A CRITIQUE OF HOMOLOGY AS A MORPHOLOGICAL CONCEPT

Two sequences of nucleotides are homologous if they are descended thro ugh a chain of replication from a common precursor molecule. Since org ans are not copies or transcriptions of organs, the concept of morphol ogical homology has no such simple and unambiguous definition. The the oretical vagueness of morphological homology is reflected in its many and inconsistent criteria of identification. Structures may be convent ionally deemed homologous even though they are radically dissimilar in form, relationships, or function, or develop via dissimilar ontogenet ic processes, or originate from nonhomologous embryological precursors . Hypotheses of homology are conventionally rejected when they are con tradicted by known patterns of phylogenetic relationships, even if the structures in question are minutely similar in their form, function, and development. The dependence of interspecific homology on phylogeny is often expressed by saying that two structures are homologous if th ey are inherited from corresponding structures in a common ancestor. H owever, this is a circular definition (what counts as a ''correspondin g'' structure is itself a question of homology), and it falsely assume s that structures can be inherited. At bottom, homology is an essentia list concept; two things are homologous only if they are in some essen tial sense the ''same'' thing and properly called by the same word. Th e concept can be made intelligible in an evolutionary context only by giving it a cladistic interpretation that makes homology judgments dep endent on the outcome of a phylogenetic analysis. It follows that such judgments cannot play a role in evaluating conflicting phylogenetic h ypotheses. (C) 1994 Wiley-Liss, Inc.