Fooling With Mother Nature

So a creationist museum has a display on natural selection. This shouldn't be too surprising, since not even a creationist can deny that nature selects for certain traits, as this selection can be observed directly. Their argument is that it can't create a new species.

This sort of makes sense for young earth creationists, since 6000 years or so isn't really enough time to create the diversity we see. But young earth arguments run into so many other problems, it's hardly worth the trade-off. I don't understand, however, how other creationists can maintain this argument.

What is the difference between closely related species? They've got the same basic skeleton, organs, musculature (or the equivalents for plants). They've got the same cell structure. Underneath it all, they share the same DNA. The only really differences tend to fall into categories like shape, size, color and behavior--the very things natural selection works on. Given enough time, what's stopping one species from turning into a new one? What is this insuperable barrier creationists believe in? Is there some sort of as-yet undiscovered rubber band mechanism inside each species that makes it snap back to normal if it stretches too far?

The day these are no longer rhetorical questions is the day the creationists actually have an argument.