Homology (biology)

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 taxa.^[1] A common example of homologous structures in evolutionary biology are the wings of bats and the arms of primates.^[1] Evolutionary theory explains the existence of homologous structures adapted to different purposes as the result of descent with modification from a common ancestor.

In the context of sexual differentiation—the process of development of the differences between males and females from an undifferentiated fertilized egg — the male and female organs are homologous if they develop from the same embryonic tissue.^[2] A typical example is the ovaries of female humans and the testicles of male humans.^[2]

In the context of morphological differentiation, organs that developed in the same embryological manner from similar origins, such as from matching primordia in successive segments of the same organism, may be said to 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. In contrast, a sesamoid bone such as the patella is not homologous to a neighbouring skeletal bone such as the femur.

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.

Etymology

The word homology, coined in about 1656, derives from the Greek ὁμόλογος homologos from ὁμός homos "same" and λόγος logos "relation".^{[3][4][lower-alpha 1]}

Definition

Homology is the relationship between biological structures or sequences that are derived from a common ancestor. Homologous traits of organisms are therefore explained by descent from a common ancestor. The opposite of homologous organs are analogous organs which do similar jobs in two taxa that were not present in the last common ancestor but rather evolved separately. An example of an analogous trait would be the wings of bats and birds, which evolved independently in each lineage separately after diverging from ancestors with forelimbs not used as wings (terrestrial mammals and theropod dinosaurs, respectively).

A structure can be homologous at one level, but only analogous at another. For example, the 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.^[7] Homology can also be described at the level of the gene. In genetics homology can refer to both the gene (DNA) and the corresponding protein product. 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.^[8]

Anatomical homology

The wings of pterosaurs (1), bats (2) and birds (3) are analogous as wings, but homologous as forelimbs.

Evolutionary ancestry means that structures evolved from some structure in a common ancestor; for example, the wings of bats and the arms of primates are homologous in this sense. Developmental ancestry means that structures arose from the same tissue in embryonal development; the ovaries of female humans and the testicles of male humans are homologous in this sense.

In evolutionary developmental biology (evo-devo), homology can also be partial. New structures can evolve through the combination of multiple developmental pathways, or parts of them. As a result, hybrid or mosaic structures can evolve that exhibit partial homologies. For example, certain compound leaves of flowering plants are partially homologous both to leaves and shoots because their development has evolved from a combination of leaf and shoot development.^[9][10]

Determining homology

Systematists identify two forms of homology: 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.^[11] 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.^[12]

Homologous structures in other phyla

Introductory discussions of homology commonly limit themselves to the limbs of tetrapod vertebrates, occasionally touching on other structures, such as modified teeth as in whales and elephants. Homologies provide the fundamental basis for all biological classification, although some may be highly counter-intuitive. The embryonic body segments (somites) of different arthropod taxa are an example:^[13]

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

Sequence homology

A sequence alignment, produced by ClustalO, of mammalian histone proteins.
Sequences are the amino acids for residues 120-180 of the proteins. Residues that are conserved across all sequences are highlighted in grey. Below the protein sequences is a key denoting conserved sequence (*), conservative mutations (:), semi-conservative mutations (.), and non-conservative mutations ( ).^[14]

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).^[15] 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. Homologous proteins make up protein families and superfamilies, encoded by 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 evolutionist Walter Fitch.^[16]

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.^[17]

Homology between sexes and forms

The term homology is sometimes applied to reproductive structures that share a common embryonic origin, but become spectacularly different between the two sexes in the adult. Those listed below are some of the more commonly cited examples.

Among insects, the stinger used by infertile female worker honey bees is a modified ovipositor and is thus homologous with ovipositors in other Hymenoptera.

Difference between homology and analogy

Homology is different from analogy, which describes the relation between characters that are apparently similar yet phylogenetically independent. The wings of a maple seed and the wings of an albatross are analogous, and perform somewhat similar functions, but not homologous (they both allow the organism to travel on the wind, but they didn't both develop from the same structure). Analogy is commonly also referred to as homoplasy (convergent evolution), which is further distinguished into parallelism, reversal, and convergence.^[18][19]

Homologies across phyla

See also

• Analogy (biology)
• Cladistics

Notes

References

Further reading

Wikimedia Commons has media related to Homology.

• Carroll, Sean B. (2006). Endless Forms Most Beautiful. New York: W.W. Norton & Co. ISBN 0-297-85094-6. 
• Carroll, Sean B. (2006). The making of the fittest: DNA and the ultimate forensic record of evolution. New York: W.W. Norton & Co. ISBN 0-393-06163-9. 
• DePinna, M.C. (1991). "Concepts and tests of homology in the cladistic paradigm". Cladistics. 7 (4): 367–94. doi:10.1111/j.1096-0031.1991.tb00045.x. 
• Dewey, C.N.; Pachter, L. (April 2006). "Evolution at the nucleotide level: the problem of multiple whole-genome alignment". Human Molecular Genetics. 15 (Spec No 1): R51–6. doi:10.1093/hmg/ddl056. PMID 16651369.  CS1 maint: Multiple names: authors list (link)
• Fitch, W.M. (May 2000). "Homology a personal view on some of the problems". Trends in Genetics. 16 (5): 227–31. doi:10.1016/S0168-9525(00)02005-9. PMID 10782117. 
• Gegenbaur, G. (1898). Vergleichende Anatomie der Wirbelthiere... Leipzig. 
• Haeckel, Е. (1866). Generelle Morphologie der Organismen. Bd 1-2. Вerlin. 
• Owen, R. (1847). On the archetype and homologies of the vertebrate skeleton. London. 
• Mindell D.P., Meyer A. (2001). "Homology evolving" (PDF). Trends in Ecology and Evolution. 16 (8): 434–40. doi:10.1016/S0169-5347(01)02206-6. 
• Kuzniar, A.; van Ham, R.C.; Pongor, S.; Leunissen, J.A. (November 2008). "The quest for orthologs: finding the corresponding gene across genomes". Trends Genet. 24 (11): 539–51. doi:10.1016/j.tig.2008.08.009. PMID 18819722.  CS1 maint: Multiple names: authors list (link)

External links

• Brigandt, Ingo (2011) "Essay: Homology." In: The Embryo Project Encyclopedia. ISSN 1940-5030. http://embryo.asu.edu/handle/10776/1754

Fins, limbs and wings
Fins	Aquatic locomotion Cephalopod fin Fish locomotion Fin and flipper locomotion Caudal fin Dorsal fin Fish fin flipper Lobe-finned fish Ray-finned fish Pectoral fins
Limbs	Limb development Limb morphology digitigrade plantigrade unguligrade uniped biped facultative biped triped quadruped Arthropod Cephalopod Tetrapod dactyly Digit
Wings	Flying and gliding animals Bat wing Bird wing keel skeleton feathers Insect wing Pterosaur wing Wingspan
Evolution	Evolution of fish Evolution of tetrapods Evolution of birds Origin of birds Origin of avian flight Evolution of cetaceans Comparative anatomy Convergent evolution Analogous structures Homologous structures
Related	Animal locomotion Gait Robot locomotion Samara Terrestrial locomotion Tradeoffs for locomotion in air and water Rotating locomotion Undulatory locomotion