Evolution has been demonstrated in the real world. I know there are several examples, but the one that comes to mind is the peppered moth. Creationists complain that this isn't valid, since both kinds of moths were there to begin with, but that just shows they don't understand the principle. Evolution doesn't "change" one thing into another thing. Survival of the fittest (remember, that doesn't always mean the biggest or strongest) works on a population existing all at the same time, with the small natural variations and tiny mutations that exist in all species.

Just looking at how different people look just on the outside, how many small variations of the basic human design do you think exist right here and now? We don't evolve physically much any more since we tend to protect our "culls," but I suspect something like a wide-ranging new disease would shove the naturally resistant people forward in short order.