There are two different types of mice. One mouse is a white while the other mouse is brown. Both these mice live deep in a forest.
The different colors of the mice happens because of genetic variation. Genetic variation are the changes in genes. The genes changes because there was a mutation during meiosis or crossing over. The mutation allows the fur color to be be another color than the rest. Some of colors allow the mice to blend with its habitat like the brown mouse, while some colors all allow you to stand out like the white mouse.
Because the brown mouse is able to blend in with its habitat, its easier for it to hide from prey which is the total opposite for the white mice. Its light fur is seen easier by the predators, which allows them to be eaten faster. This is an example of the white mice having to struggle for survival.
Overtime, there becomes less of the white mouse because they are being eaten by predator. The brown mice had the advantage with their different fur color to live longer than the white mice because their ability to blend in with their habitat.
If the brown mice keep producing offspring, the brown mice population would grow and eventually there would be a population full of brown mice.