I can always tell when an opponent of Darwin’s “theory of evolution” has never read Darwin, because they try to claim that Darwin’s “theory of evolution” means that “evolution is a theory.”
Unfortunately for them, the phrase “Darwin’s theory of evolution” does not mean evolution was Darwin’s theory. Scientists recognized evolution long before Darwin came along, and they understood it not as a theory, but as a fact that they observed both in nature and in the laboratory. They could see changes over time to species in nature, and they could manifest evolutionary changes themselves in their labs using species that reproduced very rapidly, such as fruit flies. To a scientist, “evolution” is merely another word for “mutation,” and there simply is no question that mutation is a fact — whether we are talking about a human who is born with a birth defect (or a unique talent), or a bird flu virus that mutates to be able to spread by person-to-person contact. Evolution simply means “mutation” or “change over time.” Such “evolutions”…such “mutations”…such “changes over time”…are proven facts.
The critical quesion is what causes such evolutionary changes to occur? The reason Darwin drew attacks from Christians for his “theory of evolution” was not because he proposed the idea of evolution; the reason he drew attacks was because he suggested that nature itself, not God, was the driving force behind evolution. Thus, just as Newton’s “theory of gravity” was not gravity itself, but rather the rule by which gravity operates; Darwin’s “theory of evolution” was not evolution itself, but rather the rule by which he believed evolution operates — namely, natural selection. Just as we do not say, “Newton had a ‘theory of gravity,’ therefore gravity is a theory,” we should also not say, “Darwin had a ‘theory of evolution,’ therefore evolution is a theory.”
So, why was natural selection (Darwin’s “theory of evolution”) so earth shattering in the 19th century, and why is it still so controversial today? Mainly because the reasoning he set forth in his book, “Origin of Species,” was VERY convincing. It was so convincing that the Christian church was unable to effectively contradict it. Indeed, it was so convincing that it is what gave rise to Christian Fundamentalism. Fundamentalism was specifically created in the 19th century by people who refused to believe Darwin’s theory of natural selection, but they had no equally convincing argument with which to oppose it, so they created their own version of Christianity — Fundamentalism — that declared the Bible to be literally true. It was the only argument they could think of to contradict Darwin, and it consisted essentially of saying that any scientific data which contradicts their own pre-conceived interpretation of the Bible must be wrong.
In sum, Darwin’s great contribution to science was not in being the first person to suggest that evolution happened; his great contribution was in being the first to convincingly suggest a convincing mechanism for why evolution happened as it had been seen to happen. He was the first to propose a theory that seemed to fit the available data: the theory of natural selection. That is Darwin’s “theory of evolution.”
So, what is “natural selection?” A detailed explanation can be found here.