Answer
The mice in rocky areas will have dark colored coats while the mice in sandy areas will have white colored coats.
Work Step by Step
According to survival of the fittest, only the mice that adapt to the ecosystem will survive. If we had light colored mice in rocky areas, they would soon be eliminated by the owls due to lack of camouflage. The same goes for the dark colored mice. And to test this, we can switch the places of some mice, and see if they survive. If they don't, then we will be proven correct.
