Parallel Evolution in Opisthobranch Gastropods and its Implications for Phylogenetic Methodology

Abstract

-Parallel 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 serious challenge to those systematists who claim that cladogram with minimal number of steps represents 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 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 genealogy based on shared-derived features, which is independent of theoretical basis, represents resurfacing of typological thinking which has little value in explaining or even depicting the patterns of diversity of life. [Parallel evolution; parsimony; methodology; opisthobranch gastropods.] Phylogenetics is the sun around which revolves, carrying with it the whole of comparative biology. This was one of Darwin's most important contributions. Our classifications will come to be, as far as they can be made so, genealogies (Darwin, 1859:486). What may be called classical phylogenetics was founded by Darwin and has continued as part of ever since. However, the sunlight of genealogy has often been obscured by the umbrage of typology, often motivated by efforts to discredit evolutionary biology in general. Repeatedly, it has been necessary to return to fundamentals and devise methodologies adequate to the task. A major figure in such endeavors was Hennig (1950, 1966), whose phylogenetic systematics has been the inspiration for the so-called cladist tradition. The result has been continued polarization among factions; legitimate methodologies, long accepted and used by competent evolutionary biologists, have been treated as if they did not exist. We believe that it is time for these issues to be more fully examined. We contend that many cladists have systematically dismissed some major operational and philosophical difficulties with their methods and ignored some alternative or supplementary techniques which might achieve better results. We find much of what cladists say perfectly acceptable. There is no question that one of the goals of is to infer genealogical relationships. There being just one historical reality, there is one (and only one) true tree for any group of organisms. Employing innovations or apomorphies to infer lineages is, or should be, operationally routine. Such matters as whether to allow paraphyletic taxa are not the bones of contention they might seem, being matters of formal classification, not history proper. The thesis that species and other taxa are inAdividuals (Ghiselin, 1966a, 1969a, 1974a, 1981) has been accepted by many cladists (e.g., Eldredge and Cracraft, 1980;