Polar Bear Evolution
The Polar Bear is a member of the Ursidae family and shares a common ancestor with the black and brown bear. As a family, bears are thought to have evolved some 4.6 million years ago, with the oldest Polar Bear fossil being between 110 and 130 thousand years old. Although not genetically isolated from other bears, evidence of 20,000 year old polar bear jaws shows a distinct shift in tooth shape and size, pointing to the evolution of a species more carnivorous than omnivorous.
Polar Bears are thought to have diverged from other bears anywhere from 70,000 to 1.5 million years ago. Using carbon dating of Polar bear bones in the arctic, researchers suggest that it was during the ice age between 200 and 125 thousand years ago that the Polar Bear took on its distinct white colouring. It is proposed that when a population of brown bears began moving north in search of food, they became cut off from other members of there species due to Glacial movement. Although most of these bears died off, a generally accepted hypothesis is that a mutation causing some bears to have a lighter coloured and thicker coats allowed them to survive and pass on their mutated gene. With no gene flow with other species due to there isolation, these lighter, heavier furred, bears began to dominate due to their ability to hunt unseen and retain body heat exceptionally well. This mutation proved to be a positive one and crucial to survival in the Arctic. As a result, these white bears had a much higher fitness in the arctic ice fields and the characteristic manifested leading to, generations later, all polar bears being white.
The trait of white fur is not an ancestral trait, as it is derived from a population of bears isolated from the remainder of their species.
This trait was a successful way of adapting to brutal arctic conditions, and as those conditions still exist in Polar Bear habitat today, it continues to be relied on for survival.
As the mutation causing white fur occurred in a geographically isolated population of organisms, and has not spread into related species, it is an example of Microevolution. However, he trait for white fur is homologous among the Ursidae family (see "Spirit Bear") and is proposed to have begun in Polar Bears as a single, isolated, gene mutation of the DNA coding for hair colour. Proof is found in a similar mutation in the white Kermode bear of British Columbia's west coast. "Spirit" bears, as they are known, gain there white fur from a parent who carries the mutated gene. Like the early Polar Bears, when both parents are mutated (white) the offspring will also be white. However, unlike the Kermode bear, the Polar Bear relied on its mutation for survival and thus white fur became the dominant, and ultimately only colouring for the species.
Polar Bears are thought to have diverged from other bears anywhere from 70,000 to 1.5 million years ago. Using carbon dating of Polar bear bones in the arctic, researchers suggest that it was during the ice age between 200 and 125 thousand years ago that the Polar Bear took on its distinct white colouring. It is proposed that when a population of brown bears began moving north in search of food, they became cut off from other members of there species due to Glacial movement. Although most of these bears died off, a generally accepted hypothesis is that a mutation causing some bears to have a lighter coloured and thicker coats allowed them to survive and pass on their mutated gene. With no gene flow with other species due to there isolation, these lighter, heavier furred, bears began to dominate due to their ability to hunt unseen and retain body heat exceptionally well. This mutation proved to be a positive one and crucial to survival in the Arctic. As a result, these white bears had a much higher fitness in the arctic ice fields and the characteristic manifested leading to, generations later, all polar bears being white.
The trait of white fur is not an ancestral trait, as it is derived from a population of bears isolated from the remainder of their species.
This trait was a successful way of adapting to brutal arctic conditions, and as those conditions still exist in Polar Bear habitat today, it continues to be relied on for survival.
As the mutation causing white fur occurred in a geographically isolated population of organisms, and has not spread into related species, it is an example of Microevolution. However, he trait for white fur is homologous among the Ursidae family (see "Spirit Bear") and is proposed to have begun in Polar Bears as a single, isolated, gene mutation of the DNA coding for hair colour. Proof is found in a similar mutation in the white Kermode bear of British Columbia's west coast. "Spirit" bears, as they are known, gain there white fur from a parent who carries the mutated gene. Like the early Polar Bears, when both parents are mutated (white) the offspring will also be white. However, unlike the Kermode bear, the Polar Bear relied on its mutation for survival and thus white fur became the dominant, and ultimately only colouring for the species.