Gene flow and melanism in Lake Erie garter snake populations

Abstract

Abstract Melanistic garter snakes (Thamnophis sirtalis) are unusually common near Lake Erie, apparently because selection for thermoregulatory ability in cool lake-shore habitats (which favours melanistic morphs) outweighs selection for crypsis (which favours striped morphs). However, morph frequencies are highly variable among sites, suggesting that random genetic drift also influences colour pattern. In an effort to better understand the evolutionary processes influencing garter snake colour patterns, we estimatedFstand Nm (the number of migrants per generation) among island and mainland populations from patterns of allozymic variation detected using electrophoresis. Estimates of Nm were high, ranging from 2·7 to 37·6 between pairs of study sites and making it unlikely that differences in morph frequencies among sites were solely the result of random genetic drift. Furthermore, differences inFstestimates between colour pattern (a one-locus two-allele trait) and allozyme loci suggest that colour pattern alleles are not in Hardy-Weinberg equilibrium, most likely as a result of natural selection. Comparison of allozymic data from Lake Erie with those from more distant sites suggests that gene flow occurs over long distances inT.sirtalis.