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Introduction to evolution

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Evolution is how life changes over many generations. All dogs once looked the same, but groups of dogs have branched off and evolved over time to become very different breeds.

Evolution is the change in groups of living things over time. Living things (organisms) have children (offspring) which differ from their parents in minor random ways. Many of these differences, called traits, can be passed down to future generations of offspring during reproduction. Evolution is the process of these inheritable differences becoming more common or rare within large groups (populations) of organisms.

Evolution occurs in two different ways. The first way is random — when a population's traits change by chance. The second way is called selection. Selection happens when a trait helps an organism to have more offspring, such as by keeping the organism from dying young. This helpful trait will tend to become more common in the population, because organisms with the trait produce more offspring — who may inherit the same trait.[1]

Selection and random change can cause more and more differences to accumulate in a population, eventually resulting in new species.[2] Every living thing is distantly related — every organism is part of an enormous family tree. This means that all differences between species have arisen through this gradual process of change, as different populations have evolved in different ways.[3]

What evolution is not

In clearly defining evolution, it is helpful to also clarify what evolution is not.

Evolution is not a theory, as defined in biology. Evolution is an observed process and natural phenomenon in the world, akin to gravity or aging. As such, it is a scientific fact. The word evolution is also sometimes used as shorthand for "theory of evolution", a well-supported scientific theory which describes and explains how the observed process of evolution occurs.[4][5] To avoid ambiguity, the term evolution will here signify the observed fact, and not the broader theory.

Evolution is not progress. There are no "goals" in evolution, it is change seen in populations from generation to generation. These changes can be positive, negative, or neutral, all depending on the situation. Evolution may seem progressive at times, because beneficial traits tend to out-compete less helpful traits under selection. However, evolution does not aspire toward any goal; there is no such thing as 'backward evolution' or 'de-evolution' because there is also no 'forward evolution' — evolution does not move in any particular direction. Even natural selection is not progress, since a trait that is helpful in one environment may be harmful after the environment changes.

Evolution is not sudden. It is an extremely gradual, incremental process, requiring several hundreds to several thousands generations, to effect obvious or dramatic change. Any major evolutionary change requires thousands of intermediary transitional forms. However, an organism can only be called "transitional" in retrospect, looking back over countless millennia; evolution does not make long-term plans, and is only guided by short-term, opportunistic selection on an individual level.[6]

Evolution, then, is a directly observed occurrence. Although selection can result in adaptations which help organisms flourish in their environment, most evolution is not particularly helpful. And although evolution can eventually result in dramatic changes, in the short term it is a meandering process of minor modifications.

How does evolution work?

Evolution is the process of organisms' inherited traits changing from generation to generation. As such, evolution follows from two simple facts:

  1. Variation: Organisms in a population are different from each other.
  2. Heredity: Some of these differences are passed on to the next generation.

This means that populations can change — and that they do change, whenever a different trait increases or decreases in commonness. This change is evolution. Traits can even vanish entirely, and new traits can appear.

Variation

Variation: Organisms have differences.

There are two ways that organisms can be different from each other: genetically and environmentally. These are equivalent to "nature" and "nurture", respectively. If an organism's environment changes, it will develop differently. However, on its own this change is not evolution: Dogs will grow bigger if they are fed more, but this size increase will not be passed on to offspring. The "nature" part of organisms — what they are born with — is what can evolve.

This type of variation, genetic diversity, is directly passed down to offspring during reproduction. Although this diversity is encoded in a tiny molecule called DNA stored in each cell of an organism, it has a major impact on the visible traits of an organism. The chemical properties of DNA cause it to build other molecules which make up the organism's body. Because every organism in a population has slightly different DNA, every organism also has a slightly different body.

Heredity

Heredity: Parents pass differences down to their offspring.

When parents produce offspring during reproduction, the offspring will be very similar to the parents. This is because parents create copies of their DNA during reproduction. These copies are modeled after the parent's DNA, so the newly-made DNA molecule will build a nearly identical new organism.

However, evolution can only occur because offspring are not perfect clones of their parents: Parents and children are different. If the parent's DNA and the offspring's DNA were always exactly alike, it would be impossible for populations to change — and such change is evolution.

There are several reasons why DNA is not perfectly copied, and small differences can arise. DNA is very complex, and even tiny errors in copying (mutations) can lead to differences in the new organism. However, the source of most genetic diversity in plants and animals is sexual reproduction. Through this process, multiple DNA molecules are combined in a random fashion, resulting in a "deliberately" unique new organism. Female and male sex cells each have their different DNA strands split in half during meiosis, and one half from each is combined to form a brand new DNA molecule. As a result, sexual species have much more diverse populations than asexual species do. This in turn makes it easier for sexual species to evolve quickly.

Fitness

Fitness: Some differences help organisms have more offspring.

If all the different traits which DNA could produce were equally beneficial, evolution would be a strictly random process. However, since some traits are helpful and some are harmful, this is not the case.

Fitness is the ability of an organism to reproduce. It has little to do with physical fitness, since, depending on the environment, a big and strong organism may be worse at reproducing than a small and stealthy organism. A trait can make an organism more fit by increasing how many viable offspring it has, by improving how well it cares for these offspring, or by simply ensuring that the organism survives long enough to reproduce.

A trait can only be "fit" or "unfit" within the context of a certain environment. If the environment changes, or if the organism migrates to a new environment or ecological niche, the fitness (i.e., the usefulness) of many traits is likely to change. This is because different survival strategies will be effective in different environments.

Selection

Selection: These differences become more common because they are passed on to more offspring.

Selection is the phenomenon of fit traits becoming more common, and unfit traits becoming less common. When some traits are helpful and other traits are unhelpful for producing new organisms (fitness), such traits become common or uncommon in a population (selection).

Selection is simply the consequence of organisms having some traits that affect how many viable offspring they have. Because of this, the number of offspring with such traits will change in each new generation, as traits that are helpful for survival and reproduction are copied through more offspring, and traits that are harmful are copied less. When a trait becomes so common that every organism in a population consistently has it, the trait has reached fixation.

Selection can occur in many different ways. When humans select for certain traits in other species, it is called artificial selection. Otherwise, it is called natural selection. In sexual organisms, one form of natural selection is sexual selection. In sexual selection, a trait is favored and becomes more common because it makes the organisms which have it more desirable to the opposite sex, allowing such organisms to reproduce more. In such cases, traits can be fit even if they would otherwise slightly hinder an organism's survival, such as the peacock's attractive but cumbersome tail. This is because merely being able to survive is useless if an individual cannot attract a mate and reproduce.[7]

What does evolution result in?

Adaptation

Although such mutations in DNA are random, natural selection is not a process of chance: the environment determines the probability of reproductive success. The end products of natural selection are organisms that are adapted to their present environments. Natural selection does not involve progress towards an ultimate goal. Evolution does not necessarily strive for more advanced, more intelligent, or more sophisticated life forms.[8] For example, fleas (wingless parasites) are descended from a winged, ancestral scorpionfly, and snakes are lizards that no longer require limbs - although pythons still grow tiny structures that are the remains of their ancestor's hind legs.[9][10] Organisms are merely the outcome of variations that succeed or fail, dependent upon the environmental conditions at the time.

Competition and cooperation

Bees and flowers have evolved to cooperate, improving both sides' fitness.

The environment of an organism does not only consist of the local climate and terrain. Other organisms also make up part of the environment. Every species in an ecosystem is a part of every other species' environment, and vice versa. As a result, organisms influence each others' evolution: If one species happens to change, this will alter the environment of every species in the area. Because the environment has changed, different traits may be selected for, causing other species to change as well. As a result, species continually evolve and adapt to each other, even when their nonliving environment remains stable.[11]

Individuals within a species also influence each others' fitness. Organisms of the same species form groups, or populations, in which both competition and cooperation take place. Because organisms in the same population are very similar, there is often a fierce competition for the same limited pool of resources. However, this similarity also means that particularly closely related individuals, especially close family members, will be likely to share much of their DNA. In such cases, selection can favor DNA which codes for the trait of altruism, or helping other organisms without benefiting oneself in the process. This is because DNA does not "care" which body it gets passed down in; if siblings are likely to have copies of the same piece of DNA, then that piece of DNA will be more fit if it helps the siblings reproduce a lot than if it helps its own organism reproduce a little bit.

Organisms often cooperate by forming colonies, groups that live together for mutual benefit. The more similar individuals within a colony are, the more likely they are to make sacrifices for each other. Altruistic traits in clone colonies and ant societies, for example, can have high fitness because the DNA which codes such traits will be nearly identical in other members of the colony.

The most extreme example of this is a multicellular organism. Billions of years ago, each organism had only one cell. However, groups of cells that formed complex colonies had high fitness, so much so that the cells eventually lost the ability to survive as separate organisms. Just as worker ants in an ant colony are sterile, and live to help the queen survive rather than to reproduce themselves, so do body cells in most multicellular organisms survive only to help that organism's sex cells reproduce. The DNA coding for making new worker ants is passed down by the queen, just as the DNA coding for making new body cells is passed down by the sex cells.

Competition and cooperation also commonly take place between organisms of different species. When different species compete, small improvements in one competitor's fitness will increase the selective pressure on the other species. This can result in an evolutionary arms race in which each species' evolution forces the other species to continue evolving. Different species also cooperate; when a random difference arises in one species which causes it to help both itself and another species, the other species will often evolve to further encourage the mutually beneficial behavior. Thus, flowers and bees have both evolved to cooperate in a mutualism in which the flowers feed the bees, and in exchange the bees help the flowers reproduce. Such co-evolution does not imply that flowers and bees ever chose or planned to cooperate. Rather, small DNA changes across populations result in cooperative traits which, being useful for reproduction, have slightly higher chances of being passed on to the next generation. Over time, small successive changes result in the complex relationships seen in ecosystems today.

New species

There are numerous species of cichlids that demonstrate dramatic variations in morphology.

Given the right circumstances, and enough time, evolution leads to the emergence of new species. Scientists have struggled to find a precise and all-inclusive definition of species. Ernst Mayr (1904–2005) defined a species as a population or group of populations whose members have the potential to interbreed naturally with one another to produce viable, fertile offspring. (The members of a species cannot produce viable, fertile offspring with members of other species).[12] Mayr's definition has gained wide acceptance among biologists, but does not apply to organisms such as bacteria, which reproduce asexually.

Speciation is the lineage-splitting event that results in two separate species forming from a single common ancestral population.[13] A widely accepted method of speciation is called allopatric speciation. Allopatric speciation begins when a population becomes geographically separated.[14] Geological processes, such as the emergence of mountain ranges, the formation of canyons, or the flooding of land bridges by changes in sea level may result in separate populations. For speciation to occur, separation must be substantial, so that genetic exchange between the two populations is completely disrupted. In their separate environments, the genetically isolated groups follow their own unique evolutionary pathways. Each group will accumulate different mutations as well as be subjected to different selective pressures. The accumulated genetic changes may result in separated populations that can no longer interbreed if they are reunited.[13] Barriers that prevent interbreeding are either prezygotic (prevent mating or fertilization) or postzygotic (barriers that occur after fertilization). If interbreeding is no longer possible, then they will be considered different species.[15]

Usually the process of speciation is slow, occurring over very long time spans; thus direct observations within human life-spans are rare. However speciation has been observed in present day organisms, and past speciation events are recorded in fossils.[16][17][18] Scientists have documented the formation of five new species of cichlid fishes from a single common ancestor that was isolated fewer than 5000 years ago from the parent stock in Lake Nagubago.[19] The evidence for speciation in this case was morphology (physical appearance) and lack of natural interbreeding. These fish have complex mating rituals and a variety of colorations; the slight modifications introduced in the new species have changed the mate selection process and the five forms that arose could not be convinced to interbreed.[20]

How has life evolved in the past?

Common descent

Taxonomy is the branch of biology that names and classifies all living things. Scientists use morphological and genetic similarities to assist them in categorizing life forms based on ancestral relationships. For example, orangutans, gorillas, chimpanzees, and humans all belong to the same taxonomic grouping referred to as a family – in this case the family called Hominidae. These animals are grouped together because of similarities that come from common ancestry (called homology).[21]

How is ancient evolution studied?

Scientific evidence for evolution comes from many aspects of biology, and includes fossils, homologous structures, and molecular similarities between species' DNA.

Research in the field of paleontology, the study of fossils, supports the idea that all living organisms are related. Fossils provide evidence that accumulated changes in organisms over long periods of time have led to the diverse forms of life we see today. A fossil itself reveals the organism's structure and the relationships between present and extinct species, allowing paleontologists to construct a family tree for all of the life forms on earth.[22]

The comparison of similarities between organisms of their form or appearance of parts, called their morphology, has long been a way to classify life into closely related groups. This can be done by comparing the structure of adult organisms in different species or by comparing the patterns of how cells grow, divide and even migrate during an organism's development.

Evolutionary biology

Darwin and Mendel

The understanding of evolutionary biology began with the 1859 publication of Charles Darwin's On the Origin of Species. In addition, Gregor Mendel's work with plants helped to explain the hereditary patterns of genetics, which led to an understanding of the mechanisms of inheritance.[23] Further discoveries on how genes mutate, together with advances in population genetics, have explained more details of how evolution occurs. Scientists now have a good understanding of the origin of new species (speciation), and they have observed the speciation process happening both in the laboratory and in the wild. This modern view of evolution is the principal theory that scientists use to understand life.

Genetics

The missing information needed to help explain how new features could pass from a parent to its offspring was provided by the pioneering genetics work of Gregor Mendel. Mendel’s experiments with several generations of pea plants demonstrated that inheritance works by separating and reshuffling hereditary information during the formation of sex cells and recombining that information during fertilization. This is like mixing different hands of cards, with an organism getting a random mix of half of the cards from one parent, and half of the cards from the other. Mendel called the information factors; however, they later became known as genes. Genes are the basic units of heredity in living organisms. They contain the information that directs the physical development and behavior of organisms.

Genes are made of DNA, a long molecule that carries information. This information is encoded in the sequence of nucleotides in the DNA, just as the sequence of the letters in words carries information on a page. The genes are like short instructions built up of the "letters" of the DNA alphabet. Put together, the entire set of these genes gives enough information to serve as an "instruction manual" of how to build and run an organism. The instructions spelled out by this DNA alphabet can be changed, however, by mutations, and this may alter the instructions carried within the genes. Within the cell, the genes are carried in chromosomes, which are packages for carrying the DNA, with the genes arranged along them like beads on a string. It is the reshuffling of the chromosomes that results in unique combinations of genes in offspring.

Every living organism (with the possible exception of RNA viruses) contains molecules of DNA, which carries genetic information. Genes are the pieces of DNA that carry this information, and they influence the properties of an organism. Genes determine an individual's general appearance and to some extent their behavior. If two organisms are closely related, their DNA will be very similar.[11] On the other hand, the more distantly related two organisms are, the more differences they will have. For example, brothers are closely related and have very similar DNA, while cousins share a more distant relationship and have far more differences in their DNA. Similarities in DNA are used to determine the relationships between species in much the same manner as they are used to show relationships between individuals. For example, comparing chimpanzees with gorillas and humans shows that there is as much as a 96 percent similarity between the DNA of humans and chimps. Comparisons of DNA indicate that humans and chimpanzees are more closely related to each other than either species is to gorillas.[24][25]

Notes

  1. ^ Gould, Stephen J. (2002). The Structure of Evolutionary Theory. Harvard University Press. p. 1433. ISBN 0674006135, 9780674006133. {{cite book}}: Check |isbn= value: invalid character (help)
  2. ^ "An introduction to evolution", Understanding Evolution: your one-stop source for information on evolution (web resource), The University of California Museum of Paleontology, Berkeley, 2008, retrieved 2008-01-23
  3. ^ Cavalier-Smith T (2006). "Cell evolution and Earth history: stasis and revolution" (pdf). Philos Trans R Soc Lond B Biol Sci. 361 (1470): 969–1006. doi:10.1098/rstb.2006.1842. PMID 16754610. Retrieved 2008-01-24.
  4. ^ "NCSE Resource". Cans and Can`ts of Teaching Evolution. National Center for Science Education. 2001-02-13. Retrieved 2008-01-01.
  5. ^ Science and Creationism: A View from the National Academy of Sciences, Second Edition (1999), National Academy of Sciences (NAS), National Academy Press, Washington DC, 2006.
  6. ^ Gould, Stephen Jay. "Punctuated Equilibrium's Threefold History". The Structure of Evolutionary Theory. Harvard University Press. pp. 1006–1021. {{cite book}}: |access-date= requires |url= (help); External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
  7. ^ Johnstone, R.A. (1995) Sexual selection, honest advertisement and the handicap principle: reviewing the evidence" Biological Reviews 70 1-65.
  8. ^ (Gould (a) 1981, p. 24)
  9. ^ Bejder L, Hall BK (2002). "Limbs in whales and limblessness in other vertebrates: mechanisms of evolutionary and developmental transformation and loss". Evol. Dev. 4 (6): 445–58. doi:10.1046/j.1525-142X.2002.02033.x. PMID 12492145.
  10. ^ Boughner JC, Buchtová M, Fu K, Diewert V, Hallgrímsson B, Richman JM (2007). "Embryonic development of Python sebae - I: Staging criteria and macroscopic skeletal morphogenesis of the head and limbs". Zoology (Jena). 110 (3): 212–30. PMID 17499493.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ^ a b Kennedy, Donald (1998). "Teaching about evolution and the nature of science". Evolution and the nature of science. The National Academy of Science. Retrieved 2007-12-30. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  12. ^ (Mayr 2001, pp. 165–69)
  13. ^ a b Quammen, David (2004). "Was Darwin Wrong?". National Geographic Magazine. National Geographic. Retrieved 2007-12-23.
  14. ^ Drummond, A; Strimmer, K (2001), "Evolution Library" (web resource), Bioinformatics (Oxford, England), 17 (7), WGBH Educational Foundation: 662–3, ISSN 1367-4803, PMID 11448888, retrieved 2008-01-23 {{citation}}: |contribution= ignored (help); Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link).
  15. ^ Sulloway, Frank J (2005). "The Evolution of Charles Darwin". Smithsonian Magazine. Smithsonian Institution. Retrieved 2007-08-31. {{cite web}}: Unknown parameter |month= ignored (help)
  16. ^ Jiggins CD, Bridle JR (2004). "Speciation in the apple maggot fly: a blend of vintages?". Trends Ecol. Evol. (Amst.). 19 (3): 111–14. doi:10.1016/j.tree.2003.12.008. PMID 16701238. {{cite journal}}: |access-date= requires |url= (help)
  17. ^ Boxhorn, John (1995). "Observed Instances of Speciation". TalkOrigins Archive. Retrieved 2007-05-10.
  18. ^ Weinberg JR, Starczak VR, Jorg, D (1992). "Evidence for Rapid Speciation Following a Founder Event in the Laboratory". Evolution. 46 (4): 1214–20. doi:10.2307/2409766. {{cite journal}}: |access-date= requires |url= (help)CS1 maint: multiple names: authors list (link)
  19. ^ (Mayr 1970, p. 348)
  20. ^ (Mayr 1970)
  21. ^ (Diamond 1992, p. 16)
  22. ^ "The Fossil Record - Life's Epic". The Virtual Fossil Museum. Retrieved 2007-08-31.
  23. ^ Rhee, Sue Yon (1999). "Gregor Mendel". Access Excellence. National Health Museum. Retrieved 2008-01-05.
  24. ^ Lovgren, Stefan (2005-08-31). "Chimps, Humans 96 Percent the Same, Gene Study Finds". National Geographic News. National Geographic. Retrieved 2007-12-23. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  25. ^ (Carroll, Grenier & Weatherbee 2000)

References

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