Plasticity and constraints in development and evolution.

Morphological similarities between organisms may be due to either homology or homoplasy. Homologous structures arise by common descent from an ancestral form, whereas homoplasious structures are independently derived in the respective lineages. The finding that similar ontogenetic mechanisms underlie the production of the similar structures in both lineages is not sufficient evidence of homology, as such similarities may also be due to parallel evolution. Parallelisms are a class of homoplasy in which the two lineages have come up with the same solution independently using the same ontogenetic mechanism. The other main class of homoplasy, convergence, is superficial similarity in morphological structures in which the underlying ontogenetic mechanisms are distinct. I argue that instances of convergence and parallelism are more common than is generally realized. Convergence suggests flexibility in underlying ontogenetic mechanisms and may be indicative of developmental processes subject to phenotypic plasticity. Parallelisms, on the other hand, may characterize developmental processes subject to constraints. Distinguishing between homology, parallelisms and convergence may clarify broader taxonomic patterns in morphological evolution.