A critique of homology as a morphological concept.
Two sequences of nucleotides are homologous if they are descended through a chain of replication from a common precursor molecule. Since organs are not copies or transcriptions of organs, the concept of morphological homology has no such simple and unambiguous definition. The theoretical vagueness of morphological homology is reflected in its many and inconsistent criteria of identification. Structures may be conventionally deemed homologous even though they are radically dissimilar in form, relationships, or function, or develop via dissimilar ontogenetic processes, or originate from nonhomologous embryological precursors. Hypothesis of homology are conventionally rejected when they are contradicted 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 they are inherited from corresponding structures in a common ancestor. However, this is a circular definition (what counts as a "corresponding" structure is itself a question of homology), and it falsely assumes that structures can be inherited. At bottom, homology is an essentialist concept; two things are homologous only if they are in some essential sense the "same" thing and properly called by the same word. The concept can be made intelligible in an evolutionary context only by giving it a cladistic interpretation that makes homology judgments dependent on the outcome of a phylogenetic analysis. It follows that such judgments cannot play a role in evaluating conflicting phylogenetic hypotheses.
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