Forelimbs in mammals provide one example of homology.

A homologous^{[Etymology 1]} trait is any characteristic of organisms that is derived from a common ancestor. This is contrasted to analogous traits: similarities between organisms that were not in the last common ancestor of the taxa being considered but rather evolved separately. As defined by Owen (1843), a homology is a "structural correspondence", whereas an analogy is a "non-correspondent similarity".

Whether or not a trait is homologous depends on both the taxonomic and anatomical level at which the trait is examined. For example, the bird and bat wing are homologous as forearms in tetrapods. However, they are not homologous as wings, because the organ served as a forearm (not a wing) in the last common ancestor of tetrapods.^[1] By definition, any homologous trait defines a clade'a monophyletic taxon in which all the members have the trait (or have lost it secondarily); and all non-members lack it.^[1]

A homologous trait may be homoplasious ' that is, it has evolved independently, but from the same ancestral structure ' plesiomorphic ' that is, present in a common ancestor but secondarily lost in some of its descendants ' or (syn)apomorphic ' present in an ancestor and all of its descendants.^[1]

A homologous trait is often called a homolog (also spelled homologue). In genetics, the term "homolog" is used both to refer to a homologous protein, and to the gene (DNA sequence) encoding it.

[edit] Homology of structures

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

Shared ancestry can be evolutionary or developmental. 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.

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 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, which is further distinguished into parallelism, reversal, and convergence.^[2]

From the point of view of evolutionary developmental biology (evo-devo) where evolution is seen as the evolution of the development of organisms, Rolf Sattler emphasized that homology can also be partial. New structures can evolve through the combination of 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 they combine some traits of leaves and shoots.^[3][4]

[edit] Homology of sequences in genetics

Homology among proteins and DNA is often concluded on the basis of sequence similarity, especially in bioinformatics. For example, in general, if two or more genes have highly similar DNA sequences, it is likely that they are homologous. But sequence similarity may also arise without common ancestry: short sequences may be similar by chance, and sequences may be similar because both were selected to bind to a particular protein, such as a transcription factor. Such sequences are similar but not homologous. Sequence regions that are homologous are also called conserved. This is not to be confused with conservation in amino acid sequences in which the amino acid at a specific position has been substituted with a different one with functionally equivalent physicochemical properties.

A sequence alignment of two proteins, produced by ClustalW

The phrase "percent homology" is sometimes used but is incorrect. "Percent identity" or "percent similarity" should be used to quantify the similarity between the biomolecule sequences. For two naturally occurring sequences, percent identity is a factual measurement, whereas homology is a hypothesis supported by evidence. One can, however, refer to partial homology where a fraction of the sequences compared (are presumed to) share descent, while the rest does not. For example, partial homology may result from a gene fusion event.

Many algorithms exist to cluster protein sequences into sequence families, which are sets of mutually homologous sequences. (See sequence clustering and sequence alignment.) Some specialized biological databases collect homologous sequences in animal genomes: HOVERGEN,^[5] HOMOLENS,^[6] HOGENOM.^[7]

Homologous sequences are of two types: orthologous and paralogous.^[8]

[edit] Orthology

Homologous sequences are orthologous if they were separated by a speciation event: when a species diverges into two separate species, the divergent copies of a single gene in the resulting species are said to be orthologous. Orthologs, or orthologous genes, are genes in different species that are similar to each other because they originated from a common ancestor. The term "ortholog" was coined in 1970 by Walter Fitch.

The strongest evidence that two similar genes are orthologous is the result of a phylogenetic analysis of the gene lineage. Genes that are found within one clade are orthologs, descended from a common ancestor. Orthologs often, but not always, have the same function.

Orthologous sequences provide useful information in taxonomic classification and phylogenetic studies of organisms. The pattern of genetic divergence can be used to trace the relatedness of organisms. Two organisms that are very closely related are likely to display very similar DNA sequences between two orthologs. Conversely, an organism that is further removed evolutionarily from another organism is likely to display a greater divergence in the sequence of the orthologs being studied.

Some other specialized biological databases provide tools to identify and collect orthologous sequences: OrthoMCL^[9] for eukaryotes, OrthoMaM^[10] for mammals, OrthologID^[11] and GreenPhylDB^[12] for plants.

[edit] Paralogy

Homologous sequences are paralogous if they were separated by a gene duplication event: if a gene in an organism is duplicated to occupy two different positions in the same genome, then the two copies are paralogous.

A set of sequences that are paralogous are called paralogs of each other. Paralogs typically have the same or similar function, but sometimes do not: due to lack of the original selective pressure upon one copy of the duplicated gene, this copy is free to mutate and acquire new functions.

Paralogous sequences provide useful insight to the way genomes evolve. The genes encoding myoglobin and hemoglobin are considered to be ancient paralogs. Similarly, the four known classes of hemoglobins (hemoglobin A, hemoglobin A2, hemoglobin B, and hemoglobin F) are paralogs of each other. While each of these genes serves the same basic function of oxygen transport, they have already diverged slightly in function: fetal hemoglobin (hemoglobin F) has a higher affinity for oxygen than adult hemoglobin. Function is not always conserved, however. Human angiogenin diverged from ribonuclease, for example, and while the two paralogs remain similar in tertiary structure, their functions within the cell are now quite different.

Paralogous genes often belong to the same species, but this is not necessary: for example, the hemoglobin gene of humans and the myoglobin gene of chimpanzees are paralogs. This is a common problem in bioinformatics: when genomes of different species have been sequenced and homologous genes have been found, one can not immediately conclude that these genes have the same or similar function, as they could be paralogs whose function has diverged.

[edit] Ohnology

Ohnologous genes are paralogous genes that have originated by a process of whole-genome duplication (WGD). The name was first given in honour of Susumu Ohno by Ken Wolfe.^[13] Ohnologs are interesting for evolutionary analysis because they all have been diverging for the same length of time since their common origin.

[edit] Xenology

Homologs resulting from horizontal gene transfer between two organisms are termed xenologs. Xenologs can have different functions, if the new environment is vastly different for the horizontally moving gene. In general, though, xenologs typically have similar function in both organisms.^[14]

[edit] Gametology

Gametology denotes the relationship between homologous genes on nonrecombining, opposite sex chromosomes. Gametologs result from the origination of genetic sex determination and barriers to recombination between sex chromosomes. Examples of gametologs include CHDW and CHDZ in birds.

[edit] See also

• Protein homology
• Cladistics
• List of Homologues of the Reproductive System
• Deep homology

[edit] Notes

[edit] References

[edit] Further reading

• 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 MC (1991). "Concepts and tests of homology in the cladistic paradigm". Cladistics 7: 367'94. doi:10.1111/j.1096-0031.1991.tb00045.x. 
• Dewey CN, Pachter L (April 2006). "Evolution at the nucleotide level: the problem of multiple whole-genome alignment". Hum. Mol. Genet. 15 (Spec No 1): R51'6. doi:10.1093/hmg/ddl056. PMID 16651369. http://hmg.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=16651369. 
• Fitch WM (May 2000). "Homology a personal view on some of the problems". Trends Genet. 16 (5): 227'31. doi:10.1016/S0168-9525(00)02005-9. PMID 10782117. http://linkinghub.elsevier.com/retrieve/pii/S0168-9525(00)02005-9. 
• 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 DP, Meyer A (2001). "Homology evolving". Trends in Ecology and Evolution 16: 434'40. doi:10.1016/S0169-5347(01)02206-6. 
• Kuzniar A, van Ham RC, Pongor S, Leunissen JA (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. http://linkinghub.elsevier.com/retrieve/pii/S0168-9525(08)00227-8. 

[edit] External links

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