The mountains are a Madrean sky island range that is typical of southern Arizona, specifically south-central Arizona, and especially the complete southeastern quadrant of Arizona, from Tucson, and Globe to Nogales, Douglas, and the Chiricahuas. Sky island ranges are mountains isolated by desert valleys. The deserts, as well as differences in elevation, prevent flora and fauna from traveling to or from nearby ecosystems. As a result, the mountain ecosystems are isolated, and distinct subspecies can develop. This is similar to what Charles Darwin discovered with species he collected from different islands in the Galápagos, a discovery that played a major role in his theory of natural selection. The Mount Graham red squirrel is an isolated population of red squirrels and possibly a subspecies as well.