Homology
A homologous[1] trait is any characteristic which is derived by evolution from a common ancestor. This is contrasted to analogous traits: similarities between organisms that were evolved separately.
The term existed before 1859, but got its modern meaning after Darwin established the idea of common descent.[2]p45 The pre-Darwinian naturalists Cuvier, Geoffroy and Richard Owen, also used the idea.
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[change] Homology vs analogy
According to Russell,[3] we owe to Richard Owen the first clear distinction between homologous and analogous organs. 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 the ear ossicles of mammals. These little bones have, in the course of several hundred million years of evolution, made their way from the gill covers of fish to the rear jaw bones of Synapsids to their present position in the ear of mammals. In the fossil record is evidence of this, and also in embryology.[5] As the embryo develops, the cartilage hardens to form bone. Later in development, tiny bone structures break loose from the jaw and migrate to the inner ear area.[6][7]
This rather extraordinary story was first proposed nearly two hundred years ago by Geoffroy, who looked at fish and tried to discover the homologies of their bones with that of land vertebrates.[8]
[change] Level of analysis
A trait may be both homologous and analogous, depending on the level at which the trait is examined. For example, the wings of birds and bats 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.[9]
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.[9]
[change] Related terms
- Homoplasy: evolved independently, but from the same ancestral structure.[10]
- Plesiomorphy: present in a common ancestor but secondarily lost in some of its descendants.
- Synapomorphy: present in an ancestor and all of its descendants.[9]
- Orthology: genes or sequences of DNA which are similar because they came from a common ancestor.
- Paralogy: when a gene is duplicated to occupy two different places in the same genome.
- Xenology: Homologs resulting from horizontal gene transfer between two organisms.
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.
[change] References
- ↑ Greek ομολογειν = 'to agree'
- ↑ Mayr, Ernst 1982. The growth of biological thought. Harvard.
- ↑ Russell E.S. 1916. Form and function: a contribution to the history of animal morphology. Murray, London. p108
- ↑ Owen, Richard 1843. Lectures on invertebrate animals. London. p374 & 379
- ↑ Palaeos, the gill arches:[1]
- ↑ Meng, Jin. 2003. The journey from jaw to ear. Biologist, 50, 154-158.
- ↑ Goodrich E.S. 1930. Studies on the structure and development of vertebrates. Macmillan, London.
- ↑ Saint-Hilaire, Étienne Geoffroy 1818. Philosopie anatomique. Paris.
- ↑ 9.0 9.1 9.2 Scotland R.W. 2010. "Deep homology: a view from systematics". BioEssays : news and reviews in molecular, cellular and developmental biology 32 (5): NA–ME. [2] [3]
- ↑ Butler A.B. 2009. Homology and homoplasty. In: Squire, Larry R. (ed) Encyclopedia of neuroscience, Academic Press. 1195–1199.
