Homology

Abstract

The biological concept of homology in most widespread use today is one based on common ancestry. A number of definitions have been proposed. A commonly quoted definition was one which defined homology as “resemblance due to inheritance from a common ancestry.” Because homologous structures are known which do not resemble one another, a preferable definition is one by Ghiselin which states that “structures or other entities are homologous when they could, in principle, be traced back through a genealogical series to a stipulated common ancestral precursor irrespective of morphological similarity.” Recently, E.O. Wiley has proposed another definition. He suggests that “a character of two or more taxa is homologous if this character is found in the common ancestor of these taxa, or, two characters (or a linear sequence of characters) are homologues if one is directly (or sequentially) derived from the other(s).” It is important to keep in mind that the structures need not resemble each other in order to be considered homologous, although most often this will be the case. Shared common ancestry is the essential feature. This definition of homology is the phylogenetic definition. The idea of homology was introduced to biology by the British comparative anatomist Richard Owen prior to the announcement and general acceptance of the Darwin-Wallace theory of evolution by natural selection. Consequently, Owen’s concept was based on morphological similarity and not on common ancestry. Since modern biology has become centered around the belief that evolution has occurred, the phylogenists have redefined Owen’s original notion. Although a minority of structuralists have advocated a return to Owen’s view, their arguments have not had much impact.