The Grand Canyon was carved out by the Colorado River several million years ago. This caused the separation of members of one squirrel species. The Abert’s Squirrel lives on the southern side of the canyon and in other parts of the United States. A second species, the Kaibab Squirrel, has been found around the northern rim of the canyon and nowhere else in the country. According to the process of natural selection, we know that there must have been some genetic variation among the existing squirrel population, but how did that lead to the development of a second species?