И насчет гибридизации: да, белые и бурые медведи могут скрещиваться и давать плодовитое потомство. Сначала это наблюдалось в зоопарках, а в 2006 году был найден первый гибридный экземпляр в природе.
Пруфлинк и цитата оттуда:
... Based on the relatively few specimens of polar bears in the fossil history Kurten (1964) estimated that polar bears may have evolved as recently as 70,000 to 100,000 years ago. Recent mtDNA information supported Kurten’s supposition of a relatively late polar bear evolution from within the range of grizzly bear population (Yu, L. et al. 2004)... polar bears are believed to have originated from a group of grizzly bears (Ursus arctos) which became isolated during the glacial periods of the mid-Pleistocene approximately 200,000 - 250,000 years ago (Talbot and Shields 1996b). Age models based on molecular studies of evolutionary relationships among extant species of bears differ considerably as to the divergence time of polar bears from grizzly bears. Wayne et al. (1991) suggested this happened 70,000 – 100,000 yrs ago while Yu et al. (2004) concluded this might have happened 100,000 – 150,000 yrs ago. Only in portions of northern Canada and northern Alaska do the ranges of polar bears and grizzly bears overlap. Cross breeding of grizzly bears and polar bears in captivity has produced reproductively viable offspring (Gray 1972). The first documented case of cross breeding in the wild was reported in the spring of 2006. A sport hunter in the Canadian southern Beaufort Sea region harvested a hybrid and genetic testing by Wildlife Genetics International in May 2006 confirmed breeding between a polar bear female and grizzly bear male had occurred.