(Note: the blue horizontal menu bar directly above lists the subsections of "Evolution According to Contemporary Science." Beginning with "How Has Life Evolved?," be sure to read each of these subsections before moving on to the next primary section, "Evil, Ethics, and Human Values in an Evolving World.")

What Is a Theory?

Let's dispose of some semantic confusion about evolution being "just a theory" and not a "fact." In common parlance, a "theory" is often no better than a guess, and hence the weakest of assertions—what a scientist would call a mere "hypothesis." But scientists themselves consider a theory to be one of the strongest of assertions, so well tested and supported by so many facts that it is actually more worthy of belief (and harder to refute) than any single "fact" or observation that contributes to it. A theory in this sense could even be called a "metafact," a fact about facts: it is not only itself true beyond reasonable doubt, it also explains and gives meaning to a multitude of relatively trivial, seemingly unrelated facts.

Thus, the atomic theory, the theory of relativity, quantum theory, the heliocentric theory, and the germ theory of disease are correctly spoken of as theories, and at the same time they are among the biggest, best-supported facts we know. They are true only within limits (relativity and quantum mechanics will someday be subsumed under a more general theory; the sun is only the center of our solar system, not of the whole universe; some but not all maladies are caused by germs), but within those limits they are incontrovertibly true.

Evolution As a Theory. Likewise, evolution (Darwin's "descent with modification") is both a theory in the strictest sense, and a rock-solid fact. All organisms living today have evolved from past ones that were very different. Darwin's more specific theory of natural selection (his explanation of how evolution occurs) is also a well-tested theory, as well as a fact as far as it goes (natural selection does occur and does produce adaptation and evolution). The only thing biologists still argue about is whether it explains all adaptive evolution.

Observations, facts, hypotheses, and theories in science always remain, in principle, subject to rechecking, retesting, and fine-tuning, but once theories have passed as many tests as Darwin's, they are fully entitled to be ranked among the best-established facts we have.