Polar bears can breed with brown bears to produce fertile grizzly–polar bear hybrids. Rather than indicating that they have only recently diverged, new evidence suggests more frequent mating has continued over a longer period of time. Thus the two bears remain genetically similar. However, because neither species can survive long in the other's ecological niche, and because they have different morphology, metabolism, social / feeding behaviors, and other phenotypic characteristics, the two bears are generally classified as separate species.