Levels of Heterozygosity in the Amphipod Gammarus minus in an Area Affected by Pleistocene Glaciation

Abstract

Populations of aquatic animal species that now occupy areas subjected to Pleistocene glaciation are colonists from southerly or protected refuges. Colonization by successive steps should lead to a reduction of genetic variability in the most distant sites through founder effect. This geographic pattern is evident in populations of the amphopod Gammarus minus along a corridor extending from S-central Pennsylvania [USA] northeastward to very near the position of the Pleistocene ice margins. Three-locus heterozygosity averages 0.37 in 35 populations S of the Juiata River. It decreases NE of the river to near zero in an irregular cline. This decline in heterozygosity in the direction of increasing Pleistocene climatic severity is most consistent with the hypothesis that alleles were lost in successive founding episodes as Gammarus minus established itself in previously vacated northerly habitats.