In this forest, there are multiple variations among the two rabbit breeds living there. One breed is a dark brown, and the other is a light brown. The dark brown breed is harder to see, and it is much faster and more athletic than the light brown breed.
Since the dark brown rabbits are fast and hard to see, they are harder to be caught by predators. The light brown rabbits do not blend in, so they are seen easier. They are also sleepy, very slow, and are fatter than the dark brown ones.
One day, a new animal was introduced to the forest. This animal was a predator, and it was a fox. This caused chaos among the rabbits, and caused them all to flee from the fox, since the rabbits were all very alarmed by the new predator.
Most of the lighter colored rabbits got eaten since they were fatter and more appetizing to the fox. They also were slower, and easier to be seen, so the fox could catch them easily. The darker rabbits were the exact opposite of the light ones, so they survived.
Since the darker rabbits survived better, there were more of them alive. Those rabbits reproduced and became more common since they passed down their genes to their offspring, creating more better adapted, dark brown rabbits, than light brown ones.
Eventually, only the dark rabbits survived in the forest since they were the best adapted to the environment. The light ones were caught by the fox. Now, the good traits are passed down from the dark ones, to their offspring, showing natural selection.