Ariver separates two groups of mice one black and the other all white a beaverdam allows the mice to cross the river overtime midi grade mice are born. the river lined with gray stones. gray mice are less visible to hawks that fly overhead and feed on mice.

Its natural selection. The reason why is because there were two traits, right? Black and white, and then over time some were able to mate due to being able to reunite with the other color. So it created grey mice. The grey mice were less visible to hawks on stones, so that is the trait that was favored, due to it being the trait that is best for its environment.

Fossil found in strata reveal that unused body part decrease in size