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{{Other uses|Homology (disambiguation)}}

[[File:Homology vertebrates-en.svg<!-- This image, Homology vertebrates-en.svg, is referred to in the text! Please don't change it without also updating the text with something equally or more suitable. -->|thumb|300px|The principle of homology: The biological relationships (shown by colors) of the bones in the forelimbs of vertebrates were used by [[Charles Darwin]] as an argument in favor of [[evolution]].]]

In the context of [[biology]], '''homology''' is the existence of shared ancestry between a pair of structures, or genes, in different [[Taxon|taxa]]. A common example of homologous structures is the forelimbs of [[vertebrates]], where the [[Bat wing development|wings of bats]], the arms of [[primate]]s, the front flippers of [[whale]]s and the forelegs of [[Canidae|dogs]] and [[Equidae|horses]] are all derived from the same ancestral [[tetrapod]] structure. [[Evolutionary biology]] explains homologous structures [[adaptation (biology)|adapted]] to different purposes as the result of descent with modification from a [[Common descent|common ancestor]].

In [[developmental biology]], organs that developed in the embryo in the same manner and from similar origins, such as from matching [[Primordium|primordia]] in successive segments of the same animal, must be homologous. Examples include the legs of a [[centipede]], the [[Insect mouthparts|maxillary palp]] and [[Insect mouthparts|labial palp]] of an insect, and the [[Vertebral column|spinous processes]] of successive [[Vertebral column|vertebrae]] in a [[vertebral column]].

[[Sequence homology]] between [[protein]] or [[DNA sequence]]s is similarly defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either a [[speciation]] event (orthologs) or a [[Gene duplication|duplication event]] (paralogs). Homology among proteins or DNA is inferred from their sequence similarity. Significant similarity is strong evidence that two sequences are related by divergent evolution from a common ancestor. [[Sequence alignment|Alignments]] of multiple sequences are used to discover the homologous regions.

In the [[sexual differentiation|development of the differences between males and females]] in the embryo, male and female [[reproductive organ]]s are homologous if they develop from the same embryonic tissue, as do the [[ovaries]] and [[testicles]] of mammals including humans.

==Definition==
[[File:Eupatorus gracilicornis Vol.jpg|thumb|left|The front wings of [[beetle]]s have evolved into [[elytron|elytra, hard wing-cases]]. These are homologous with the front wings of other
[[insect]]s.]]

The word homology, coined in about 1656, derives from the [[Ancient Greek|Greek]] ὁμόλογος ''homologos'' from ὁμός ''homos'' "same" and λόγος ''logos'' "relation".<ref>{{cite book |title=Congress of Arts and Science: Universal Exposition, St. Louis, 1904 |first=Frederick Orpen |last=Bower |chapter=Plant Morphology |publisher=Houghton, Mifflin |year=1906 |page=64 |url=https://books.google.com/books?id=Xe4ZAQAAIAAJ&pg=PA64#v=onepage&f=false}}</ref><ref>{{cite book |title=Milestones in Systematics |first1=David Malcolm |last1=Williams |first2=Peter L. |last2=Forey |publisher=CRC Press |year=2004 |isbn=0-415-28032-X |page=198}}</ref>{{efn|The alternative terms "homogeny" and "homogenous" were also used in the late 1800s and early 1900s. However, these terms are now archaic in biology, and the term "homogenous" is now generally found as a misspelling of the term "[[Homogeneous (chemistry)|homogeneous]]" which refers to the uniformity of a mixture.<ref>"homogeneous, adj.". OED Online. March 2016. Oxford University Press. http://www.oed.com/view/Entry/88045? (accessed April 09, 2016).</ref><ref>"homogenous, adj.". OED Online. March 2016. Oxford University Press. http://www.oed.com/view/Entry/88055? (accessed April 09, 2016).</ref>}}

[[File:Nephrotoma guestfalica.jpg|thumb|The hind wings of [[Diptera]]n flies such as this [[cranefly]] have [[divergent evolution|evolved divergently]] to form small club-like [[haltere]]s. These are homologous with the hind wings of other insects.]]

Homology is the relationship between biological structures or sequences that are derived from a [[common descent|common ancestor]]. For example, many [[insect]]s (such as [[dragonfly|dragonflies]]) possess two pairs of flying [[wing (insect)|wings]]. In [[beetle]]s, the first pair of wings has evolved into a pair of [[elytron|hard wing covers]],<ref>{{cite book|last=Wagner|first=Günter P.|title=Homology, Genes, and Evolutionary Innovation|url=https://books.google.com/books?id=g7vzAgAAQBAJ&pg=PA53|year=2014|publisher=Princeton University Press|isbn=978-1-4008-5146-1|pages=53–54 |quote=elytra have very little similarity with typical wings, but are clearly homologous to forewings. Hence butterflies, flies, and beetles all have two pairs of dorsal appendages that are homologous among species.}}</ref> while in [[Diptera]]n flies the second pair of wings has evolved into small [[halteres]] used for balance.{{efn|If the two pairs of wings are considered as interchangeable, homologous structures, this may be described as a parallel reduction in the number of wings, but otherwise the two changes are each divergent changes in one pair of wings.}}<ref>{{cite book |last=Lipshitz |first=Howard D. |title=Genes, Development and Cancer: The Life and Work of Edward B. Lewis |url=https://books.google.com/books?id=8g4GCAAAQBAJ&pg=PA240 |year=2012 |publisher=Springer |isbn=978-1-4419-8981-9 |page=240 |quote=For example, wing and haltere are homologous, yet widely divergent, organs that normally arise as dorsal appendages of the second thoracic (T2) and third thoracic (T3) segments, respectively.}}</ref>

Similarly, the forelimbs of ancestral [[vertebrate]]s have evolved into the front flippers of [[whale]]s, the wings of [[bird]]s, the running forelegs of [[dog]]s, [[deer]], and [[horse]]s, the short forelegs of [[frog]]s and [[lizard]]s, and the grasping [[hand]]s of [[primate]]s including humans. The same major forearm bones ([[humerus]], [[radius]], and [[ulna]]{{efn|These are coloured in the lead image: humerus brown, radius pale buff, ulna red.}}) are found in fossils of [[lobe-finned fish]] such as ''[[Eusthenopteron]]''.<ref>{{cite web|title=Homology: Legs and Limbs|url=http://evolution.berkeley.edu/evolibrary/article/0_0_0/homology_02|publisher=UC Berkeley|accessdate=15 December 2016}}</ref>

===Homology vs analogy===
[[File:Acer pseudoplatanus MHNT.BOT.2004.0.461.jpg|thumb|upright=0.7|[[Sycamore maple]] fruits have wings [[Analogy (biology)|analogous]] to a bird's.]]
{{further|Convergent evolution}}

The opposite of homologous organs are [[Analogy (biology)|analogous]] organs which do similar jobs in two taxa that were not [[phylogeny|present in their last common ancestor]] but rather [[convergent evolution|evolved separately]]. For example, the [[insect wing|wings of insects]] and birds evolved independently in [[phylum (biology)|widely separated groups]], and converged functionally to support powered [[flight]], so they are analogous. Similarly, the wings of a [[sycamore maple]] seed and the wings of a bird are analogous but not homologous, as they develop from quite different structures.

A structure can be homologous at one level, but only analogous at another. For example, [[pterosaur]], [[Bird flight|bird]] and [[bat wing]]s are analogous as wings, but homologous as forelimbs because the organ served as a forearm (not a wing) in the last common ancestor of [[tetrapod]]s, and evolved in different ways in the three groups. For example, in the pterosaurs, the "wing" involves both the forelimb and the hindlimb.<ref name=Scotland2010>{{Cite journal | last1=Scotland | first1=R. W. | title=Deep homology: A view from systematics | doi=10.1002/bies.200900175 | journal=BioEssays | volume=32 | issue=5 | pages=438–449 | year=2010 | pmid=20394064}}</ref>

Analogy is called [[homoplasy]] in cladistics, and [[convergent evolution|convergent or parallel evolution]] in evolutionary biology.<ref>Cf. Butler, A. B.: ''Homology and Homoplasty.'' In: Squire, Larry R. (Ed.): ''Encyclopedia of Neuroscience'', Academic Press, 2009, pp. 1195–1199.</ref><ref>{{cite web |url=http://explainry.com/difference-between/homologous-analogous-structures/ |title=Homologous structure vs. analogous structure: What is the difference? |access-date=27 September 2016}}</ref>

===In cladistics===
{{further|Cladistics}}

Specialised terms are used in taxonomic research. Primary homology is that initially conjectured by a researcher based on similar structure or anatomical connections, who states a hypothesis that two characters share an ancestry. Secondary homology is implied by [[parsimony analysis]], where a character that only occurs once on a tree is taken to be homologous.<ref name="de Pinna1991">{{Cite journal| doi=10.1111/j.1096-0031.1991.tb00045.x| title=Concepts and Tests of Homology in the Cladistic Paradigm| year=1991| last1=Pinna | first1=M. C. C.| journal=Cladistics| volume=7| issue=4| pages=367–394 }}</ref> As implied in this definition, many [[cladistics|cladists]] consider homology to be synonymous with [[synapomorphy]], a shared derived character or [[Phenotypic trait|trait]] state that distinguishes a [[clade]] from other organisms.<ref name="PageHolmes2009">{{cite book |author1=Page, Roderick D.M. |author2=Holmes, Edward C. |title=Molecular Evolution: A Phylogenetic Approach |url=https://books.google.com/books?id=p2lWhjuK8m8C |date=2009|publisher=John Wiley & Sons |isbn=978-1-4443-1336-9}}</ref>

==In different taxa==

{{anchor|In arthropods}}
===In arthropods===
Homologies provide the fundamental basis for all biological classification, although some may be highly counter-intuitive. The embryonic body segments ([[somite]]s) of different [[arthropod]]s taxa have diverged from a simple body plan with many similar appendages, into a variety of body plans with fewer segments equipped with specialised appendages. The homologies between these have been discovered by comparing [[gene]]s in [[evolutionary developmental biology]].<ref>Brusca, R.C. & Brusca, G.J. 1990. Invertebrates. Sinauer Associates, Sunderland: P. 669</ref>

{| class="wikitable"
|-
! [[Somite]]<br>(body<br>segment)
! [[Trilobite]]<br>([[Trilobitomorpha]])<br>[[File:Acadoparadoxides sp 4343.JPG|50px]]
! [[Spider]]<br>([[Chelicerata]])<br>[[File:Araneus quadratus MHNT.jpg|50px]]
! [[Centipede]]<br>([[Myriapoda]])<br>[[File:Scolopendridae - Scolopendra cingulata.jpg|50px]]
! [[Insect]]<br>([[Hexapoda]])<br>[[File:Cerf-volant MHNT Dos.jpg|50px]]
! [[Shrimp]]<br>([[Crustacea]])<br>[[File:GarneleCrystalRed20.jpg|50px]]
|-
| 1
| antennae
| [[chelicerae]] (jaws and fangs)
| antennae
| antennae
| 1st antennae
|-
| 2
| 1st legs
| [[pedipalps]]
| -
| -
| 2nd antennae
|-|
| 3
| 2nd legs
| 1st legs
| [[Mandible (arthropod mouthpart)|mandibles]]
| mandibles
| mandibles (jaws)
|-
| 4
| 3rd legs
| 2nd legs
| 1st [[maxillae]]
| 1st maxillae
| 1st maxillae
|-
| 5
| 4th legs
| 3rd legs
| 2nd maxillae
| 2nd maxillae
| 2nd maxillae
|-
| 6
| 5th legs
| 4th legs
| collum (no legs)
| 1st legs
| 1st legs
|-
| 7
| 6th legs
| -
| 1st legs
| 2nd legs
| 2nd legs
|-
| 8
| 7th legs
| -
| 2nd legs
| 3rd legs
| 3rd legs
|-
| 9
| 8th legs
| -
| 3rd legs
| -
| 4th legs
|-
| 10
| 9th legs
| -
| 4th legs
| -
| 5th legs
|-
|}

Among insects, the [[stinger]] of the female [[honey bee]] is a modified [[ovipositor]], homologous with ovipositors in other insects such as the [[Orthoptera]], [[Hemiptera]], and those [[Hymenoptera]] without stingers.<ref>{{cite journal |last1=Shing |first1=H. |last2=Erickson |first2=E. H. |title=Some ultrastructure of the honeybee (''Apis mellifera'' L.) sting |journal=Apidologie |date=1982 |volume=13 |issue=3 |pages=203-213 |url=https://hal.archives-ouvertes.fr/hal-00890568/document}}</ref>

{{anchor|In mammals}}

===In mammals===
{{further|Comparative anatomy}}

The three small bones in the [[middle ear]] of mammals including humans, the [[malleus]], [[incus]], and [[stapes]], are today used to transmit sound from the [[eardrum]] to the [[inner ear]]. The malleus and incus develop in the embryo from structures that form jaw bones (the quadrate and the articular) in lizards, and in fossils of lizard-like ancestors of mammals. Both lines of evidence show that these bones are homologous, sharing a common ancestor.<ref>{{cite web |title=Homology: From jaws to ears — an unusual example of a homology|url=http://evolution.berkeley.edu/evolibrary/article/homology_06|publisher=UC Berkeley |accessdate=15 December 2016}}</ref>

Among the many [[List of homologues of the human reproductive system|homologies in mammal reproductive systems]], [[ovary|ovaries]] and [[testicle]]s are homologous.<ref name="Hyde2010">{{Citation | last=Hyde | first=Janet Shibley | last2=DeLamater | first2=John D. | title=Understanding Human Sexuality | place=New York | publisher=[[McGraw-Hill]] |date=June 2010 | edition=11th | chapter=Chapter 5 | chapterurl=http://highered.mcgraw-hill.com/sites/dl/free/0072986360/238525/hyd86360_ch05.pdf | page=103 | isbn=978-0073382821}}</ref>

{{anchor|In plants}}

===In plants===
In many plants, defensive or storage structures are made by modifications of the development of primary leaves, stems, and roots.

{| class="wikitable"
|-
! Primary organs
! Defensive structures
! Storage structures
|-
| Leaves
| [[Thorns, spines, and prickles|Spines]]
| Swollen leaves (e.g. [[succulents]])
|-
| Stems
| [[Thorns, spines, and prickles|Thorns]]
| Tubers (e.g. [[potato]]), rhizomes (e.g. [[ginger]]), fleshy stems (e.g. [[cactus|cacti]])
|-
| Roots
| -
| Root tubers (e.g. [[sweet potato]]), taproot (e.g. [[carrot]])
|}

Certain [[compound leaf|compound leave]]s of [[flower]]ing plants are partially homologous both to leaves and shoots, because their [[evolutionary developmental biology|development has evolved]] from a [[mosaic (genetics)|genetic mosaic]] of leaf and shoot development.<ref>{{cite journal |author=Sattler R |title=Homology — a continuing challenge |journal=Systematic Botany |volume=9 |pages=382–94 |year=1984 |doi=10.2307/2418787 |issue=4 |jstor=2418787}}</ref><ref>{{cite book |author=Sattler, R. |chapter=Homology, homeosis, and process morphology in plants |editor=Hall, Brian Keith |title=Homology: the hierarchical basis of comparative biology |publisher=Academic Press |year=1994 |pages=423–75 |isbn=0-12-319583-7 }}</ref>

[[File:Pachyrhachis problematicus 45.JPG|thumb|The [[Cretaceous]] snake ''[[Pachyrhachis problematicus]]'' had hind legs (circled).]]

==Developmental biology==

[[Developmental biology]] can identify homologous structures that arose from the same tissue in [[embryogenesis]]. For example, adult [[snake]]s have no legs, but their early embryos have limb-buds for hind legs, which are soon lost as the embryos develop. The implication that the ancestors of snakes had hind legs is confirmed by [[fossil]] evidence: the [[Cretaceous]] snake ''[[Pachyrhachis problematicus]]'' had hind legs complete with hip bones ([[ilium (bone)|ilium]], [[pubis (bone)|pubis]], [[ischium]]), thigh bone ([[femur]]), leg bones ([[tibia]], [[fibula]]) and foot bones ([[calcaneum]], [[astragalus]]) as in tetrapods with legs today.<ref>{{cite web|title=Homologies: developmental biology|url=http://evolution.berkeley.edu/evolibrary/article/lines_07|publisher=UC Berkeley|accessdate=15 December 2016}}</ref>

==Sequence homology==<!-- This section is linked from [[Primary structure]] -->
[[File:Histone Alignment.png|thumb|300px|left|A [[sequence alignment]] of [[Amino acid#Table of standard amino acid abbreviations and side chain properties|amino acids]] for mammalian [[histone]] proteins. [[Conserved sequence|Sequences conserved]] across all 5 species analysed are highlighted in grey. [[Conservative mutation|Conservative]], semi-conservative, and [[segregating site|non-conservative mutations]] are indicated.<ref>{{cite web|url=http://www.ebi.ac.uk/Tools/msa/clustalw2/help/faq.html#23|website=Clustal |title= Clustal FAQ #Symbols|accessdate=8 December 2014}}</ref>]]
{{main article|Sequence homology}}
{{further information|Deep homology|Evolutionary developmental biology}}

As with anatomical structures, [[sequence homology]] between [[protein]] or [[DNA sequence]]s is defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either a [[speciation]] event (orthologs) or a [[Gene duplication|duplication event]] (paralogs).<ref>{{cite journal |author=Koonin EV |title=Orthologs, paralogs, and evolutionary genomics |journal=Annual Review of Genetics |volume=39 |issue= |pages=309–38 |year=2005 |pmid=16285863 |doi=10.1146/annurev.genet.39.073003.114725}}</ref> Homology among proteins or DNA is typically inferred from their sequence similarity. Significant similarity is strong evidence that two sequences are related by divergent evolution of a common ancestor. [[Sequence alignment|Alignments]] of multiple sequences are used to indicate which regions of each sequence are homologous. Homologous proteins make up protein [[protein family|families]] and [[protein superfamily|superfamilies]], encoded by [[gene family|gene families]].

Homologous sequences are orthologous if they are descended from the same ancestral sequence separated by a [[speciation]] event: when a species diverges into two separate species, the copies of a single gene in the two resulting species are said to be orthologous. Orthologs, or orthologous genes, are genes in different species that originated by vertical descent from a single gene of the last common ancestor. The term "ortholog" was coined in 1970 by the [[molecular evolution]]ist [[Walter M. Fitch|Walter Fitch]].<ref name="Fitch WM 99–113">{{cite journal |author=Fitch WM |title=Distinguishing homologous from analogous proteins |journal=Systematic Zoology |volume=19 |issue=2 |pages=99–113 |date=June 1970 |pmid=5449325 |doi=10.2307/2412448}}</ref>

Homologous sequences are paralogous if they were created by a duplication event within the genome. For [[gene duplication]] events, if a gene in an organism is duplicated to occupy two different positions in the same genome, then the two copies are paralogous. Paralogous genes often belong to the same species. They can shape the structure of whole genomes and thus explain genome evolution to a large extent. Examples include the [[Homeobox]] ([[Hox gene|Hox]]) genes in animals. These genes not only underwent gene duplications within [[chromosome]]s but also [[Genome evolution|whole genome duplications]]. As a result Hox genes in most vertebrates are clustered across multiple chromosomes with the HoxA-D clusters being the best studied.<ref name=Zakany>{{Cite journal |last=Zakany |first=Jozsef |last2=Duboule |first2=Denis |date=2007-08-01 |title=The role of Hox genes during vertebrate limb development |journal=Current Opinion in Genetics & Development |volume=17 |issue=4 |pages=359–366 |doi=10.1016/j.gde.2007.05.011|issn=0959-437X |pmid=17644373}}</ref>

==In behavior==
{{main article|Homology (psychology)}}

It has been suggested that [[Homology (psychology)|some behaviors might be homologous]], based on either shared behavior across related taxa or common origins of the behavior in an individual’s development, though this remains controversial.<ref>{{cite journal |last=Moore |first=David S |title=Importing the homology concept from biology into developmental psychology |journal=Developmental Psychobiology |year=2013 |volume=55 |issue=1|pages=13–21 |doi=10.1002/dev.21015 |url=http://onlinelibrary.wiley.com/doi/10.1002/dev.21015/full |pmid=22711075}}</ref>

==Notes==
{{notelist}}

==References==
{{reflist|30em}}

==Further reading==
{{commons|Homology}}
* Brigandt, Ingo (2011) [http://embryo.asu.edu/handle/10776/1754 "Essay: Homology."] In: ''The Embryo Project Encyclopedia''. {{ISSN|1940-5030}}. [http://embryo.asu.edu/handle/10776/1754 http://embryo.asu.edu/handle/10776/1754]
* {{cite book |author=Carroll, Sean B. |authorlink=Sean B. Carroll |title=Endless Forms Most Beautiful |publisher=W.W. Norton & Co |location=New York |year=2006 |isbn=0-297-85094-6 }}
* {{cite book |author=Carroll, Sean B. |title=The making of the fittest: DNA and the ultimate forensic record of evolution |publisher=W.W. Norton & Co |location=New York |year=2006 |isbn=0-393-06163-9 }}
* {{cite journal |author=DePinna, M.C. |title=Concepts and tests of homology in the cladistic paradigm |journal=Cladistics |volume=7 |pages=367–94 |year=1991 |doi=10.1111/j.1096-0031.1991.tb00045.x |issue=4}}
* {{cite journal |author=Dewey, C.N.; [[Lior Pachter|Pachter, L.]] |title=Evolution at the nucleotide level: the problem of multiple whole-genome alignment |journal=Human Molecular Genetics |volume=15 |issue=Spec No 1 |pages=R51–6 |date=April 2006 |pmid=16651369 |doi=10.1093/hmg/ddl056}}
* {{cite journal |author=Fitch, W.M. |title=Homology a personal view on some of the problems |journal=Trends in Genetics |volume=16 |issue=5 |pages=227–31 |date=May 2000 |pmid=10782117 |doi=10.1016/S0168-9525(00)02005-9}}
* {{cite book |author=Gegenbaur, G. |title=Vergleichende Anatomie der Wirbelthiere... |location=Leipzig |year=1898 }}
* {{cite book |author=Haeckel, Е. |title=Generelle Morphologie der Organismen |location=Bd 1-2. Вerlin |year=1866 }}
* {{cite book |author=Owen, R. |title=On the archetype and homologies of the vertebrate skeleton |location=London |year=1847 }}
* {{cite journal |author=Mindell D.P., Meyer A. |title=Homology evolving |journal=Trends in Ecology and Evolution |volume=16 |pages=434–40 |year=2001 |url=http://euplotes.biology.uiowa.edu/web/IBS593/week4/Homologyevolving.pdf |doi=10.1016/S0169-5347(01)02206-6 |issue=8}}
* {{cite journal |author1=Kuzniar, A. |author2=van Ham, R.C. |author3=Pongor, S. |author4=Leunissen, J.A. |title=The quest for orthologs: finding the corresponding gene across genomes |journal=Trends Genet. |volume=24 |issue=11 |pages=539–51 |date=November 2008 |pmid=18819722 |doi=10.1016/j.tig.2008.08.009}}

==External links==
{{fins, limbs and wings}}

{{DEFAULTSORT:Homology (Biology)}}
[[Category:Evolutionary biology]]
[[Category:Phylogenetics]]
[[Category:Comparative anatomy]]

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