Richard Dawkins makes this clear in his book The Greatest Show on Earth (p. 17).
Biologists often make a distinction between the fact of evolution (all living things are cousins) and the theory of what drives it (they usually mean natural selection, and they may contrast it with rival theories such as Lamarck's theory of 'use and disuse' and 'the inheritance of acquired characteristics'). ... Nowadays it is no longer possible to dispute the fact of evolution itself—it has graduated to become a theorum or obviously supported fact—but it could still bedoubted (just) that natural selection is its major driving force.The distinction is important. Things like common descent and the history of life are the facts that demonstrate evolution. Evolutionary theory offers a solid, widely-accepted, explanation of how evolution happens.
Douglas Futuyma has written one of the most respected textbooks on evolution. He agrees with this distinction—as do all other textbook authors that I know of. Here's what Futuyma says in Evolution 2nd ed. p. 4.
The explanation of how modification occurs and how ancestors gave rise to diverse descendants constitutes the theory of evolution. We now know that Darwin's hypothesis of natural selection on hereditary variation was correct, but we also know that there are more causes of evolution than Darwin realized, and that natural selection and hereditary variation themselves are more complex than he imagined. A body of ideas about the causes of evolution, including mutation, recombination, gene flow, isolation, random genetic drift, the many forms of natural selection, and other factors, constitute our current theory of evolution or "evolutionary theory." Like all theories in science, it is a work in progress, for we do not yet know the causes of all of evolution, or all the biological phenomena that evolutionary biology will have to explain. Indeed, some details may turn out to be wrong. But the main tenets of the theory, as far as it goes, are so well supported that most biologists confidently accept evolutionary theory as the foundation of the science of life.No doubt you're puzzled about the purpose of this posting. You are probably saying to yourself. "So what? We all know that, already."
Apparently, not all of us agree. In an otherwise excellent review of Richard Dawkin's book, Jerry Coyne says the following [see: The Improbability Pump].
Demonstrating the truth of natural selection is just one of Dawkins's aims, for the theory of evolution is composed of several more or less independent parts, which I like to describe in one longish sentence: "Life on earth evolved gradually, beginning with one primitive species; it then branched out over time, throwing off many new and diverse species--and the process producing the illusion of design in organisms is natural selection." This sentence constitutes a scientific theory, which is not just a guess but an informed statement about the general principles that explain many observations about nature.I think that's very wrong. First, it's wrong because it states that the history of life is a theory. Second, it's wrong because it states that the "illusion of design" is part of modern evolutionary theory (it isn't). Third, it's wrong because it only mentions natural selection and modern evolutionary theory is much more than that.
I hope this was just an attempt to (over-)simplify evolution for the readers of The Nation. In that case it might be (just) excusable. But I can't wait until the creationists get a hold of this review. They'll be delighted to learn that, according to Jerry Coyne, the gradual descent and diversification of life is only a theory.
They'll also be happy to learn from a prominent evolutionary biologist that design is part of modern evolutionary theory.