Geographic Distance, Environmental Difference, and Divergence Between Isolated Populations

Evolutionary theory predicts that population divergence should be correlated with both geographic distance and environmental difference. This prediction was tested using 25 populations of Goniobasis proximo (Say), a snail restricted to small creeks in the piedmont and mountains of the southern Appalachians. Geographic distance was measured both over land and through water. Genetic difference was measured in three ways: an analysis of allozyme frequencies at seven polymorphic enzyme loci using gel electrophoresis, and two measures based on different variance components from 23 specially-screened morphological variables. Three measures of environmental difference were also calculated: one based on 11 water chemical variables; a second based on standardized water chemical variables plus four physical and biological variables; and a third based on diatom floral similarity. The correlations between all pairs of the resulting eight symmetric matrices were examined by calculating Kc statistics and tested using permutation. Population divergence was found highly correlated with geographic distance by all measures. Morphological divergence was also correlated with environmental difference. Isolation may, however, have precluded a correlation between divergence in allozyme frequencies and environmental difference. Thus, allozyme divergence among isolated populations seems more a function of gene flow restriction or time since isolation than selection.