The Michigan Physidae revisited: A population genetic survey

We report an analysis of gene frequencies at 7 polymorphic allozyme-encoding loci in 16 populations of physid snails collected from Michigan, surveyed as a step toward integrating Te's (1978) influential classification of the Physidae with a more comprehensive system based on genetic interrelationships and breeding data. Analysis of a genetic distance matrix revealed three groups - two populations of Aplexa hypnorum together, five populations of Physa acuta together, and nine populations of P. gyrina, P. sayii, and P. parkeri combined. Allozyme divergence among the populations of this last cluster, referred to as the "gyrina group," was comparable to that seen among the five populations of the well-characterized P acuta cluster, which breeding experiments have demonstrated biologically conspecific. These results suggest that Michigan populations assigned to P gyrina, P sayii, and P parkeri may comprise a single biological species, the globose and often shouldered shell morphology of the latter resulting from local and perhaps phenotypically plastic responses to lacustrine environments. The 14 "taxonomic units" from Michigan that Te included in his analysis may represent as few as four biological species. A reduction in nominal higher levels of classification within the Physidae is called for.