Consider the following scenario. You suddenly realize that you have a population of mice living in your attic. You do not want to put out traps or poison, so you decide to get a cat. The cat is fast; however, the cat is not as fast as the very fast mice. There are a total of 20 mice living in the attic. Of the 20 mice, 10 are very fast, 5 are medium fast, and 5 are very slow.
A few weeks pass, and there are fewer mice; however, you still have some mice in the attic. After a few months, it seems that there are even more mice than there were to begin with. Why do you have more mice than you did when you first bought the cat? Why do we not see situations such as this happening in humans as often as we see them in other organisms?