Two animals that can be found at the Grand Canyon include the Abert's and Kaibab squirrels. The Abert's squirrel is found at the south rim of the canyon and in a few surrounding states. The Kaibab squirrel, however, can only be found at the canyon's north rim, mainly on the Kaibab Plateau. The Kaibab squirrel is not found anywhere else in the world. Scientists believe that the squirrels are actually closely related and have developed separately as a result of geographic isolation. It is possible that the squirrels were separated by plateau erosion that separated the forests and isolated the Kaibab population. Some scientists are studying if the squirrels are now two separate species or if the Kaibab squirrel is simply a subspecies of the Abert's squirrel. Separate species are unable to reproduce with each other. What evidence would be used to support the claim that the squirrels are two separate species?