RESUMO
Population loss is often a harbinger of species extinction, but few opportunities exist to follow a species' demography and genetics through both time and space while this occurs. Previous research has shown that the northern fur seal (Callorhinus ursinus) was extirpated from most of its range over the past 200-800 years and that some of the extirpated populations had unique life history strategies. In this study, widespread availability of subfossils in the eastern Pacific allowed us to examine temporal changes in spatial genetic structure during massive population range contraction and partial recovery. We sequenced the mitochondrial control region from 40 ancient and 365 modern samples and analyzed them through extensive simulations within a serial Approximate Bayesian Computation framework. These analyses suggest that the species maintained a high abundance, probably in subarctic refugia, that dispersal rates are likely 85% per generation into new breeding colonies, and that population structure was not higher in the past. Despite substantial loss of breeding range, this species' high dispersal rates and refugia appear to have prevented a loss of genetic diversity. High dispersal rates also suggest that previous evidence for divergent life history strategies in ancient populations likely resulted from behavioral plasticity. Our results support the proposal that panmictic, or nearly panmictic, species with large ranges will be more resilient to future disturbance and environmental change. When appropriately verified, evidence of low population structure can be powerful information for conservation decision-making.