The prospect of polar bears becoming extinct by 2075 because of the loss of the North Polar icecap is a stark warning of what we are doing to the life-support system we call Earth as we pump out more and more of our toxic waste, like bacteria polluting our culture medium. It also serves to illustrate a couple of things about how life evolves and changes over time, pushed as often as not by environmental change, not genetic change.
Polar bears (Ursus maritimus) are close relatives of the Brown bears (Ursus arctos) of which there are about 16 recognised subspecies, including the grizzly bear, with disagreement amongst taxonomists about the exact status of some populations. Their closest relatives appear to be the brown bears of the ABC (Admiralty, Baranof and Chichagof) Islands off the Southeast Alaskan coast. Genetic evidence suggests an ancestral brown bear which diversified to give brown and polar bears. The ancestral brown bear was then pushed south during the last Ice Age where it diversified into all the other subspecies in different parts of its range, leaving only a remnant population in Southeast Alaska which became isolated (and secure) as sea levels rose cutting off the Alexander Archipelago from the mainland. At the end of the Ice Age one of the browns from further south moved back north as the ice sheet retreated and the polar bears also retreated with the ice, having adapted to the Ice Age conditions and living entirely on meat.
The fact that the browns are regarded as subspecies and can interbreed, and the fact that polars bears and grizzlies interbreed, producing fertile 'grolars', is a clue that this is actually evolution in progress. This branch of the bear family, which is itself a branch of the Carnivora order of mammals, which includes cats, dogs, seals and sea lions, racoons, stoats, weasels, badgers, meerkats and mongooses, is very recently evolved and has diversified according to local environmental conditions and in response to climate changes which produced the last Ice Age which is still drawing to a close.
What will cause the polar bear's extinction, if we don't reverse climate change in time to save it, will be evolution itself. Evolution is a one-way process with no reverse gear, which, being completely blind and unplanned, can take a species down an evolutionary cul-de-sac from which there is no escape. As the environment changes around it, adaptations which were once an advantage and so came to dominate in the species gene pool, can be a disadvantage without any genetic change whatsoever.
To the shagrin of creationist pseudoscientists who witter on about Shannon Information Theory and the Second Law of Thermodynamics whilst obviously not understanding either or confident their willing dupes won't, there need be no change whatsoever in the genetic information let alone any new information needing to be created. An environmental change alters the meaning of the information entirely, so turning adaptive and beneficial change into a lethal maladaptation which can lead to extinction.
The fact that it is human activity which is changing the environment and so casually exterminating a species - one amongst many caused by human activity and global warming - in the blink of an eye on a geological time scale, doesn't change that basic principle of Darwinian adaptive evolution one iota. It serves merely to illustrate that evolution is an unplanned, unintelligent, undirected natural process which can produce everything from the magnificence of apex predators like polar bears to ebola viruses and slug mites, and exterminate them without a second's thought.
There is nobody to care for this planet if we don't take care of it ourselves.