Both the Grey and Brown mice live in the forest. Their main predator is the Red fox which uses its sharp hearing and sense of smell to track his prey.
The population of mice is growing much faster than they can survive in the forest. Both the Brown and Grey mice fight for the same limited resources to live. Both the Grey and Brown mice are eaten by the foxes.
There 2 main types of mice in the forest. Some mice are born with the Brown color trait, while others are born with the Grey color trait. All of the mice population in the forest is competing for their survival.
The Grey mice are much more visible in the dark forest. This makes it easier for the Foxes to spot it. The Brown mice are much more difficult to spot due to how well they camouflage to their environment.
Since the mice color is an inherited adaptation, the Brown mice are less likely to be detected by the foxes, therefore, increasing their survival. This means that the Brown mice will be eaten less than the Grey mice.
Because the Brown mice have a much higher survival chance than the Grey mice, they will be able to reproduce and pass on their genes. The Brown color trait will start becoming more prevalent. This is due to the ability of the Brown mice to reproduce more than the Grey mice. The next generation will see an increase in the Brown mice population and a decrease in the Grey mice population.