SPECIES-WIDE POPULATION STRUCTURE IN A SOUTHEASTERN U.S. FRESHWATER FISH,HETERANDRIA FORMOSA: GENE FLOW AND BIOGEOGRAPHY

The phylogeography of the freshwater fish fauna of the southeastern United States has almost achieved paradigm status in evolutionary biology (Avise 1992), and the major geographic features responsible for shaping species distributions are well-characterized. Nevertheless, variation among species in distributions of allele or haplotype frequencies suggests that species-specific processes (e.g., migration) may also play a role in establishing those distributions. There has also been relatively little investigation into how population structure may differ among subregions in the Southeast, for example, on the Florida peninsula versus the U.S. mainland to the northwest and/or northeast. The geology of the peninsula is such that both physical and biotic fluctuations may have been (and still be) particularly important in establishing the population structure of freshwater taxa. This possibility leads to two interesting questions in population genetics. (1) Does gene flow in freshwater species of the region better approximate a one- or two-dimensional pattern? (2) Are populations on the peninsula farther from migration-genetic drift equilibrium than their counterparts on the mainland? These questions were addressed by examining the population strucuture of a livebearing fish, Heterandria formosa; several features of the biology of the species make it particularly likely that recent gene flow has been important in its evolution. I surveyed electrophoretic variation in 34 populations distributed throughout the species range. The phylogeographic patterns observed are in general concordance with those found in other species, although with some differences. A two-dimensional hypothesis of gene flow on the Florida peninsula better explains the data than does a one-dimensional one. There is no evidence that populations on the peninsula are farther from migration-drift equilibrium than those to the northwest. Populations in the northeast have lower genetic diversity than those to the south and west and show no isolation by distance; those results are consistent with a recent range expansion into the northeast, although smaller historical effective population sizes could also explain the pattern.