The evolutionary consequences of interspecies hybridization: insights from ancient and modern bear genomes

Mayr’s “biological species concept” defines species as groups of interbreeding natural populations that are reproductively isolated from other groups. From a morphological perspective, brown bears and polar bears are considered to be distinct species. However, recent observations of brown/polar bear hybrids in the North American Arctic challenge this. Using genome sequence data extracted from both living and extinct populations of brown and polar bears, we explore the evolutionary history of these two groups of organisms. We show that hybridization between brown bears and polar bears is not limited to the present day, but took place at least twice and possibly more frequently during the last 100,000 years. Focusing on two hybrid populations—living bears from the ABC Islands of southeastern Alaska and an extinct population of bears that lived on present-day Ireland—we explore the demographic signal and genomic consequences of hybridization.