How could physical changes to Earth such as a meteor impact, an ice age, or global warming lead to physical changes in organisms?

As changes to the Earths climate occur, organisms within will be given the natural selection all animals are. Those being, die off and not adapt, or adapt "evolve" to the new climate and survive. An example is the bear, polar bears adapted to winter climates by having a white fur coat to blend in, while grizzlies have a brown coat to blend in with shrubbery or the soil.