A biologist conducting a long-term field study noticed there were two types of mice in a valley, one type of mouse had webbed toes and one type of mouse lacked webbed toes. A dam was built in the valley turning the land into a wetland. As
the years passed, all mice living in the valley were found to have webbed toes. No mice were found with non-webbed toes.
Which statement best explains the observed change in the mouse population?