Concepts of Homology and Analogy

Abstract

In a discussion springing from Boyden''s recommendations that biologists return to Owen''s pre-evolutionary concepts of analogy and homology, Hubbs points out that the homology concept has itself evolved from static to dynamic implications during the past century. Homology has come to signify an agreement in evolutionary derivation, a dogma which has perhaps been pushed too far in critical interpretations of evolution. Unfortunately, the homology concept has been too little applied to function, although homologous function offers as dependable evidence of common origin as does homologous structure. Neither function nor structure is less subject to convergent evolution. In the best modern usage, homology indicates genetic agreement of structure and function in both phylog-eny and ontogeny. Since serial homologs, structures in one organism, are often more alike than true homologs, structures in different organisms, the use of another term for this sort of homology is ill-advised. Homology concepts would also be of value in small taxonomic groups; there is no need for restricting the use of the terms to the larger divisions. Although complexities arise in the use of any homology concept, all structures having an agreement in derivation should be included, to the extent indicated by the comparison being made. E.g., bird and bat wings are homologous as fore-limbs, but not as wings. Regeneration studies show such phenomena as the replacement of an excised crustacean eye by an antenna. Since the antenna is not a de novo feature, but due simply to a shift in developmental field, the new organ should be regarded as homologous to those in the typical position. "Independent homology" is used by Hubbs to designate similar structures arising independently, perhaps through identical mutations. It is important to recognize both such gradations between homology and analogy and the difficulty of always distinguishing poly- from monophyletic origin. "Analogy" should indicate superficial similarity of structures or functions which are not monophyletic.