Many creationists like to claim that “we’ve never seen one species come from another species; when a human is born to a chimp I’ll believe evolution,” in what they believe to be a knock-down argument against evolution. Obviously this claim is indicative of a fundamental misunderstanding of the process of speciation, but I’ll let that pass for now. What is really more important to the debate is that this claim is actually flatly false. We have indeed witnessed speciation numerous times.
Here’s an interesting article about an instance of observed speciation in Galapagos finches from Wired Magazine: Birth of New Species Witnessed by Scientists