How does it Work?
Imagine that you own an island that is inhabited by a population of gray rabbits. Imagine further that one day you discover a tan rabbit on the island, and for some reason you prefer it to the gray ones. You start trapping rabbits and get rid of all of the gray ones you catch. The occasional tan one that shows up in one of your traps is released back onto the island. At first tan rabbits will show up in your traps only rarely, and they will be observed infrequently. Over time, however, as you selectively remove the gray ones, the tan ones will become more and more common, if there is nothing selectively eliminating them. If you continue eliminating the gray rabbits, the tan ones will eventually become more numerous, and if you live long enough, you may succeed in reducing the gray rabbits to an occasional individual only. You may not eradicate them, but you will have succeeded in changing the population of rabbits from predominately gray to predominately tan.