Similarity, parsimony and conjectures of homology: The chelonian shoulder girdle revisited

Abstract

AbstractMuch has been written about the definition and recognition of biological homology. Homology is usually defined as similarity inherited from a common ancestor (e.g., papers in Hall, 1994). It is recognised through cladistic analysis: Patterson (1982) and de Pinna (1991) have cogently argued that homology can be equated with synapomorphy (a shared evolutionary novelty uniting a monophyletic group). Such identification involves two stages: first, a possible homology is proposed on the basis of morphological similarity. This similarity might be structural, topological, developmental, or any combination thereof. Next, a cladistic analysis is performed, involving the trait in question and all other informative traits identified. If the trait is congruent with the resultant phylogeny, it is accepted as homologous in all taxa which possess it. If the trait is incongruent with the phylogeny, it is interpreted as homoplasious in certain taxa. This has been termed the test of congruence (Patterson, 1982; de Pinna, 1991).Rieppel (1996) has recently suggested that the test of congruence might be circular, and that as a result certain inferences about the evolution of the chelonian shoulder girdle (Lee, 1996) are poorly substantiated. Here I argue that the test of congruence is not circular, and that the disputed conclusions about the evolution of chelonian shoulder girdle can be defended on the basis of parsimony. More generally, I suggest how considerations of parsimony can and should be used to arbitrate between conflicting conjectures of homology that are both congruent with an accepted phylogeny.