Genetic differentiation and estimated levels of gene flow in members of the Gammarus pulex-group (Crustacea, Amphipoda) in western Europe

Genetic differentiation of morphologically variable members of the G. pulex-group in northwestern Europe was investigated by electrophoresis at 20 enzyme loci. Five recently described related forms were examined with reference to the “classical” species G. pulex, G. wautieri and G. fossarum. G. pulex and G. wautieri were shown to be genetically homogeneous and clearly distinct species. In the other species, morphologically similar forms were shown to be genetically distant, and inter-areal intraspecific genetic differentiation may warrant recognition of subspecies or sibling species. In some cases, levels of reproductive isolation of the forms involved could be assessed by cross-breeding experiments. To estimate dispersal capabilities, gene flow levels were indirectly estimated from gene frequency data. Gene flow levels are generally low and fluctuating population sizes may enhance the occurrence of stochastic processes. Dendrograms derived from genetic distances were compared with zoogeographical and paleoclimatological evidence. The distribution areas of the species involved correspond to patterns predicted by these data.