How adaptive evolution could enable habitat transitions of alpine ground beetles (Carabidae: Nebria) during glacial-interglacial climate cycles

Oscillating glacial climates of the Plio-Pleistocene have influenced speciation in alpine environments, as cycles of isolation during interglacial periods and mixing among glacial refugia have led to the formation of new species. Less understood, however, is the role that adaptive evolution could play during habitat transitions, as some species move from lowland glacial refugia to higher elevation habitats. The carabid genus Nebria includes nearly 500 species that specialize in cold climates among montane and high-latitude regions across the Northern Hemisphere. This incredible species richness is not evenly distributed, and Mount Rainier, WA is a particular hotspot of diversity, where 12 Nebria species are distributed on and surrounding this individual mountain. These 12 species exhibit elevational zonation, with distinct high-elevation and low-elevation specialists. Genetic comparisons of high and low-elevation species have led to the identification of putative adaptive mechanisms that could enable transitions to challenging high-elevation environments. Some of these selective signals were found in genes related to hypoxia response, and our work highlights the potential for hypobaric hypoxia to restrict insect range expansion. These findings could be especially relevant in advancing understanding of how genetic variation could enable, or restrict, movement during climate change, as there is a general pattern among cold-specialized organisms in mountains to move up in elevation in response to warming.