Parallel Evolution in Opisthobranch Gastropods and its Implications for Phylogenetic Methodology

Abstract

Parallel evolution, as exemplified by data on opisthobranch mollusks (a group in which snails have repeatedly evolved into slugs), often exceeds divergent evolution. A similar level of parallelism has been described in an array of other organisms. This fact presents a serious challenge to those systematists who claim that a cladogram with a minimal number of steps represents a most parsimonious and most likely scenario of genealogical relationship. When rampant parallelism is present, pattern cladism or other congruence networks which rely solely on ingroup comparison of characters are unsuccessful in resolving the problem. We suggest that emphasis on unique innovation and divergent apomorphy presents a fruitful and scientifically consistent alternative to this dilemma. We contend that cladists have drastically altered the meaning of several terms—including “parallel evolution,” “parsimony,” and “a priori”—to suit their own methodological and philosophical paradigm. These modifications reduce the testability of their hypotheses and, therefore, the scientific credibility of their methods. The construction of a genealogy based on shared-derived features, which is independent of a theoretical basis, represents a resurfacing of typological thinking which has little value in explaining or even depicting the patterns of diversity of life.