Considerable genetic diversity and structure despite endemism and limited ecological specialization in the Hayden's ringlet, Coenonympha haydenii

Understanding the processes that underlie the development of population genetic structure is central to the study of evolution. Patterns of genetic structure, in turn, can reveal signatures of local adaptation, barriers to gene flow, or even the genesis of speciation. However, it is unclear whether the processes that dominate the development of genetic structure differ in populations with a severely restricted range relative to widespread species. For example, in narrowly endemic species, is population structure likely to be adaptive in nature (e.g., via niche specialization), or rather the result of genetic drift (e.g., isolation by distance)? In this study, we investigated patterns of genetic diversity and structure in the narrow endemic Haydens ringlet butterfly. Specifically, we asked to what degree genetic structure in the Haydens ringlet can be explained by isolation by distance, barriers to gene flow, and host association. We employed a genotyping-by-sequencing (GBS) approach coupled with host preference assays, Bayesian modeling, and population genomic analyses to answer these questions. Our results suggest that despite their restricted range, levels of genetic diversity in the Haydens ringlet are comparable to those seen in non-endemic butterfly species. Haydens ringlets showed a strong preference for feeding on grasses vs. sedges, but neither host preference nor potential host availability at sampling sites correlated with genetic structure. We conclude that geography, in the form of barriers to migration and simple isolation by distance, were the major drivers of differentiation in this endemic species.