Rock pocket mice are interesting in many ways. They make a living in the intense heat of the desert. They also come in different fur colors, from tan to black. Scientists have a good understanding of what causes fur color in individual mice. The amount, type, and distribution of a pigment molecule called melanin gives fur its color. But why are almost all the mice in some populations black and those in other populations mostly tan? What determines these patterns? The American Southwest is a fantastic place to study rock pocket mice with different coat colors. Lava flows created patches of dark rock among the surrounding light-colored sand. Researchers noticed that black rock pocket mice were more common on the dark lava flows, whereas the tan rock pocket mice were more common on the light-colored sand. They suggested that these patterns could be explained by evolution by natural selection. But what is the evidence for this claim?