The evolutionary history and migratory behavior of polar bears.
Throughout life’s long history evolution has been demonstrated over and over again. Competition for resources and geographic and climatic changes have lead to natural selection, which since the beginning of time has molded our world into what it is. The polar bear, Ursus maritimus, is an excellent example of a natural survivor. Since divergence from their most closely related extant ancestor during the mid-Pleistocene Epoch (300 – 400 TYA), the brown bear (Ursus arctos), (Talbot and Shields, 1996) polar bears have acquired several adaptive traits. By what is believed to be 100,000 years ago the polar bear came into existence.
They finally had diverged enough from brown bears to be classified as there own species, although in captivity the two currently produce fertile offspring (Rosing, N., 1996). It has even been stated that polar bears have “been shown to possess the most advanced evolutionary adaptation of any previously studied hibernating mammal to conditions of no food or water” (Talbot and Shields, 1996). These adaptive traits not only include unique hibernation patterns, but also physiological and morphological advantages, and behavioral radiation. Unfortunately, not all adaptive traits remain beneficial throughout the course of life’s history, and as the biosphere changes an organism must continuously adapt as well. Whether prior adaptations can be reversed or continue to be modified is a question not easily answered (I. B. Weiner., et al., 2003). For instance, will global warming lead to the end of the polar bear?
An inference made from the study of mitochondrial DNA strongly implies that polar bears have diverged from the same linage as brown bears currently living on the islands of southeast Alaska. Both extant groups are believed to have descended from brown bears in the proximity of Siberia that had expanded into North America (Talbot and Shields, 1996). Since their initial divergence they have come to inhabit such areas as Canada, Greenland, Norway, Russia, and the USA (Howell-Skalla, L. A., et al., 2002). Despite geographical differences experiments have shown little genetic variation among these populations indicating minute independent adaptations (Paetkau, D., et al., 1999), which can be complimented with the fact that polar bears are a relatively new species; the last descendant of the Ursine evolutionary linage (Struzek, Ed, 2003, Rosing, N., 1996). Since other bears, especially modern brown bears most likely share a common ancestor with that of polar bears, brown bears are a great model to use in comparison due to their similarities. Many traits unique to polar bears may be due to the initial adaptations that were gradually introduced by their ancestors as their habitat shifted closer and closer to that which polar bears now occupy. Modern brown bears have proven to be capable of living very close to the harsh environment of the winter arctic, which basically means that the first generations of the evolving white bears started with the same good genes.