The polar bear (Ursus maritimus) and their close relatives the brown bear (Ursus arctos horribilis) were thought to have diverged from a common ancestor around 150-200,000 years ago, an estimate based on DNA sequences from mitochondria (small structures which are the ‘energy powerhouses’ within our cells and contain some genes that are independent from those found in the nucleus). However, researchers have recently examined genetic markers from the nuclear DNA of polar and brown bears and found a whole different story. The study has revealed that polar bears actually diverged around 600,000 years ago - giving them a much longer period than was previously thought to evolve their unique adaptations to Arctic conditions. This makes sense, as the previous estimate would have meant the polar bears had evolved their adaptations at a rate that would be exceptionally rapid for a large mammal. The false story told by the mitochondrial DNA is likely to be due to hybridisation between polar and brown bears that has occurred since the species diverged (see previous post). Mitochondrial DNA is inherited from the mother, so it is likely that along the way, polar bears may have picked up mitochondrial DNA from a brown bear mother which is now mixed with genetic sequences typical of the polar bear. DNA in the nucleus, however, has been less affected by hybridisation.
The genetic analysis also flagged up much lower genetic diversity across polar bears than is seen in brown bears. This is a common feature in endangered species. The reason is often that the species has faced difficult environmental pressures in the past, leading to large reductions in population size, then followed by inbreeding between related individuals in order for the species to recover. A lack of genetic diversity will make it all the more difficult for polar bears to survive the pressures they currently face, such as climatic warming, human interference, and pollution in their food chain. If one individual struggles to cope, and all individuals are genetically similar, then it is certain that many others will also lack the features needed to survive and prosper through these difficult times. You can help support the polar bear by becoming a member or adopting an animal through WWF.
Ref: Senckenburg Research Institute and Natural History Museum, 2012. Polar bears evolutionarily five times older and genetically more distinct: Ancestry traced back 600,000 years. ScienceDaily [link]