The sandy and lava flow substrates are entirely natural phenomena.'' Mike Majerus, an evolutionary geneticist at Cambridge University, who was not part of the study. ''This work is very important,'' said Dr. Scientists point out that other well-known cases involve evolution caused by humans some have suggested that those changes may be atypical of natural evolutionary change, since they have typically involved intense, directed pressures destroying most of a population, like the spraying of pesticides or the application of antibiotics. Researchers say the study is the first documentation of the genetic changes underlying an adaptive change where the evolutionary forces were natural. Now, in a recent study in The Proceedings of the National Academy of Sciences, researchers report identifying the gene responsible for the evolution of dark coat coloration in these mice, pinpointing the DNA sequence changes that underlie this classic story of evolutionary change, the cute and furry counterpart to the famous case of the peppered moth. There the same species of rock pocket mouse has only dark coats, having evolved an entirely distinct and, for their surroundings, equally well-disguised pelage. But in some areas, ancient lava flows have left behind swaths of blackened rock.

Nearly everywhere these mice are sandy-colored, well camouflaged as they scurry across beige-colored outcrops. But while these mice may seem to scamper haphazardly across the desert floor, their arrangement in nature is strikingly orderly. In the deserts of the Southwest, among the towering saguaros and the spiny cholla cactuses, rock pocket mice hop and dash in search of a meal of seeds.

Pocket mouse evolution