Molecular homology and DNA hybridization.

We reviewed the concept of homology, which can broadly be defined as a correspondence between characteristics that is caused by continuity of information (Van Valen 1982). The concept applies widely in molecular biology when correspondence is taken to mean a genetic relationship resulting from a unique heritable modification of a feature at some previous point in time. Such correspondence can be established for features within a single organism as well as between organisms, making paralogy a valid form of molecular homology under this definition. Molecular homology can be recognized at a variety of organizational levels, which are interdependent. For example, the recognition of homology at the site level involves a statement of homology at the sequence level, and vice versa. This hierarchy, the potential for nonhomologous identity at the site level, and such processes as sequence transposition combine to yield a molecular equivalent to complex structural homology at the anatomical level. As a result, statements of homology between heritable units can involve a valid sense of percent homology. We analyzed DNA hybridization with respect to the problems of recognizing homology and using it in phylogenetic inference. Under a model requiring continuous divergence among compared sequences, DNA hybridization distances embed evolutionary hierarchy, and groups inferred using pairwise methods of tree reconstruction are based on underlying patterns of apomorphic homology. Thus, symple-siomorphic homology will not confound DNA hybridization phylogenies. However, nonhomologous identities that act like apomorphic homologies can lead to inaccurate reconstructions. The main difference between methods of phylogenetic analysis of DNA sequences is that parsimony methods permit hypotheses of nonhomology, whereas distance methods do not.