Suppose that a large river separates two populations of mice. one population is all black, and the other all white. a beaver dam that is built allows the mice to cross the river. over time, many gray mice are born. what is this an example of?

In this case two kinds of selection might be applicable to this situation. For example, if the population of grey mice are more exposed to predators due some materials used in the dam then that would be an example of disruptive selection. But if the population of gray mice is less visible to predators and if the population grow, then it will be an example of Stabilizing selection. Basicaly meant, this type of situation is also known as codominancy or codominant type.