Homology (biology)

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For other uses, see Homology (disambiguation).
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 biology, homology is the existence of shared ancestry between a pair of structures, or genes, in different taxa. A common example of homologous structures is the forelimbs of vertebrates, where the wings of bats, the arms of primates, the front flippers of whales and the forelegs of dogs and horses are all derived from the same ancestral tetrapod structure. Evolutionary biology explains homologous structures adapted to different purposes as the result of descent with modification from a common ancestor.

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

Sequence homology between protein or DNA sequences 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 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. Alignments of multiple sequences are used to discover the homologous regions.

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


The front wings of beetles have evolved into elytra, hard wing-cases. These are homologous with the front wings of other insects.

The word homology, coined in about 1656, derives from the Greek ὁμόλογος homologos from ὁμός homos "same" and λόγος logos "relation".[1][2][a]

The hind wings of Dipteran flies such as this cranefly have evolved divergently to form small club-like halteres. 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 ancestor. For example, many insects (such as dragonflies) possess two pairs of flying wings. In beetles, the first pair of wings has evolved into a pair of hard wing covers,[5] while in Dipteran flies the second pair of wings has evolved into small halteres used for balance.[b][6]

Similarly, the forelimbs of ancestral vertebrates have evolved into the front flippers of whales, the wings of birds, the running forelegs of dogs, deer, and horses, the short forelegs of frogs and lizards, and the grasping hands of primates including humans. The same major forearm bones (humerus, radius, and ulna[c]) are found in fossils of lobe-finned fish such as Eusthenopteron.[7]

Homology vs analogy[edit]

Sycamore maple fruits have wings analogous to a bird's.
Further information: Convergent evolution

The opposite of homologous organs are analogous organs which do similar jobs in two taxa that were not present in their last common ancestor but rather evolved separately. For example, the wings of insects and birds evolved independently in 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.[8] A structure can be homologous at one level, but only analogous at another. For example, pterosaur, bird and bat wings are analogous as wings, but homologous as forelimbs because the organ served as a forearm (not a wing) in the last common ancestor of tetrapods, and evolved in different ways in the three groups. For example, in the pterosaurs, the "wing" involves both the forelimb and the hindlimb.[9] Analogy is called homoplasy in cladistics, and convergent or parallel evolution in evolutionary biology.[10][11]

In cladistics[edit]

Further information: 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.[12] As implied in this definition, many cladists consider homology to be synonymous with synapomorphy, a shared derived character or trait state that distinguishes a clade from other organisms.[13]

In different taxa[edit]

The Cretaceous snake Pachyrhachis problematicus had hind legs (circled).

In arthropods[edit]

Homologies provide the fundamental basis for all biological classification, although some may be highly counter-intuitive. The embryonic body segments (somites) of different arthropods 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 genes in evolutionary developmental biology.[14]

Acadoparadoxides sp 4343.JPG
Araneus quadratus MHNT.jpg
Scolopendridae - Scolopendra cingulata.jpg
Cerf-volant MHNT Dos.jpg
1 antennae chelicerae (jaws and fangs) antennae antennae 1st antennae
2 1st legs pedipalps - - 2nd antennae
3 2nd legs 1st legs 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.[15]

In mammals[edit]

Further information: 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.[16]

Among the many homologies in mammal reproductive systems, ovaries and testicles are homologous.[17]

In plants[edit]

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

Primary organs Defensive structures Storage structures
Leaves Spines Swollen leaves (e.g. succulents)
Stems Thorns Tubers (e.g. potato), rhizomes (e.g. ginger), fleshy stems (e.g. cacti)
Roots - Root tubers (e.g. sweet potato), taproot (e.g. carrot)

Certain compound leaves of flowering plants are partially homologous both to leaves and shoots, because their development has evolved from a genetic mosaic of leaf and shoot development.[18][19]

Developmental biology[edit]

Developmental biology can identify homologous structures that arose from the same tissue in embryogenesis. For example, adult snakes 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, pubis, ischium), thigh bone (femur), leg bones (tibia, fibula) and foot bones (calcaneum, astragalus) as in tetrapods with legs today.[20]

Sequence homology[edit]

Main article: Sequence homology
A sequence alignment of amino acids for mammalian histone proteins. Sequences conserved across all five species analysed are highlighted in grey. Conservative, semi-conservative, and non-conservative mutations are indicated.[21]

As with anatomical structures, sequence homology between protein or DNA sequences is defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either a speciation event (orthologs) or a duplication event (paralogs). 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. Alignments of multiple sequences are used to indicate which regions of each sequence are homologous.[22]

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 evolutionist Walter Fitch.[23]

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) genes in animals. These genes not only underwent gene duplications within chromosomes but also 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.[24]

In behavior[edit]

Main article: Homology (psychology)

It has been suggested that 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.[25]


  1. ^ 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" which refers to the uniformity of a mixture.[3][4]
  2. ^ 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.
  3. ^ These are coloured in the lead image: humerus brown, radius pale buff, ulna red.


  1. ^ Bower, Frederick Orpen (1906). "Plant Morphology". Congress of Arts and Science: Universal Exposition, St. Louis, 1904. Houghton, Mifflin. p. 64. 
  2. ^ Williams, David Malcolm; Forey, Peter L. (2004). Milestones in Systematics. CRC Press. p. 198. ISBN 0-415-28032-X. 
  3. ^ "homogeneous, adj.". OED Online. March 2016. Oxford University Press. http://www.oed.com/view/Entry/88045? (accessed April 09, 2016).
  4. ^ "homogenous, adj.". OED Online. March 2016. Oxford University Press. http://www.oed.com/view/Entry/88055? (accessed April 09, 2016).
  5. ^ Wagner, Günter P. (2014). Homology, Genes, and Evolutionary Innovation. Princeton University Press. pp. 53–54. ISBN 978-1-4008-5146-1. 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. 
  6. ^ Lipshitz, Howard D. (2012). Genes, Development and Cancer: The Life and Work of Edward B. Lewis. Springer. p. 240. ISBN 978-1-4419-8981-9. 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. 
  7. ^ "Homology: Legs and Limbs". UC Berkeley. Retrieved 15 December 2016. 
  8. ^ "Secret Found to Flight of 'Helicopter Seeds'". LiveScience. 11 June 2009. Retrieved 2 March 2017. 
  9. ^ Scotland, R. W. (2010). "Deep homology: A view from systematics". BioEssays. 32 (5): 438–449. doi:10.1002/bies.200900175. PMID 20394064. 
  10. ^ Cf. Butler, A. B.: Homology and Homoplasty. In: Squire, Larry R. (Ed.): Encyclopedia of Neuroscience, Academic Press, 2009, pp. 1195–1199.
  11. ^ "Homologous structure vs. analogous structure: What is the difference?". Retrieved 27 September 2016. 
  12. ^ Pinna, M. C. C. (1991). "Concepts and Tests of Homology in the Cladistic Paradigm". Cladistics. 7 (4): 367–394. doi:10.1111/j.1096-0031.1991.tb00045.x. 
  13. ^ Page, Roderick D.M.; Holmes, Edward C. (2009). Molecular Evolution: A Phylogenetic Approach. John Wiley & Sons. ISBN 978-1-4443-1336-9. 
  14. ^ Brusca, R.C. & Brusca, G.J. 1990. Invertebrates. Sinauer Associates, Sunderland: P. 669
  15. ^ Shing, H.; Erickson, E. H. (1982). "Some ultrastructure of the honeybee (Apis mellifera L.) sting". Apidologie. 13 (3): 203–213. 
  16. ^ "Homology: From jaws to ears — an unusual example of a homology". UC Berkeley. Retrieved 15 December 2016. 
  17. ^ Hyde, Janet Shibley; DeLamater, John D. (June 2010), "Chapter 5" (PDF), Understanding Human Sexuality (11th ed.), New York: McGraw-Hill, p. 103, ISBN 978-0-07-338282-1 
  18. ^ Sattler R (1984). "Homology — a continuing challenge". Systematic Botany. 9 (4): 382–94. doi:10.2307/2418787. JSTOR 2418787. 
  19. ^ Sattler, R. (1994). "Homology, homeosis, and process morphology in plants". In Hall, Brian Keith. Homology: the hierarchical basis of comparative biology. Academic Press. pp. 423–75. ISBN 0-12-319583-7. 
  20. ^ "Homologies: developmental biology". UC Berkeley. Retrieved 15 December 2016. 
  21. ^ "Clustal FAQ #Symbols". Clustal. Retrieved 8 December 2014. 
  22. ^ Koonin EV (2005). "Orthologs, paralogs, and evolutionary genomics". Annual Review of Genetics. 39: 309–38. doi:10.1146/annurev.genet.39.073003.114725. PMID 16285863. 
  23. ^ Fitch WM (June 1970). "Distinguishing homologous from analogous proteins". Systematic Zoology. 19 (2): 99–113. doi:10.2307/2412448. JSTOR 2412448. PMID 5449325. 
  24. ^ Zakany, Jozsef; Duboule, Denis (2007-08-01). "The role of Hox genes during vertebrate limb development". Current Opinion in Genetics & Development. 17 (4): 359–366. doi:10.1016/j.gde.2007.05.011. ISSN 0959-437X. PMID 17644373. 
  25. ^ Moore, David S (2013). "Importing the homology concept from biology into developmental psychology". Developmental Psychobiology. 55 (1): 13–21. doi:10.1002/dev.21015. PMID 22711075. 

Further reading[edit]

External links[edit]

  • Media related to Homology at Wikimedia Commons