Rock pocket mice live in the deserts of the American southwest. Ancestral pocket mice had light-colored coats that blended in with the region's rocks and sandy soil, keeping the mice hidden from their owl predators. Starting about 1.7 million years ago, a series of volcanic eruptions spewed out wide trails of black lava that wove right through the middle of pocket-mouse territory.
Today there are two forms of pocket mice: light-colored mice that live on sandy soil, and dark-colored mice that live on black lava rock. The dark mice came about through the process of evolution. Naturally occurring mutations to coat-color genes produced mice with dark fur. On black rocks, dark mice had an advantage over light mice: they were better-hidden from predators. They survived and reproduced, passing their dark-fur genes to their offspring, which still survive today.
Once a favorable variation occurs, it can quickly become the major form in a population. Each year, mice produce more offspring than will reach adulthood. Thanks to natural selection, the offspring with favorable characteristics are more likely to survive and reproduce.