Genomic erosion in Svalbard reindeer. Long-term reduced genetic variation in Svalbard reindeer did not preclude adaptation to its high-arctic island environment

Understanding the evolution of declining and endangered populations is of crucial significance in evolutionary and conservation biology. Since island populations are typically much smaller than those from their mainland range, they are ideal systems to examine threats such as loss of genetic diversity and adaptive potential as well as accumulation of deleterious variation. The Svalbard reindeer (Rangifer tarandus platyrhynchus) is an endemic wild subspecies that colonised the Svalbard archipelago ca. 7,000 years ago and that shows numerous physiological and morphological adaptations to its challenging high-arctic habitat. Here, we leverage a de-novo chromosome-level assembly for R. t. platyrhynchus and analyse 132 reindeer genomes spanning most of its Holarctic range, including 91 genomes from Svalbard to examine the genomic consequences of long-term isolation and small population size in this insular subspecies. Empirical data, demographic reconstructions, and forward simulations show that long-term isolation and an associated high level of inbreeding may have facilitated the reduction of highly deleterious—and to a lesser extent, moderately deleterious—variation. Importantly, our study shows that long-term reduced genetic diversity did not preclude local adaptation to the arctic environment. Thus, even a severely bottlenecked and isolated population can retain evolutionary potential.