Relative influences of historical and contemporary forces shaping the distribution of genetic variation in the Atlantic killifish, Fundulus heteroclitus.

A major goal of population genetics research is to identify the relative influences of historical and contemporary processes that serve to structure genetic variation. Most population genetic models assume that populations exist in a state of migration-drift equilibrium. However, in the past this assumption has rarely been verified, and is likely rarely achieved in natural populations. We assessed the equilibrium status at both local and regional scales of the Atlantic killifish, Fundulus heteroclitus. This species is a model organism for the study of adaptive clinal variation, but has also experienced a complicated history of range expansion and secondary contact following allopatric divergence, potentially obscuring the influence of contemporary evolutionary processes. Presumptively neutral genetic markers (microsatellites) demonstrated zones of secondary intergradation among coastal populations centred around northern New Jersey and the Chesapeake Bay region. Analysis of genetic variation indicated isolation by distance among some populations and provided supporting evidence that the Delaware Bay, but not the Chesapeake Bay, has acted as a barrier to dispersal among coastal populations. Bayesian estimates indicated large effective population sizes and low migration rates, and were in good agreement with empirically derived estimates of population and neighbourhood size from mark-recapture studies. These data indicate that populations are not in migration-drift equilibrium at a regional scale, and suggest that contributing factors include large population size combined with relatively low migration rates. These conditions should be considered when interpreting the evolutionary significance of the distribution of genetic variation among F. heteroclitus populations.

Microsatellite analysis of the phylogeography, Pleistocene history and secondary contact hypotheses for the killifish, Fundulus heteroclitus.

The mummichog, Fundulus heteroclitus, exhibits extensive latitudinal clinal variation in a number of physiological and biochemical traits, coupled with phylogeographical patterns at mitochondrial and nuclear DNA loci that suggest a complicated history of spatially variable selection and secondary intergradation. This species continues to serve as a model for understanding local and regional adaptation to variable environments. Resolving the influences of historical processes on the distribution of genetic variation within and among extant populations of F. heteroclitus is crucial to a better understanding of how populations evolve in the context of contemporary environments. In this study, we analysed geographical patterns of genetic variation at eight microsatellite loci among 15 populations of F. heteroclitus distributed throughout the North American range of the species from Nova Scotia to Georgia. Genetic variation in Northern populations was lower than in Southern populations and was strongly correlated with latitude throughout the species range. The most common Northern alleles at all eight loci exhibited concordant latitudinal clinal patterns, and the existence of an abrupt transition zone in allele frequencies between Northern and Southern populations was similar to that observed for mitochondrial DNA and allozyme loci. A significant pattern of isolation by distance was observed both within and between northern and southern regions. This pattern was unexpected, particularly for northern populations, given the recent colonization history of post-Pleistocene habitats, and was inconsistent with either a recent northward population expansion or a geographically restricted northern Pleistocene refugium. The data provided no evidence for recent population bottlenecks, and estimates of historical effective population sizes suggest that post-Pleistocene populations have been large throughout the species distribution. These results suggest that F. heteroclitus was broadly distributed throughout most of its current range during the last glacial event and that the abrupt transition in allele frequencies that separate Northern and Southern populations may reflect regional disequilibrium conditions associated with the post-Pleistocene colonization history of habitats in that region.