Genetic divergence among populations of pleurocerid snails inhabiting rivers of the southern Appalachians: Evidence of a two-stage process

Exceptionally high levels of genetic divergence have recently been documented within and among populations of pleurocerid snails inhabiting streams of the Older Appalachian physiographic provinces, apparently accompanied by little morphological differentiation.  Here I report the opposite phenomenon in pleurocerid populations inhabiting streams of the Younger Appalachian provinces in southwestern Virginia, east Tennessee, and north Georgia.  Populations of Goniobasis simplex, G. carinifera, G. acutocarinata, G. clavaeformis, Pleurocera unciale and P. vestita were sampled from the Powell, Little, and Hiwassee subdrainages of the Tennessee River and the Coahulla subdrainage of the Alabama River.  Gene frequencies at 10 polymorphic allozyme-encoding loci (15 populations, 30 individuals per population) confirmed that G. simplex populations are distinct, but suggested that pleurocerids heretofore identified as G. carinifera, G. acutocarinata, G. claveformis, P. unciale and P. vestita may be conspecific, even across nominal genera.  Taxonomists seem to have been misled by plasticity in shell morphology, populations of the carinifera group in all subdrainages bearing narrow and carinate shells in the headwaters and broad, heavy shells in the main rivers.  Such high levels of morphological variation, unaccompanied by commensurate levels of genetic divergence, suggest that pleurocerid populations may have colonized streams of the Ridge and Valley province much later than the streams of the Piedmont and Blue Ridge provinces immediately to their east.