Patterns of Electrophoretic and Morphologic Variability in a Widely Distributed Unionid: an Initial Survey

Results from an initial survey of electrophoretic and morphologic variability expressed by a highly polytypic and widely distributed unionid species, Elliptio complanata, indicate that two distict races exist on the northern Atlantic Slope. The Delmarva Peninsula is a present site of introgressive hybridization between these races, and various patterns of electrophoretic and morphologic intermediacy are observed. It is proposed that this hybrid zone constitutes one of a series of contacts between geographic races of E. complanata presently distributed as a "Rassenkreis" around the Appalachian mountains. Such races were probably formed as a result of alternate phases of range expansion and contraction during Pleistocene glacial episodes, and patterns of differentiation now present among these races are consistent with initial stages of speciation as predicted by a reticulate model. Continued study of this species is likely to provide insight into patterns of transspecific change in this group of organisms.