Genome-wide evidence for a hybrid origin of modern polar bears

Interspecific hybridization is recognized as a widespread phenomenon but measuring its extent, directionality, and adaptive importance in the evolution of species remain challenging. Polar bears possess unique adaptations to life on the Arctic sea ice, whereas their closest relatives -brown bears - are boreal and subarctic generalists. Despite largely non-overlapping modern distributions, genomic evidence demonstrates ancient admixture between these species. Here, we analyze new genomes from contemporary zones of species overlap as well as a previously sequenced 120,000-year old polar bear subfossil. We use explicit statistical fitting of data to admixture graphs to provide a framework for testing alternative scenarios of population relationships and gene flow directionality. Our analyses favor a single, parsimonious introgression event from relatives of extant Southeast Alaskan coastal brown bears into the ancestor of extant polar bears, which inverts the current paradigm of unidirectional gene flow from polar into brown bear. This conclusion has clear implications for our understanding of the impact of climate change: a specialist Arctic lineage may have been the recipient of generalist, boreal genetic variants at crucial times during critical phases of Northern Hemisphere glacial oscillations.