Polar bears’ ancestor was an Irish brown bear

INTERBREEDING: Historically, polar bears interbred with brown bears

Female brown bear is ancestor of all living polar bears

July 2011: The female ancestor of all living polar bears was a brown bear that lived near present-day Britain and Ireland just prior to the peak of the last ice age - 20,000 to 50,000 years ago, according to an international team of scientists.

Beth Shapiro, one of the team's leaders, explained that climate changes affecting the North Atlantic ice sheet probably gave rise to periodic overlaps in bear habitats.

These overlaps then led to interbreeding - which is what led to maternal DNA from brown bears being introduced into polar bears.

Vastly different species - but interbreeding possible
The research, which is led by Shapiro and Daniel Bradley is expected to help guide future conservation efforts for polar bears, which are listed as threatened under the Endangered Species Act.

Polar and brown bears are vastly different species in terms of body size, skin and coat color, fur type, tooth structure, and many other physical features. Behaviorally, they are also quite distinct: Polar bears are expert swimmers that have adapted to a highly specialised, Arctic lifestyle, while brown bears - a species that includes Grizzlies and Kodiaks - are climbers that prefer the mountain forests, wilderness regions, and river valleys of Europe, Asia, and North America.

‘Despite these differences, we know that the two species have interbred opportunistically and probably on many occasions during the last 100,000 years,' Shapiro said.

Because of this interbreeding, the modern polar bear's mitochondrial DNA probably underwent fixation - a drastic reduction in genetic variation and a transition to a state in which the entire gene pool includes only one form of a particular gene.

Environmental changes brings bears together
After performing genetic analyses of 242 brown-bear and polar-bear mitochondrial lineages sampled throughout the last 120,000 years and across multiple geographic ranges, Shapiro's team found that the fixation of the mitochondrial genome probably happened during or just before the peak of the last ice age, as early as 50,000 years ago, near present-day Ireland.

Shapiro noted that the specific population of brown bears that shared its maternal DNA with polar bears has been extinct for 9,000 years. However, her data offer clear genetic evidence that the two species were in contact long before the brown bear's disappearance from the British Isles.

'Shapiro explained that, although both polar bears and brown bears have experienced long periods of geographic stability, episodes of both warming and cooling during the last 500,000 years or more likely led to environmental conditions that encouraged interbreeding as geographic ranges of polar and brown bears to overlap.

‘There seems to be little barrier to their mating'
‘Polar and brown bears likely came into contact intermittently, in particular in coastal regions where the effects of climate change may have been more pronounced,' Shapiro said.

‘Whenever they come into contact, there seems to be little barrier to their mating.'

During a warming period, elevated air temperatures, melting glacial ice, and rising sea levels probably forced polar bears to spend more time onshore in search of food, and thus closer to their brown-bear neighbours.

Likewise, during a cooling glacial period, brown bears living farther from the coast may have been forced into habitat normally occupied by polar bears. The British-Irish Ice Sheet reached its maximum extent about 20,000 years ago, with major tidewater glaciers on the western shelf and down the Irish Sea Basin into the Celtic Sea. During this period, parts of Ireland were probably uninhabitable because of glaciation, pushing brown bears toward ice shelves and land exposed by lower sea levels. 

With today's climate change, this is happening again
‘The two species bumped up against one another for extended periods of time on different occasions, sharing both habitats and genes,' Shapiro said.

Climatological data suggest that the planet now is experiencing another warming period - known as the Holocene or the Present Interglacial - which is even warmer than the period that marked the beginning of the last ice age.

‘Today we are seeing a similar change in the arctic climate,' Shapiro said. ‘And once again that change is providing polar and brown bears the opportunity to share habitats and to hybridise.

'In fact, several adult hybrid bears have been reported in the past five years.' 

For this reason, Shapiro said, scientists should reconsider conservation efforts focused not just on polar bears, but also on hybrids, since they may play an underappreciated role in the survival of certain species.