Diagram of the skulls of aMonitor lizard and aCrocodile: homologous bones have the same colours.
Ahomologous[1]trait is any characteristic which is derived byevolution from a commonancestor. This is contrasted toanalogous traits: similarities between organisms that wereevolved separately.
A homologous trait is often called ahomologue (also spelledhomolog). In genetics, the term "homologue" is used both to refer to a homologous protein, and to the gene (DNA sequence) encoding it.
According to Russell,[3] we owe to Richard Owen the first clear distinction between homologous and analogousorgans. Owen's definitions were:
Analogue: a part or organ in one animal which has the same function as another part or organ in a different animal.
Homologue: the same organ in different animals under every variety of form and function.[4]
The distinction is made clear by examples such as theear ossicles of mammals. These little bones have, in the course of several hundred million years ofevolution, made their way from thegill covers of fish to the rearjaw bones ofSynapsids to their present position in the ear of mammals. In the fossil record is evidence of this, and also in embryology.[5] As theembryo develops, thecartilage hardens to form bone. Later in development, tiny bone structures break loose from the jaw and migrate to the inner ear area.[6][7] The ear ossicles are homologous with the jaw bones and the gill covers, but not analogous.
This rather extraordinary story was first proposed in 1818 byÉtienne Geoffroy Saint-Hilaire, who looked at fish and tried to discover the homologies of their bones with that of land vertebrates.[8]
The wings ofpterosaurs (1),bats (2) andbirds (3) are analogous as wings, but homologous as forearms.
A trait may be both homologous and analogous, depending on the level at which the trait is examined. For example, the wings of birds andbats are homologous asforearms intetrapods. However, they are not homologous aswings, because the organ served as a forearm (not a wing) in the last common ancestor oftetrapods.[9]
By definition, any homologous trait defines aclade–amonophyletictaxon in which all the members have the trait (or have lost it secondarily); and all non-members lack it.[9]
Conserved sequences of DNA, RNA and proteins can be used to decide homologies between organisms.
Orthology:genes or sequences ofDNA which are similar because they came from a common ancestor. They were originally separated by aspeciation event.Orthologs (orthologous genes) are genes in different species which originated by vertical descent from a single gene of the last common ancestor. The term "ortholog" was coined in 1970 by Walter Fitch.[11]
Paralogy: when agene is duplicated to occupy two different places in the same genome, the two copies are paralogous. Paralogous genes often belong to the same species, but this is not necessary: for example, thehaemoglobin gene of humans and themyoglobin gene ofchimpanzees are paralogs. Paralogs typically have the same or similar function, but sometimes do not. At least one of the copies will be under less selection pressure, and may mutate and acquire a new function.
Xenology: Homologs resulting fromhorizontal gene transfer between two organisms. 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.[12]
Analgorithm identifies deeply homologous genetic modules in unicellular organisms, plants, and non-human animals based onphenotypes (such as traits and developmental defects). The technique aligns phenotypes across organisms based on the homology of genes involved in the phenotypes.[14][15]
↑9.09.19.2Scotland R.W. 2010. "Deep homology: a view from systematics".BioEssays : news and reviews in molecular, cellular and developmental biology32 (5): NA–ME.[1][2]
↑Butler A.B. 2009.Homology and homoplasty. In: Squire, Larry R. (ed)Encyclopedia of neuroscience, Academic Press. 1195–1199.
