Homology and Phylogeny: Morphology and Systematics

Abstract

A definition of homology, similarity in structure in a group of organisms that delimits that group alone and that was found in their common ancestor, is discussed. This definition clarifies the level of universality at which structures are homologous and various kinds of similarity relationships between structures. Similarities are 1:1 correspondences in the structures being compared; similar structures that appear to be derived after using outgroup analysis are potentially homologous. Final hypotheses of homology (synapomorphies) are made after the evaluation of all potentially homologous characters in the formation of an hypothesized phylogeny. Similarity refers simply to some commonality in the structures being compared, not to structures in their entirety; failure to make this distinction clear is responsible for much of the confusion over the term "homology." The emphasis on the necessity of 1:1 correspondence highlights the danger of constructing morphoclines via chains of similarity relationships, the two ends of which show no commonality. Kinds of evidence used in establishing similarity are discussed, with emphasis on the use, as well as the limitations, of developmental evidence. Such evidence is particularly valuable in establishing transformational relationships between homologies characterizing different taxonomic levels. Similarly, both the significance of terata and the nature of the serially repeating construction of plants can be understood best by emphasizing levels of developmental and evolutionary complexity; results of confusing the level of analysis are shown. Reification of an unsuitable terminology is shown to affect initial assessments of similarity and hence final hypotheses of homology. Any connection between the definition of homology proposed and essentialistic typology is dismissed. Finally, notions that the concept of homology is necessarily circular and untestable are discussed. It is suggested that the connection between assessments of similarity and aspects of the evolutionary process is a necessary one in any attempt to uncover groups that does not involve direct observation of their formation. In the general discussion it is emphasized that the phylogenetic framework in which the definition of homology is placed specifies levels between and within which comparisons can most usefully be made, so enabling problems of interest to both systematists and morphologists to be tackled. The mechanisms by which changes occur in an organism during evolution are, however, not necessarily clarified by the phylogenetic or, indeed, any approach. It is suggested that the common meeting place of the systematist and morphologist is the analysis of similarity and its conversion into hypotheses of homology.