Squirrels and other small mammals near Arizona's Grand Canyon are shown to have experienced speciation. Two populations of antelope squirrels that were originally a single species became separated by the canyon. Presently, these squirrel populations represent two different species: the white-tailed antelope squirrel inhabits the north side of Arizona's Grand Canyon, and the closely related Harris's antelope squirrel can be found on the south side of the canyon. Which statement BEST explains why the antelope squirrels experienced speciation?