Environmental stress responses and adaptive evolution in the alpine ground beetle Nebria vandykei

The oscillating glacial-interglacial climate has had well-characterized effects on alpine species, driving rapid distributional and demographic shifts that have led to lineage diversification. It is unclear whether adaptive evolution has occurred during these rapid demographic changes, because strong genetic drift can overcome the force of selection. Here, using the alpine ground beetle Nebria vandykei, we test for evidence of adaptive evolution. Initially, we explore the genetic pathways induced during environmental stress responses through RNA sequencing, showing that cold, heat and desiccation stress activate a largely non-overlapping set of molecular pathways. Using additional transcriptome sequencing, we estimate the evolutionary relationship of N. vandykei to related species in the subgenus Catonebria and several outgroups. Phylogenetic analyses suggest that a history of admixture or very rapid diversification underlies the evolution of N. vandykei. Finally, using tests for selection polarized by high- and low-elevation relatives, we demonstrate selection acting on stress response pathways and on pathways known to function in tolerance to cold and hypoxic environments. These results support the role of environmental adaptation in alpine species despite rapid demographic change, while demonstrating that admixture might play a key role in facilitating adaptive diversification of alpine species.