Environmental suitability throughout the late quaternary explains population genetic diversity

Genetic variation among populations is reflected in biogeographic patterns for many species, but general rules of spatial genetic variation have not been established. In this paper, we establish a theoretical framework based on projecting environmental Grinellian niches back through time to relate the present geographic distribution of population genetic structure to a given species' historical evolutionary context. Thanks to advances in next-generation sequencing technologies, as well as more accurate climate models and the amassing of information stored in biological collections, it is possible to implement this theoretical framework directly. We develop a case study of the tassel-eared squirrel Sciurus aberti to jointly analyze spatial, environmental, and genetic data to predict the historical endemic area of this species. Our results reveal that in cases of genetic isolation by geographic distance, the prevalence of environmental suitability over time corresponds to the genetic fixation index (F_st) of populations with respect to a source population. Populations closer to the historical endemic area show higher genetic diversity and a lower F_st value. This empirical example relates back to the theoretical framework, allowing two further advances: 1) a layer of biogeographic explanation for the results obtained from population genomic methods; and 2) predictive maps of this genetic structure to support biodiversity conservation efforts. Overall, this work advances a perspective that integrates population genetics with historical patterns of species distribution. The limitations posed in the theoretical framework should be considered before implementing the suitability prevalence area (SPA) in a general way over different taxa. Otherwise, the predictability of the genetic diversity of populations as a product of environmental stability over time may not be adequate.