Evolution within a species is a proven fact. That's what Darwin observed. One type of bird in one environment had a different beak than the same type of bird in a different environment. Each species changes to fit into its environment, and those changes allow it to survive and thrive.

The thing Creationists disagree with is the idea that one species can become another, which is not proven by the fossil record. That's why the search for the "missing link" continues... that "link" would be the fossil that proves, at least in some part, cross-species evolution.

But logic dictates that there must be BILLIONS of these fossils to be found, with the fourty-million year period ascribed to by most evolutionists. So far, we're still looking.

PS: Not all creationists are "young earth" creationists. The bible does not even espouse the idea.