THE INFLUENCE OF DISPERSAL AND DIET BREADTH ON PATTERNS OF GENETIC ISOLATION BY DISTANCE IN PHYTOPHAGOUS INSECTS

To determine the effects of dispersal ability and diet breadth on popu lation-genetic structure, we reviewed the allozyme literature and esti mated genetic isolation by distance (IBD) for 43 species/host races of phytophagous insects. Subsequently, we tested two opposing hypotheses regarding the influence of dispersal ability on IBD: that IBD slopes do not vary with mobility, but that intercepts increase with mobility, and, alternatively, that IBD slopes vary with dispersal ability. We f ound that from tens of kilometers to more than 1,000 km, IBD is weak i n sedentary and highly mobile species but pronounced in moderately mob ile species. We attribute the weak IBD in strong dispersers to the hom ogenizing effects of gene flow, whereas in sedentary species, limited gene flow allows nearly all populations to diverge. In intermediate di spersers, genetic homogeneity is achieved at small spatial scales, but limited dispersal promotes genetic divergence over long distances. We also tested the hypothesis that IBD increases with decreasing diet br eadth. We discovered no such pattern, casting doubt on the supposition that specialization promotes speciation by influencing population-gen etic subdivision. Finally, we found that the number of populations is a more important consideration than the number of polymorphic loci in studies of IBD.