Dispersal, gene flow, and population structure.

The accuracy of gene flow estimates is unknown in most natural populations because direct estimates of dispersal are often not possible. These estimates can be highly imprecise or even biased because population genetic structure reflects more than a simple balance between genetic drift and gene flow. Most of the models used to estimate gene flow also assume very simple patterns of movement. As a result, multiple interpretations of population structure involving contemporary gene flow, departures from equilibrium, and other factors are almost always possible. One way to isolate the relative contribution of gene flow to population genetic differentiation is to utilize comparative methods. Population genetic statistics such as FST, heterozygosity and Nei's D can be compared between species with differing dispersal abilities if these species are otherwise phylogenetically, geographically and demographically comparable. Accordingly, the available literature was searched for all groups that meet these criteria to determine whether broad conclusions regarding the relationships between dispersal, population genetic structure, and gene flow estimates are possible. Allozyme and mtDNA data were summarized for 27 animal groups in which dispersal differences can be characterized. In total, genetic data were obtained for 333 species of vertebrates and invertebrates from terrestrial, freshwater and marine habitats. Across these groups, dispersal ability was consistently related to population structure, with a mean rank correlation of -0.72 between ranked dispersal ability and FST. Gene flow estimates derived from private alleles were also correlated with dispersal ability, but were less widely available. Direct-count heterozygosity and average values of Nei's D showed moderate degrees of correlation with dispersal ability. Thus, despite regional, taxonomic and methodological differences among the groups of species surveyed, available data demonstrate that dispersal makes a measurable contribution to population genetic differentiation in the majority of animal species in nature, and that gene flow estimates are rarely so overwhelmed by population history, departures from equilibrium, or other microevolutionary forces as to be uninformative.