Example 1 of Natural Selection:
The fish had a continuous variation in regards to the colorful spots. The more colorful males were able to reproduce more successfully, however they were the first to be eaten. The bland guppies did not reproduce well, however they were able to escape predators easily. The middle of the bell curve is were most of the guppies end up in an unaltered experiment. However, if you add more predators, you will have more bland guppies. If you remove predators, your guppies will be brightly colored.