Over time, the brown bears' habitat becomes colder because of Earth's natural processes. As this change becomes constant and habitat of the brown bear is permanently changed, the fur of the brown bear is affected. Previously, the bear had thinner, brown fur that protected against mild cold.
The first generation of brown bears breeds in the new colder habitat. Heritability passes down their thin, brown fur to 75% of their offspring, but one of their offspring has thicker, white fur as a result of a mutation. The variability of the offspring produces a new, thicker, white fur.
Compare and Contrast
As these bears grow up into the next generation, the white, thick furred bear has grown strong and much warmer than it's siblings, surviving the first cold winter easily. Since it's strong and able to reproduce, it breeds with a common, brown bear and produces 50/50 thin and thick furred offspring.
The thicker furred bears have a much easier time surviving the cold of the new habitat and grow stronger than the thin furred bears, giving them a reproductive advantage. Since they are able to do this, they reproduce together and, as pictured here, their collective thick furred mutations is passed on to thier offspring through heritability.
The brown bear that had lived in a warmer, forest climate had thinner, brown fur which sustained them well in that warmer climate. As Earth's processes changed their habitat from a warmer, forest climate into a colder, tundra setting, the mutation of thicker, white fur gives this new breed of bears a reproductive advantage, created through variety and passed down through heritability.
The presence of these thick furred bears was made through the natural selection process, and in this generation, you can see that now, the thick furred mutation to the thin furred bear has become preferable to the habitat and species. This is an example of how natural selecion can increase biodiversity and give way to