A scientific "theory" is a logical explanation for something that we cannot observe directly. There were many theories about the size, shape, and nature of the ATOM, and ultimately we reached the point where we could observe some of them (electron microscope), so all the theories went out the window, replaced by factual knowledge.
We cannot observe the process by which different species come into being; they simply take too long. "Evolution" is a nice, neat theory that is more or less consistent with what we can observe, so we use it as a teaching tool. But like a sixty-year-old beauty, its value decreases the closer we look. Whole books have been written about the things that Evolution cannot explain, and the proofs that it just doesn't work in many specific instances (some THING cannot be explained by natural selection).
The staunchest defenders of Evolution are Leftists with no knowledge or understanding of any science. They have converted Evolution from a scientific theory to a "religion" (something that is accepted on faith), and woe to anyone who challenges it. HERETICS! But I have news for them: Just because one debunks Evolution does not necessarily mean that one accepts Genesis as a historical or scientific explanation of the origin of species.