Geographic ranges are a complex product of both ecological and evolutionary processes, reflecting current biotic and abiotic conditions as well as gene flow, drift, adaptation, and history. A commonly invoked hypothesis for the inhibition of range expansion centers around the idea that gene flow from densely populated range center prevents local adaptation at the range periphery. In this study, we assessed the potential for gene flow to inhibit adaptation to the local environment by using a single species, Plethodon ouachitae, which occupies multiple mountaintops in Oklahoma and Arkansas with population density decreasing with elevation and valleys most likely representing a barrier to contemporary gene flow. We were able to quantify genetic connectivity along elevational transects and between mountains using microsatellite markers. Additionally, we characterized environmental differences between mountains in the niches occupied by our study species using our own field sampling and museum locality records. Finally, we determined whether populations inhabiting different environments were adapted to their local conditions; we measured metabolic rate thermal sensitivity and seasonal acclimation ability for multiple individuals from all populations. These ecophysiological traits have been shown to be important in determining the elevational range limits for mountaintop Plethodon.
Results/Conclusions
Contrary to the hypothesized downslope neutral diffusion of genes, within mountain, on the majority of transects movement appeared to be biased upslope, away from the less populated range edge. Within mountain, we found sampling sites were highly connected supporting a single panmictic population. Between mountains, we found an overall strong signature of genetic structure with populations segregating by mountain, supporting the hypothesis that warmer valleys represent a contemporary barrier to movement. Correlative niche models built on occurrence records for each individual mountain indicate that the abiotic conditions occupied by populations on different mountains are distinct. These same metrics have been used in other studies to indicate niche divergence between species and population level niche adaptation within species. However, we found the neither differentiation in metabolic rate thermal sensitivity nor acclimation ability between populations on different mountains or at different elevations. These findings support that mountaintop Plethodon ouachitae, even in the absence of gene flow, shows evidence for niche conservatism in ecophysiological traits.