Ok, let's say we start out with white birds. Original mother gives birth to babies that are very much like her, but one baby has a red spot on his tail. He is still very white except for that one little red spot. So he mates with other white birds, and their babies have red spots. Some have bigger spots, some have smaller spots, some have no spots at all. They go on mating with each other, and over the course of time birds with red spots are able to attract more attention because of their pretty red spot. Over the course of time, the red spot becomes more and more pronounced because individuals with more red are able to mate more. And the completely white birds, or birds with only a little bit of red, are able to mate less, so they produce fewer offspring. Over a long period of time all mothers have red tails and all of their babies have red tails, and a completely white version has disappeared from the gene pool.
But on the mainland there are predators that do not exist on the island, and who can see spots. So if the original mother's sister gives birth to a spotted baby, the baby gets eaten. Or if by some chance he survives to adulthood and mates with another white bird, their spotted kids get eaten. And so on the mainland the pure white birds have the advantage until all traces of red disappear from the gene pool.
Fast forward from the original parents to 10,000 generations later. The mainland predators died off. A red tailed bird flies to mainland. The girls all look at him and say - huh? what kind of bird is that? we don't like him.