At some point in the past—possibly as recently as a few thousand years ago—mice spread into open areas along both the Gulf and Atlantic coasts, where they dug their burrows into sand dunes and grassy embankments. Beach mice now raced across a vastly different background than their inland ancestors, and in these new environs dark mice got plucked from the sand. By chance, some of the beach mice carried in their DNA new mutations to one or both of two genes involved in the production of dark pigments. Mice inheriting these mutations carried copies of the pigment-influencing genes that were just a little bit different from the copies carried by other mice (alternative versions of a gene are known as alleles), and as a result they developed with lighter fur. Mice bearing the new alleles survived better than mice inheriting the ancestral versions of the genes, and these survivors populated the beaches with their pups. Over time, mice with the new alleles increased in frequency, while those with the original alleles disappeared, and the result was an evolutionary shift from dark to white.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 35