryos: new perspectives on the Cambrian explosion |url = http://dev.biologists.org/cgi/reprint/126/5/851 |journal = Development |volume = 126 |issue = 5 |pages = 851–9 |date = March 1, 1999 |pmid = 9927587 |ref = harv }}</ref> Various triggers for the Cambrian explosion have been proposed, including the accumulation of [[oxygen]] in the [[atmosphere]] from [[photosynthesis]].<ref>{{cite journal |author = Ohno S |title = The reason for as well as the consequence of the Cambrian explosion in animal evolution |series = 44 |journal = J. Mol. Evol. |volume = 1 |issue = S1 |pages = S23–7 |year = 1997 |pmid = 9071008 |doi = 10.1007/PL00000055 |ref = harv }}<br />*{{cite journal |author = Valentine J, Jablonski D |title = Morphological and developmental macroevolution: a paleontological perspective |url = http://www.ijdb.ehu.es/web/paper.php?doi=14756327 |journal = Int. J. Dev. Biol. |volume = 47 |issue = 7–8 |pages = 517–22 |year = 2003 |pmid = 14756327 |ref = harv }}</ref>

About 500 million years ago, [[plant]]s and [[fungus|fungi]] colonised the land and were soon followed by [[arthropod]]s and other animals.<ref>{{cite journal |author = Waters ER |title = Molecular adaptation and the origin of land plants |journal = Mol. Phylogenet. Evol. |volume = 29 |issue = 3 |pages = 456–63 |year = 2003 |pmid = 14615186 |doi = 10.1016/j.ympev.2003.07.018 |ref = harv }}</ref> [[Insect]]s were particularly successful and even today make up the majority of animal species.<ref>{{cite journal |author = Mayhew PJ |title = Why are there so many insect species? Perspectives from fossils and phylogenies |journal = Biol Rev Camb Philos Soc |volume = 82 |issue = 3 |pages = 425–54 |year = 2007 |pmid = 17624962 |doi = 10.1111/j.1469-185X.2007.00018.x |ref = harv }}</ref> [[Amphibian]]s first appeared around 364 million years ago, followed by early [[amniote]]s and [[bird]]s around 155 million years ago (both from "[[reptile]]"-like lineages), [[mammal]]s around 129 million years ago, [[homininae]] around 10 million years ago and [[modern humans]] around 250,000 years ago.<ref>{{cite journal | last = Carroll | first = Robert L. | title = The Palaeozoic Ancestry of Salamanders, Frogs and Caecilians | journal = Zool J Linn Soc | volume = 150 | issue = s1 | pages = 1–140 | year = 2007 | month = May | pmid = 12752770 | doi = 10.1111/j.1096-3642.2007.00246.x | ref = harv }}</ref><ref>{{cite journal |author = Wible JR, Rougier GW, Novacek MJ, Asher RJ |title = Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary |journal = Nature |volume = 447 |issue = 7147 |pages = 1003–1006 |year = 2007 |pmid = 17581585 |doi = 10.1038/nature05854 |ref = harv |bibcode = 2007Natur.447.1003W }}</ref><ref>{{cite journal |author = Witmer LM |title = Palaeontology: An icon knocked from its perch |journal = Nature |volume = 475 |issue = 7357 |pages = 458–459 |year = 2011 |pmid = 21796198 |doi = 10.1038/475458a |ref = harv }}</ref> However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both [[Biomass (ecology)|biomass]] and species being prokaryotes.<ref name=Schloss/>

== Applications ==
{{Main|Applications of evolution|Artificial selection|Evolutionary computation}}
Concepts and models used in evolutionary biology, such as natural selection, have many applications.<ref name=Bull>{{cite journal |author = Bull JJ, Wichman HA |title = Applied evolution |journal = Annu Rev Ecol Syst |volume = 32 |issue = 1 |pages = 183–217 |year = 2001 |doi = 10.1146/annurev.ecolsys.32.081501.114020 |ref = harv }}</ref>

Artificial selection is the intentional selection of traits in a population of organisms. This has been used for thousands of years in the [[domestication]] of plants and animals.<ref>{{cite journal |author = Doebley JF, Gaut BS, Smith BD |title = The molecular genetics of crop domestication |journal = Cell |volume = 127 |issue = 7 |pages = 1309–21 |year = 2006 |pmid = 17190597 |doi = 10.1016/j.cell.2006.12.006 |ref = harv }}</ref> More recently, such selection has become a vital part of [[genetic engineering]], with [[selectable marker]]s such as antibiotic resistance genes being used to manipulate DNA. In repeated rounds of mutation and selection proteins with valuable properties have evolved, for example modified [[enzyme]]s and new [[antibody|antibodies]], in a process called [[directed evolution]].<ref>{{cite journal |author = Jäckel C, Kast P, Hilvert D |title = Protein design by directed evolution |journal = Annu Rev Biophys |volume = 37 |issue = 1 |pages = 153–73 |year = 2008 |pmid = 18573077 |doi = 10.1146/annurev.biophys.37.032807.125832 |ref = harv }}</ref>

Understanding the changes that have occurred during organism's evolution can reveal the genes needed to construct parts of the body, genes which may be involved in human [[genetic disorder]]s.<ref>{{cite journal |author = Maher B. |title = Evolution: Biology's next top model? |journal = Nature |volume = 458 |issue = 7239 |pages = 695–8 |year = 2009 |doi = 10.1038/458695a |pmid = 19360058 |ref = harv }}</ref> For example, the [[mexican tetra]] is an [[albino]] cavefish that lost its eyesight during evolution. Breeding together different populations of this blind fish produced some offspring with functional eyes, since different mutations had occurred in the isolated populations that had evolved in different caves.<ref>{{cite journal |author = Borowsky R |title = Restoring sight in blind cavefish |journal = Curr. Biol. |volume = 18 |issue = 1 |pages = R23–4 |year = 2008 |pmid = 18177707 |doi = 10.1016/j.cub.2007.11.023 |ref = harv }}</ref> This helped identify genes required for vision and pigmentation.<ref>{{cite journal |author = Gross JB, Borowsky R, Tabin CJ |editor1-last=Barsh |editor1-first=Gregory S. |title = A novel role for Mc1r in the parallel evolution of depigmentation in independent populations of the cavefish Astyanax mexicanus |journal = PLoS Genet. |volume = 5 |issue = 1 |pages = e1000326 |year = 2009 |pmid = 19119422 |pmc = 2603666 |doi = 10.1371/journal.pgen.1000326 |ref = harv }}</ref>

In [[computer science]], simulations of evolution using [[evolutionary algorithm]]s and [[artificial life]] started in the 1960s and was extended with simulation of [[artificial selection]].<ref>{{cite journal |author = Fraser AS |title = Monte Carlo analyses of genetic models |journal = Nature |volume = 181 |issue = 4603 |pages = 208–9 |year = 1958 |pmid = 13504138 |doi = 10.1038/181208a0 |ref = harv |bibcode = 1958Natur.181..208F }}</ref> [[Evolutionary algorithm|Artificial evolution]] became a widely recognised optimisation method as a result of the work of [[Ingo Rechenberg]] in the 1960s. He used [[Evolution strategy|evolution strategies]] to solve complex engineering problems.<ref>{{cite book |last = Rechenberg |first = Ingo |year = 1973 |title = Evolutionsstrategie&nbsp;– Optimierung technischer Systeme nach Prinzipien der biologischen Evolution (PhD thesis) |publisher = Fromman-Holzboog |language = German }}</ref> [[Genetic algorithm]]s in particular became popular through the writing of [[John Henry Holland|John Holland]].<ref>{{cite book |last = Holland |first = John H. |year = 1975 |title = Adaptation in Natural and Artificial Systems |publisher = University of Michigan Press |isbn = 0-262-58111-6 }}</ref> Practical applications also include [[genetic programming|automatic evolution of computer programs]].<ref>{{cite book |last = Koza |first = John R. |year = 1992 |title = Genetic Programming |subtitle = On the Programming of Computers by Means of Natural Selection |publisher = MIT Press |isbn = 0-262-11170-5 }}</ref> Evolutionary algorithms are now used to solve multi-dimensional problems more efficiently than software produced by human designers and also to optimise the design of systems.<ref>{{cite journal |author = Jamshidi M |title = Tools for intelligent control: fuzzy controllers, neural networks and genetic algorithms |journal = Philosophical transactions. Series A, Mathematical, physical and engineering sciences |volume = 361 |issue = 1809 |pages = 1781–808 |year = 2003 |pmid = 12952685 |doi = 10.1098/rsta.2003.1225 |ref = harv |bibcode = 2003RSPTA.361.1781J }}</ref>

== Social and cultural responses ==

{{Further|Social effect of evolutionary theory|Objections to evolution}}
[[File:Editorial cartoon depicting Charles Darwin as an ape (1871).jpg|upright|thumb|As evolution became widely accepted in the 1870s, [[caricature]]s of [[Charles Darwin]] with an [[ape]] or [[monkey]] body symbolised evolution.<ref name=Browne2003e>{{cite book |author = Browne, Janet |title = Charles Darwin: The Power of Place |publisher = Pimlico |location = London |year = 2003 |pages = 376–379 |isbn = 0-7126-6837-3 }}</ref>]]
In the 19th century, particularly after the publication of ''[[On the Origin of Species]]'' in 1859, the idea that life had evolved was an active source of academic debate centred on the philosophical, social and religious implications of evolution. Today, the modern evolutionary synthesis is accepted by a vast majority of scientists.<ref name=Kutschera/> However, evolution remains a contentious concept for some [[Theism|theists]].<ref>For an overview of the philosophical, religious and cosmological controversies, see: {{cite book |authorlink = Daniel Dennett |last = Dennett |first = D |title = [[Darwin's Dangerous Idea|Darwin's Dangerous Idea: Evolution and the Meanings of Life]] |publisher = Simon & Schuster |year = 1995 |isbn = 978-0-684-82471-0 }}<br />*For the scientific and social reception of evolution in the 19th and early 20th centuries, see: {{cite web |last = Johnston |first = Ian C. |title = History of Science: Origins of Evolutionary Theory |work = And Still We Evolve |publisher = Liberal Studies Department, Malaspina University College |url = http://records.viu.ca/~johnstoi/darwin/sect3.htm |accessdate = May 24, 2007 }}<br />*{{cite book |authorlink = Peter J. Bowler |last = Bowler |first = PJ |title = Evolution: The History of an Idea, Third Edition, Completely Revised and Expanded |publisher = University of California Press |isbn = 978-0-520-23693-6 |year = 2003 }}<br />*{{cite journal |author = Zuckerkandl E |title = Intelligent design and biological complexity |journal = Gene |volume = 385 |pages = 2–18 |year = 2006 |pmid = 17011142 |doi = 10.1016/j.gene.2006.03.025 |ref = harv }}</ref>

While [[Level of support for evolution#Support for evolution by religious bodies|various religions and denominations]] have reconciled their beliefs with evolution through concepts such as [[theistic evolution]], there are [[creationism|creationists]] who believe that evolution is contradicted by the [[creation myths]] found in their [[religion]]s and who raise various [[objections to evolution]].<ref name=ScottEC/><ref name=Ross2005>{{cite journal |author = Ross, M.R. |year = 2005 |title = Who Believes What? Clearing up Confusion over Intelligent Design and Young-Earth Creationism |journal = Journal of Geoscience Education |volume = 53 |issue = 3 |page = 319 |url = http://www.nagt.org/files/nagt/jge/abstracts/Ross_v53n3p319.pdf |accessdate = April 28, 2008 |ref = harv }}</ref><ref>{{cite journal |doi = 10.1126/science.1163672 |volume = 322 |issue = 5908 |pages = 1637–1638 |last = Hameed |first = Salman |title = Science and Religion: Bracing for Islamic Creationism |journal = Science |accessdate = 2009 |date = December 12, 2008 |url = http://helios.hampshire.edu/~sahCS/Hameed-Science-Creationism.pdf |pmid = 19074331 |ref = harv }}</ref> As had been demonstrated by responses to the publication of ''[[Vestiges of the Natural History of Creation]]'' in 1844, the most controversial aspect of evolutionary biology is the implication of [[human evolution]] that humans share common ancestry with apes and that the mental and moral faculties of humanity have the same types of natural causes as other inherited traits in animals.<ref name=bowler>{{cite book |last = Bowler |first = Peter J. |authorlink = Peter J. Bowler |title = Evolution:The History of an Idea |publisher = University of California Press |year = 2003 |isbn = 0-520-23693-9 }}</ref> In some countries, notably the United States, these tensions between science and religion have fuelled the current [[Creation–evolution controversy|creation-evolution controversy]], a religious conflict focusing on [[politics of creationism|politics]] and [[creation and evolution in public education|public education]].<ref>{{cite journal |author = Spergel D. N. |title = Science communication. Public acceptance of evolution |journal = Science |volume = 313 |issue = 5788 |pages = 765–66 |year = 2006 |pmid = 16902112 |doi = 10.1126/science.1126746 |last2 = Scott |first2 = EC |last3 = Okamoto |first3 = S |ref = harv }}</ref> While other scientific fields such as [[physical cosmology|cosmology]]<ref name="wmap">{{cite journal |doi = 10.1086/377226 |title = First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters |first = D. N. |last = Spergel |journal = The Astrophysical Journal Supplement Series |volume = 148 |year = 2003 |issue = 1 |pages = 175–94 |last2 = Verde |first2 = L. |last3 = Peiris |first3 = H. V. |last4 = Komatsu |first4 = E. |last5 = Nolta |first5 = M. R. |last6 = Bennett |first6 = C. L. |last7 = Halpern |first7 = M. |last8 = Hinshaw |first8 = G. |last9 = Jarosik |first9 = N. |ref = harv |bibcode = 2003ApJS..148..175S |arxiv = astro-ph/0302209 }}</ref> and [[Earth science]]<ref name="zircon">{{cite journal |author = Wilde SA, Valley JW, Peck WH, Graham CM |title = Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago |journal = Nature |volume = 409 |issue = 6817 |pages = 175–78 |year = 2001 |pmid = 11196637 |doi = 10.1038/35051550 |ref = harv }}</ref> also conflict with literal interpretations of many religious texts, evolutionary biology experiences significantly more opposition from religious literalists.

The teaching of evolution in American secondary school biology classes was uncommon in most of the first half of the 20th century. The [[Scopes Trial]] decision of 1925 caused the subject to become very rare in American secondary biology textbooks for a generation, but it was gradually re-introduced about a generation later and legally protected with the 1968 ''[[Epperson v. Arkansas]]'' decision. Since then, the competing religious belief of creationism was legally disallowed in secondary school curricula in various decisions in the 1970s and 1980s, but it returned in [[Pseudoscience|pseudoscientific]] form as [[intelligent design]], to be excluded once again in the 2005 ''[[Kitzmiller v. Dover Area School District]]'' case.<ref name=BioScience>{{cite journal |last = Branch |first = Glenn |title = Understanding Creationism after ''Kitzmiller'' |journal = BioScience |date = March 2007 |volume = 57 |issue = 3 |pages = 278–284 |doi = 10.1641/B570313 |url = http://www.bioone.org/doi/full/10.1641/B570313 |accessdate = November 13, 2011 |publisher = American Institute of Biological Sciences }}</ref>

== See also ==

{{Wikipedia books|Evolution}}

* [[Applications of evolution]]
* [[Biocultural evolution]]
* [[Biological imperative]]
* [[Current research in evolutionary biology]]
* [[Evolutionary anthropology]]
* [[Evolutionary neuroscience]]
* [[Evolutionary psychology]]
* [[Evolutionary history of life]]
* [[Human evolution]]
* [[Molecular evolution]]
* [[Neuroculture]]
* [[Sociobiology]]

== References ==

{{reflist|30em}}

== Further reading ==

'''Introductory reading'''
* {{cite book |author = Carroll, S. |authorlink = Sean B. Carroll |title = [[Endless Forms Most Beautiful]] |publisher = W.W. Norton |location = New York |year = 2005 |isbn = 0-393-06016-0 }}
* {{cite book |author = [[Brian Charlesworth|Charlesworth, C.B.]] and [[Deborah Charlesworth|Charlesworth, D.]] |title = Evolution |publisher = [[Oxford University Press]] |location = Oxfordshire |year = 2003 |isbn = 0-19-280251-8 }}
* {{cite book |author = Dawkins, R. |authorlink = Richard Dawkins |title = [[The Selfish Gene|The Selfish Gene: 30th Anniversary Edition]] |publisher = Oxford University Press |year = 2006 |isbn = 0-19-929115-2 }}
* {{cite book |author = Gould, S.J. |authorlink = Stephen Jay Gould |title = [[Wonderful Life (book)|Wonderful Life: The Burgess Shale and the Nature of History]] |publisher = W.W. Norton |location = New York |year = 1989 |isbn = 0-393-30700-X }}
* {{cite book |author = Jones, J.S. |authorlink = Steve Jones (biologist) |title = [[Almost Like a Whale|Almost Like a Whale: The Origin of Species Updated]]. (''American title:'' ''Darwin's Ghost'') |publisher = Ballantine Books |location = New York |year = 2001 |isbn = 0-345-42277-5 }}
* {{cite book |last = Mader |first = Sylvia S. |others = Murray P. Pendarvis |title = Biology |edition = 9th |year = 2007 |publisher = [[McGraw Hill]] |isbn = 978-0-07-325839-3 }}
* {{cite book |author = Maynard Smith, J. |authorlink = John Maynard Smith |title = [[The Theory of Evolution|The Theory of Evolution: Canto Edition]] |publisher = [[Cambridge University Press]] |year = 1993 |isbn = 0-521-45128-0 }}
* {{cite book |author = Pallen, M.J. |title = The Rough Guide to Evolution |publisher = [[Rough Guides]] |year = 2009 |isbn = 978-1-85828-946-5 }}
* {{cite book |author = Smith, C.B. and Sullivan, C. |title = The Top 10 Myths about Evolution |publisher = [[Prometheus Books]] |year = 2007 |isbn = 978-1-59102-479-8 }}

'''History of evolutionary thought'''
* {{cite book |last = Darwin |first = Charles|author-link = Charles Darwin |year = 1859 |title = On the Origin of Species |edition = 1st|publication-place = London |publisher = John Murray |url = http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=1 |isbn = 0-8014-1319-2 }}
* {{cite book |author = Larson, E.J. |authorlink = Edward Larson |title = Evolution: The Remarkable History of a Scientific Theory |publisher = Modern Library |location = New York |year = 2004 |isbn = 0-679-64288-9 }}
* {{cite book |author = Zimmer, C. |authorlink = Carl Zimmer |title = Evolution: The Triumph of an Idea |publisher = HarperCollins |location = London |year = 2001 |isbn = 0-06-019906-7 }}

'''Advanced reading'''
* {{cite book |author = [[Nick Barton|Barton, N.H.]], [[Derek Briggs|Briggs, D.E.G.]], [[Jonathan Eisen|Eisen, J.A.]], Goldstein, D.B. and Patel, N.H. |title = Evolution |publisher = [[Cold Spring Harbor Laboratory Press]] |year = 2007 |isbn = 0-87969-684-2 }}
* {{cite book |author = [[Jerry Coyne|Coyne, J.A.]] and [[H. Allen Orr|Orr, H.A.]] |title = Speciation |publisher = Sinauer Associates |location = Sunderland |year = 2004 |isbn = 0-87893-089-2 }}
* {{cite book |author = Futuyma, D.J. |authorlink = Douglas J. Futuyma |title = Evolution |publisher = Sinauer Associates |location = Sunderland |year = 2005 |isbn = 0-87893-187-2 }}
* {{cite book |author = [[Carl Bergstrom|Bergstrom, C.T.]] and Lee Alan Dugatkin |title = Evolution|publisher = W.W. Norton |location = New York |year = 2011 |isbn = 0-393-92592-7 }}
* {{cite book | last = Gould | first = S.J. | authorlink = Stephen Jay Gould | title = [[The Structure of Evolutionary Theory]] | publisher = Belknap Press ([[Harvard University Press]]) | location = Cambridge | year = 2002 | isbn = 978-0-674-00613-3 }}
* {{cite book |author = [[John Maynard Smith|Maynard Smith, J.]] and [[Eörs Szathmáry|Szathmáry, E.]] |title = [[The Major Transitions in Evolution]] |publisher = Oxford University Press |location = Oxfordshire |year = 1997 |isbn = 0-19-850294-X }}
* {{cite book |author = Mayr, E. |authorlink = Ernst W. Mayr |title = What Evolution Is |publisher = Basic Books |location = New York |year = 2001 |isbn = 0-465-04426-3 }}
* {{cite book |author = Olson, Wendy; Hall, Brian Keith |title = Keywords and concepts in evolutionary developmental biology |publisher = Harvard University Press |location = Cambridge |year = 2003 |isbn = 0-674-02240-8 }}

== External links ==

<!-- IMPORTANT! Please do not add any links before discussing them on the talk page. -->
{{Spoken Wikipedia|Evolution.ogg|2005-04-18}} <!-- updated changed sections 2005-04-18 -->
{{Sister project links|evolution}}

;General information

* {{In Our Time|Evolution|p00545gl|Evolution}}
* {{cite web |url = http://www.newscientist.com/topic/evolution |title = Evolution |publisher = New Scientist |accessdate = May 30, 2011 }}
* {{cite web |url = http://science.howstuffworks.com/evolution/evolution.htm |title = How Evolution Works |publisher = Howstuffworks.com |accessdate = May 30, 2011 }}
* {{cite web |url = http://nationalacademies.org/evolution/ |title = Evolution Resources from the National Academies |publisher = [[U.S. National Academy of Sciences]] |accessdate = May 30, 2011 }}
* {{cite web |url = http://evolution.berkeley.edu/ |title = Understanding Evolution: your one-stop resource for information on Evolution |publisher = University of California, Berkeley |accessdate = May 30, 2011 }}
* {{cite web |url = http://www.nsf.gov/news/special_reports/darwin/textonly/index.jsp |title = Evolution of Evolution – 150 Years of Darwin's "On the Origin of Species" |publisher = [[National Science Foundation]] |accessdate = May 30, 2011 }}

;History of evolutionary thought

* {{cite web |url = http://darwin-online.org.uk/ |title = The Complete Work of Charles Darwin Online |publisher = National University of Singapore |accessdate = May 30, 2011 }}
* {{cite web |url = http://www.rationalrevolution.net/articles/understanding_evolution.htm |title = Understanding Evolution: History, Theory, Evidence, and Implications |author = Price RG |publisher = RationalRevolution.net |accessdate = May 30, 2011 }}

;Online lectures

* {{cite web |url = http://www.molbio.wisc.edu/carroll/Fittest.html |title = The Making of the Fittest |author = Carroll SB |authorlink = Sean B. Carroll |accessdate = May 30, 2011 }}
* {{cite web |url = http://oyc.yale.edu/ecology-and-evolutionary-biology/principles-of-evolution-ecology-and-behavior/ |title = Principles of Evolution, Ecology and Behavior |author = Stearns SC |authorlink = Stephen C. Stearns |accessdate = August 30, 2011 }}

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[[af:Evolusie]]
[[am:ዝግመተ ለውጥ]]
[[ar:تطور]]
[[an:Evolución]]
[[bn:বিবর্তন]]
[[zh-min-nan:Ián-hoà]]
[[be:Эвалюцыя]]
[[be-x-old:Эвалюцыя]]
[[bg:Еволюция]]
[[bs:Evolucija]]
[[ca:Evolució]]
[[cs:Evoluce]]
[[cy:Esblygiad]]
[[da:Evolution (biologi)]]
[[de:Evolution]]
[[et:Evolutsioon]]
[[el:Εξέλιξη]]
[[eml:Evoluziòun]]
[[es:Evolución biológica]]
[[eo:Evoluismo]]
[[eu:Eboluzio]]
[[fa:تکامل]]
[[hif:Evolution]]
[[fo:Menningarlæran]]
[[fr:Évolution (biologie)]]
[[fy:Evolúsje]]
[[ga:Éabhlóid]]
[[gd:Gnè-fhàs]]
[[gl:Evolución]]
[[ko:진화]]
[[hy:Էվոլյուցիա]]
[[hi:क्रम-विकास]]
[[hr:Evolucija]]
[[ilo:Ebolusion]]
[[id:Evolusi]]
[[ia:Evolution]]
[[is:Þróunarkenningin]]
[[it:Evoluzione]]
[[he:אבולוציה]]
[[krc:Эволюция]]
[[ka:ევოლუცია]]
[[kk:Эволюция]]
[[sw:Mageuko ya spishi]]
[[ht:Evolisyon]]
[[lez:Эволюция]]
[[la:Evolutio]]
[[lv:Evolūcija]]
[[lb:Evolutioun]]
[[lt:Evoliucija]]
[[li:Evolutie]]
[[hu:Evolúció]]
[[mk:Еволуција]]
[[ml:ജീവപരിണാമം]]
[[mt:Evoluzzjoni]]
[[mr:उत्क्रांती]]
[[arz:تطور]]
[[ms:Evolusi]]
[[mwl:Eiboluçon]]
[[mn:Эволюци]]
[[my:ဆင့်ကဲ့ပြောင်းလဲမှုဖြစ်စဉ်]]
[[nl:Evolutie (biologie)]]
[[ne:क्रम-विकास]]
[[new:विकासक्रम]]
[[ja:進化]]
[[no:Evolusjon]]
[[nn:Evolusjon]]
[[nov:Evolutione]]
[[oc:Evolucion]]
[[uz:Evolutsiya]]
[[pnb:ایولوشن]]
[[pap:Evolushon]]
[[pl:Ewolucja]]
[[pt:Evolução]]
[[ro:Evoluție]]
[[rm:Evoluziun]]
[[qu:Rikch'aqyay]]
[[rue:Еволуція]]
[[ru:Эволюция]]
[[sah:Эволюция]]
[[sq:Evolucioni]]
[[si:පරිණාමය]]
[[simple:Evolution]]
[[sk:Biologická evolúcia]]
[[sl:Evolucija]]
[[ckb:توخمەگەشە]]
[[sr:Еволуција (биологија)]]
[[sh:Evolucija]]
[[su:Évolusi]]
[[fi:Evoluutio]]
[[sv:Evolution]]
[[tl:Ebolusyon]]
[[ta:படிவளர்ச்சிக் கொள்கை]]
[[tt:Эволюция]]
[[te:పరిణామం]]
[[th:วิวัฒนาการ]]
[[tr:Evrim]]
[[uk:Еволюція]]
[[ur:نظریۂ ارتقا]]
[[vi:Tiến hóa]]
[[zh-classical:天演]]
[[war:Ebolusyon]]
[[yi:עוואלוציע]]
[[zh-yue:天演]]
[[bat-smg:Evuoliocėjė]]
[[zh:演化]]

Revision as of 22:42, 31 January 2013

Evolution is fake and you are stupid

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