Mice have moved into a new area with lots of dark volcanic rock. There are both tan and black mice on account of natural genetic variation.
Birds of prey in the area begin to target these mice as prey, making them a part of their diet.
The tan mice are more visable to their predators therefore, they are targeted more frequently than the black mice.
Over time, the population of tan mice decrease as they are more targeted by their predators.
The mice will mate to keep the population going. It is more likely for a black mouse to breed with another black mouse due to selective pressure.
Because the black mice had a higher chance of breeding than the ten mice, the next generation of mice has a higher ratio of black than tan mice. Because of natural genetic variation, there is a possibility for tan mice to be born