One of the common arguments trotted out by our more stupid friends is that we've never seen evolution happen so therefore it can't have ever happened. They will, if pressed, admit that there are different breeds of dogs. But (and it's a big but, believe you me) they are all still dogs. You can impregnate a Great Dane with Chihuahua sperm and get live puppies. But they are missing a key point about speciation, it's not just the inability to produce viable offspring (think mating a dog with a vole) that makes two organisms be from different species, there's also a concept called Reproductive Isolation. Reproductive Isolation means that even if two species could successfully mate if brought together, they can't, for one reason or another get together. A couple of examples might be squirrels living on opposite sides of the Grand Canyon, or two kinds of flowers that bloom at different times of the year.
Getting back to our dog example, if we released a bunch of Chihuahuas and a bunch of Great Danes into some contained range where there were no other dogs of intermediate size (say an island far from any other dog populations), the two groups of dogs would be reproductively isolated and would never never never interbreed even though they could produce viable offspring. So without a gradation of other dog breeds to connect a Chihuahua to a Great Dane, the two breeds are effectively different species.
So there, my stupider friends, you have seen evolution.