The correct answer is - geographical isolation.
With the formation of a new geographical feature, the canyon in this case, one of the results was the separation of two populations of black-tailed rats. Since the rats from the two separate populations are no longer able to contact and reproduce because of a geographical feature, this a nice example of geographical isolation.
Because the two rat populations have been separated, over time they can develop totally different trait and be unrecognizable. That of course will largely depend on the environment and evolutionary pressure.
