Edgy population genetics

Background/Question/Methods

Limits on species distributions consistently arise in nature. These manifest at the species level as range limits, and at the assemblage level as biogeographic region boundaries. Distributional limits are thought to arise from reduced availability of suitable habitat. The central-marginal hypothesis posits that, given a gradual decrease in habitat availability, species abundances should be highest at the range core and taper out away from the core until populations can no longer maintain viable sizes—this forms the range edge. Population genetic factors contribute to the reduced viability of marginal populations. Their small size and isolation are associated with stronger genetic drift and reduced gene flow from larger, more genetically diverse core populations. These processes should decrease genetic diversity in marginal populations, which may limit their capacity to adapt to new environments and extend range limits. We might expect these population dynamics to play out on biogeographic scales if we consider biogeographic regions as entities that emerge from an aggregation of many species’ ranges. Here we test the central-marginal hypothesis in mammals across the Americas using a macrogenetic dataset of georeferenced, population-level diversity and differentiation metrics. Then, we test whether central-marginal patterns hold for biogeographic regions using both macrogenetic and demographic data.

Results/Conclusions

Generalized linear mixed models show limited population genetic support for the central-marginal hypothesis across species ranges. Rather than omnidirectional gradients radiating from species’ range centers, we detected directed gradients in genetic diversity with species-specific patterns. The directionality of species level genetic diversity gradients appears to be related to species evolutionary history. For biogeographic regions we detected predicted patterns of decreased effective population sizes and genetic diversity, and increased genetic differentiation nearer to region boundaries. This suggests that central-marginal patterns may only be detectable at biogeographic scales due to the superimposition of complementary species-level population demographic patterns. These results help link microevolutionary processes to macroecological patterns and underscore the importance of population processes for broader biodiversity patterns across spatial scales and levels of biological organization. Disruptions to population demography due to human-caused changes in climate and land use have the potential to substantially rearrange and homogenize historical patterns of biodiversity.