THE CORRELATES OF DIVERGENCE IN ISOLATED POPULATIONS OF THE FRESHWATER SNAIL, GONIOBASIS PROXIMA (SAY)

Goniobasis proxima is a prosobranch snail restricted to small piedmont and mountain streams in the Blue Ridge Province south of Roanoke, Virginia. Populations of the snail are isolated both by large rivers and intervening mountain ridges. The purpose of this study was to use correlation to indirectly estimate the relative importance of natural selection and gene flow restriction in causing divergence between these populations. I selected 25 populations from a 15,000 km(\u272) area in Virginia and North Carolina. Population divergence was measured in three ways: an analysis of gene frequencies at 7 polymorphic enzyme loci using gel electrophoresis, and two multivariate analyses of 23 morphological measurements screened for genetic component. Intensity of selection for divergence was estimated using three measures of environmental difference. Two of these measures were based on 11 water chemical variables measured seasonally plus 3 physical variables and parasitism. The third environmental measurement was based on the similarity of the diatom floras at the 25 sites as determined from the guts of the snails themselves. Gene flow restriction was estimated using geographic distances through water. As a preliminary study, karyotypes and allele frequencies at 15 enzyme loci were examined in three Virginia Goniobasis and compared to Florida Goniobasis. In a single G. proxima population, allele frequencies were found to vary significantly between nine samples, both temporally and spatially over distances as small as 0.5 km. Transplant studies were initiated. Overall results suggested that there has been little gene flow between even closely neighboring populations since the Pleistocene at the latest. However, there is evidence that at one time, perhaps during the Tertiary, rates of gene flow were much higher. No significant correlation was found between geographic distance and any measure of population divergence over short distances. Nor was correlation found between overall protein divergence and any of the three overall measures of environmental difference. However, population divergence in morphology was found significantly correlated with water chemical difference among localities. This suggests that variation in local selection is more important than variation in gene flow for determining amount of divergence between populations of G. proxima