A new genetic study of polar bears and brown bears led by researchers at the University of California, Santa Cruz has overturned prevailing ideas about the evolutionary history of the two species. Brown bears and polar bears are closely related and known to produce fertile ursid hybrids. Previous studies suggested that past hybridization had resulted in all polar bears having genes that came from brown bears. But new research indicates that episodes of gene flow between the two species occurred only in isolated populations and did not affect the larger polar bear population, which remains free of brown bear genes.