Francisco Aboitiz Neuroscience Office 73-346 Center for the Health Sciences University of California Los Angeles California 90024 Ideas in Theoretical Biology "Homology: A Comparative or a Historial Concept?" 37: 27-29, 1988 ABSTRACT The meaning of the word 'homology' has changed. From being a comparative concept in pre-Darwinian times, it became a historical concept, strictly signifying a common evolutionary origin for either anatomical structures or genes. . * * * The distinction between homology and analogy was used by pre-Darwinian biologists as a criterion for animal classification. There were at least two ways of establishing homology (Russell, 1916; Aboitiz, 1987). One definition stated that, in different animals, homologous structures were those who shared a set of topographic relations to other organs. According to a second definition, homology consisted of having a common embryological origin. Although there was some disagreement about what homology was (still other definitions were seldom proposed, and also different kinds of homologies were sometimes suggested), all the proposed criteria were empirically testable, and the concept was very useful for the elucidation of taxonomies (Russell, 1916). With the rise of evolutionary ideas, homologies became 'suggestive' of a common ancestry in different groups (Darwin, 1872). Today, however, the word 'homology' is currently understood as strictly signifying a common evolutionary origin for either anatomical traits or genes or proteins (Gould, 1987, 1988; Reeck et al., 1987). In different species, two structures or genes are considered homologous only if they are derived from the same structure or gene in a (hypothetical) common ancestor. Under this definition, homology is not anymore a criterion for classification; rather, it is established after taxonomies are elucidated. There is no expression for organs sharing a relative position in body (or having the same embryological origin) in different species. If these organs happen to be originated separately in evolution, they will not be homologous anymore. Furthermore, in molecular biology there is no place for a comparative definition of homology. As expressed above, this definition bears relation to the concept of a body plan that determines the relative position of each organ. It makes no sense today to speak either of a 'genetic plan' determining the 'relative positions' of genes, or of a common embryological origin for two genes. A science of morphogenesis as the realization of a body plan is beginning its renaissance (Goodwin, 1982; Thom, 1972). In this approach, evolutionary considerations are considered as secondary. . If this turns out to be the case, the term 'homology' would have to be restricted to the discipline of morphology, and should not be used in molecular biology. REFERENCES Aboitiz, F. (1987). 'Homology' in anatomy and molecular biology. 51: 515-516. Darwin, C. (1872). ., 6th ed. New York: Collier Eds., 1962. Goodwin, B.C. (1982). Development and evolution. 97: 43-55. Gould, S.J. (1987). . Cambridge: Harvard University Press. Gould, S.J. (1988). Conference on molecular data and systematics. Symposium on "The Impact of Molecular Analyses on Our Understanding of Evolution". University of Southern California, March 11. Reeck, G.R. et al. (1987). A terminology muddle and a way out of it. 50: 667. Russell, E.S. (1916).