I’ve long been uncomfortable with explaining, in public lectures, why evolution is both a theory and a fact. To do that properly, you have to explain what a scientists really mean by the word “theory” and why it’s not just an idle guess or speculation. But that can be confusing, because I always say that a “theory” is an explanatory schema that makes sense of a body of facts, a sort of organizing principle for thinking about things that happens to explain all the data. But I’ve never been comfortable with that, as even guesses can make sense of a body of facts, but have not been tested nearly as rigorously as the theory of evolution. “String theory,” for example, makes sense of some things, but nobody knows whether it’s the correct explanation for particles. I always have the feeling that I’m confusing my audience when I explain why evolution is a theory, and then go on to show that what I see as the five pillars of evolution—evolutionary change, relatively gradualistic change of populations (i.e., change over many generations rather than a few), natural selection as the process producing “design” in nature, common descent, and speciation—are actually facts. 

In his famous essay “Evolution as fact and theory“, Steve Gould also made this distinction (my emphasis):

Well, evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world’s data. Theories are structures of ideas that explain and interpret facts. Facts do not go away when scientists debate rival theories to explain them. Einstein’s theory of gravitation replaced Newton’s, but apples did not suspend themselves in mid-air, pending the outcome. And humans evolved from apelike ancestors whether they did so by Darwin’s proposed mechanism or by some other, yet to be discovered.

Moreover, “fact” does not mean “absolute certainty.” The final proofs of logic and mathematics flow deductively from stated premises and achieve certainty only because they are not about the empirical world. Evolutionists make no claim for perpetual truth, though creationists often do (and then attack us for a style of argument that they themselves favor). In science, “fact” can only mean “confirmed to such a degree that it would be perverse to withhold provisional assent.” I suppose that apples might start to rise tomorrow, but the possibility does not merit equal time in physics classrooms.

. . . Evolutionists have been clear about this distinction between fact and theory from the very beginning, if only because we have always acknowledged how far we are from completely understanding the mechanisms (theory) by which evolution (fact) occurred.

While I don’t have a big beef with this passage, which is of course very famous, it’s also confusing. If you claim that the “mechanism” for evolutionary change that produces design-like features of organisms is natural selection, is that claim a theory or a fact? It’s not only a theory, for we have enough evidence for it—and no credible alternative theory—that we can take it as provisional truth (i.e., fact). So is there a real difference, as Gould claims, between the “idea” of natural selection (Gould’s “theory”) and the observations that it works in nature, has a sound theoretical basis, and has no other explanation when referring to adaptations (Gould’s “fact”)? Is the notion that lineages split by a process of speciation that renders them unable to exchange genes (a process), a “theory,” as Gould would have it, or a fact? I see it as a fact, and so the distinction between theory and fact that, claims Gould, all biologists accept, is nebulous.

At one point, when Darwin gestated and then birthed the theory encompassing my five points, that theory had not yet attained the status of Facthood, for there simply weren’t enough data on, say, speciation or natural selection in the wild to say that Evolution was a Fact. But over the last 150 years, Facthood has been attained.

I was thus pleased to see that Richard Dawkins has taken a break from tweeting to write a good popular analysis on his organization’s website of the confusing distinction between evolution as theory and fact: “Is it a theory? Is it a law? No, it’s a fact.” In that essay, which is well worth reading, Dawkins encapsulates the ambiguity and confusion many of us feel while teaching evolution, and resolves it in a simple way: stop arguing that evolution is a theory, and emphasize instead that it’s a fact. (It is, of course, both, but it confuses people to dwell on the “theory” claim unless you’re debating someone who argues that “evolution is only a theory.”):

The party line among scientists arguing for evolution is to promote Sense 1 [the way I define theory above, as opposed to “Sense 2,” the popular notion of a theory as an idle guess or speculation], and I have followed it until today. But now I want to depart from the party line. I now think that trying to clear up this terminological point about the meaning of “theory” is a losing battle. We should stop using “theory” altogether for the case of evolution and insist, instead, that evolution is a fact.

. . . We are failing to get across “Theory, Sense 1”. Let’s dump it and talk frankly of evolution as a fact, from which it would be perverse to hold assent.

The notion of “fact” that Dawkins uses above also comes of course from Gould’s essay, as shown above. And I think that’s a good definition of scientific fact, with the caveat that “withholding assent” refers to those people who are qualified to judge scientific evidence. (If you don’t use that caveat, then more than 70% of Americans do not give their assent to a purely naturalistic theory of evolution.)

What remains a theory—or even a hypothesis—is the claim that most of evolutionary change is driven by natural selection. As I said above, I think we have enough evidence that what Dan Dennett calls the “designoid” features of organisms—the spines of the cactus, the cryptic coloration of a flatfish, the insect-entrapping shape of a bucket orchid, the fusiform shape of dolphins, and so on—result from natural selection. But we don’t know what proportion of all evolutionary change (and that’s itself ambiguous: do we mean changes in characters, or changes in genes?) is due to selection versus other evolutionary forces like genetic drift. One can make a good case, for instance, that among all alterations in the genome of a lineage, most of the DNA changes are due to drift rather than selection. So I’m happy, with the proper caveats, to regard as a hypothesis the statement that “most of the change in an organism is due to natural selection,” while accepting as a theory (or fact) the statement “nearly all the ‘designoid’ features of organisms are due to natural selection.” This is, of course, also confusing to non-biologists, and so I agree with Richard when he says this:

In our tussles with creationists it is evolution itself rather than natural selection that bears the brunt of their attacks. So we can set aside the status of natural selection and concentrate on the fact of evolution as something so firmly established by evidence that to deny it would be perverse. It is a fact, beyond all reasonable dispute, that if you trace your ancestry and your dog’s ancestry backwards you’ll eventually hit a common ancestor. It is a fact, beyond reasonable dispute, that when you eat fish and chips you are eating distant cousin fish and even more distant cousin potato.

So I’m happy to simply avoid explaining why evolution is both a theory and a fact, and in future lectures will say it’s both, but not go on to the confusing discussion of “theory” unless someone asks in the Q&A. I’d prefer, as this is what I lecture on anyway when giving the evidence for evolution, to claim that the important idea is that evolution (at least the five tenets I give above), is a FACT. And of course in my talks to the public I do mention natural selection and the evidence supporting its pervasiveness.

Richard winds up by arguing that evolution is not a “law,” and again I fully agree with him. But that’s a minor issue, for few evolutionists would even use the word “law,” which to us means something like the “laws” of physics: regularities that are never violated. Evolution is not a law in this sense, for there are virtually no statements you can make about the process that hold across all 3.8 billion years of evolution. The only one I can think of is that “all lineages evolve genetically,” for I don’t think there were or are any exceptions to that. But all other generalizations about evolution are subject to qualifications and exceptions.