Evolutionary ecology at the extremes of species' ranges

The nature of species at the extremes of their ranges impinges fundamentally on diverse biological issues, including species' range dynamics, population variability, speciation and conservation biology. We review the literature concerning genetic and ecological variation at species' range edges, and discuss historical and contemporary forces that may generate observed trends, as well as their current and future implications. We discuss literature which shows how environmental, ecological and evolutionary factors act to limit species' ranges, and how these factors impose selection for adaptation or dispersal in peripheral populations exposed to extreme and stochastic biotic and abiotic stressors. When conditions are sufficiently harsh such that local extinction is certain, peripheral populations may represent temporary offshoots from stable core populations. However, in cases where peripheral populations persist at the range edge under divergent or extreme conditions, biologically significant differences can arise from historical and contemporary ecological and evolutionary forces. In many such cases reviewed herein, peripheral populations tended to diverge from the species' core, and to display lower genetic diversity or greater stress-adaptation. We conclude that while such populations may be of particular conservation value as significant components of intraspecific biodiversity or sources of evolutionary innovation and persistence during environmental change, small and greatly variable population size, especially combined with low genetic variability, can result in elevated extinction risk in harsh and stochastic peripheral environments. As a result, while peripheral populations should not be dismissed as evolutionary dead-ends destined for local extinction, neither should they be uncritically granted inherently superior significance based only on their peripheral position alone.