Polar bears and Brown bears are by scientific standards separate species, having a common ancestor many millions of years ago. Despite this, the Polar bear and the Brown bear are still able to reproduce and produce fertile offspring together, indicating that they are in fact of the same species.
Researchers sequenced 80 polar bear genomes, as well as a number of brown bear genomes in order to see their relationship. The DNA showed that the separation of Polar bears and Brown bears most likely occurred a lot later in evolutionary time than we thought, roughly 300,000 and 470,000 years ago.
Polar bear DNA also showed rapid evolution within this time, mainly to aid in keeping cholesterol down, as Polar bears live off a more blubber-rich and fatty diet.
This new time zone for the divergence of Polar bears and Brown bears seems to line up with the longest interglacial period of the last million years, a 50,000 year period in which forests established themselves in Southern Greenland.
It’s assumed that in this time Brown bears managed to settle in many newly forested areas, and when the glaciers grew back these populations became isolated and thus were under intense pressure to adapt to the icy environment.
A Polar bear has many physiological adaptations to the Arctic environment: broad paws for swimming, white fur, a distinctive head shape etc. When the researches tried looking into the evolution of these genes, other interesting genes caught their attention more.
One of the most interesting was a gene involved in cholesterol metabolism. Due to the high amount of fat in the diet of Polar bears in comparison to Brown bears, this gene had nine changes in it, occurring in just a short period of evolutionary time. Contrastingly, Panda bears and Brown bears have no differences in this gene, even though they split from a common ancestor longer ago.
Due to the high levels of cholesterol Polar bears must have with their diet, it could be assumed this modified gene was to help in reducing the amount of cholesterol in the blood, however this is not the case. “Cholesterol levels in blood plasma of Polar bears are extreme”, note the researchers.
Instead, this evolutionarily modified gene seems to have reworked the heart so that it can survive such high cholesterol levels. Nine of the sixteen genes which are changing the most are involved in cardiovascular development and maintenance, to help survive this pressure.
This evolutionary change between Brown bears and Polar bears has occurred in less than 20,000 generations, which is almost nothing in terms of evolution, especially for a large mammal.
Jack Di Francesco