You are just ignorant of the state of modern science. That's not a personal insult.
The proof is nicely compiled in Dawkin's recent book which I referenced. Evolution doesn't know how to make something, but as you said, there is great variation even within a species.
Here is a thought experiment. Suppose elephants needed big tusks to fight other males for the chance to mate with females. Suppose humans came along and killed all the males with big tusks for the ivory trade. The males left would have smaller tusks, and they would pass along their traits to their children.
If evolution were not true, average tusk size would remain the same. The species would remain unchanged by any selection pressure.
If evolution were true, average tusk size would decrease because only small tusked males would reproduce.
In fact, that's exactly what we see, the average tusk size of legally hunted elephants in Africa has decreased.
The proof is nicely compiled in Dawkin's recent book which I referenced. Evolution doesn't know how to make something, but as you said, there is great variation even within a species.
Here is a thought experiment. Suppose elephants needed big tusks to fight other males for the chance to mate with females. Suppose humans came along and killed all the males with big tusks for the ivory trade. The males left would have smaller tusks, and they would pass along their traits to their children.
If evolution were not true, average tusk size would remain the same. The species would remain unchanged by any selection pressure.
If evolution were true, average tusk size would decrease because only small tusked males would reproduce.
In fact, that's exactly what we see, the average tusk size of legally hunted elephants in Africa has decreased.