The homology theorem in ultrastructural research

Abstract

The task of identifying characters that have relevance to systematics and phylogeny relies on the methodology of the homology theorem. The systematist will classify organisms according to similarities that can be recognized in these organisms, and the central problem is to distinguish homologs (similarities whose causal explanation is common ancestry) from analogs (similarities independent of ancestry). This problem of distinction is especially acute for systematists dealing with ultrastructural characters. In order to identify homologs, the systematist should apply a series of criteria, each of which provides an estimate of probability of homology. We propose three criteria for homology here, developed from criteria proposed by Remane (1952). As a countercheck, the probability of analogy should be weighed against that of homology. We propose five criteria that are designed to reveal analogy. No single criterion, either homology or analogy criterion is sufficient in itself to establish homology or non-homology. The proper methodology for applying the homology theorem is first to estimate probability of homology through consideration of homology criteria as a unit, and secondly to estimate probability of non-homology through consideration of analogy criteria as a unit. The applicability of these criteria to ultrastructural characters is illustrated with examples from ultrastructural studies, especially of adhesive systems in lower metazoans. © 1979 by the American Society of Zoologists.