Biologists often say that "evolution is a fact" (see, for example, Futuyma, 1979; Edwords, 1987), and creationists often say that "evolution is just a theory." To evaluate the truth in these contradictory statements, one needs to examine fact and theory and the context in which the terms are used.
The most basic facts in science are the "brute, sensory facts" from perceptions which are shared and on which we agree. From these sensory facts, scientists build facts and concepts of increasing complexity. When there is solid agreement on the statement of a complexity, scientists may call the statement a fact. Holton reported on Einstein's use of fact:
Among facts, Einstein in various writings included inertial motion, the constancy of light velocity, the equality of gravitational and inertial mass, and the impossibility of constructing perpetual motion machines. Nevertheless in the most primitive form . . . [facts] can be thought of as simple sensory impressions.
When biologists say that "evolution is a fact," I think they mean that they accept the following statement so firmly that they consider it to be as true as any basic sensory fact: each species arose from another species that preceded it in time, and higher taxa arose by a continuation of the speciation processes. The term fact as commonly applied to such statements signifies not the kind of content in the statements but, rather, the strength of our acceptance of the statements. So, if we are willing to accept a broad definition of fact, biologists are correct in saying that "evolution is a fact."
But in the context of Darwin's The Origin of Species by Means of Natural Selection and the modern theories of evolution, "evolution is a fact" may tend to block a full view of the major theories and the hundreds of subtheories found in the study of evolution (Lewis, 1980). To consider this possible blockage, one must examine the meaning of theory, a term that is often misused to mean a notion, a deduction, a single idea or postulate, or anything an author is unsure of.
Futuyma quoted the definition of theory from the Oxford English Dictionary: "a statement of the general laws, principles, or causes of something known or observed" (Futuyma, 1979). This definition is adequate in that a theory must contain a statement of its basic premises. If one cannot state the basic premises (postulates) of a theory being discussed, one has not even begun to know the theory.
The Oxford definition, however, is inadequate because it is incomplete. A theory is better defined as a quasi-geometric pattern of reasoning containing a few ideas given in postulates and containing lines of reasoning that (1) may use facts to support a postulate or (2) may use postulates and facts to explain other facts or to predict possible new facts. This definition of theory is derived from many sources (including Braithwait, 1953; Suppe, 1974; Holton. 1979). To test this view of theory, I have studied and collected the postulates of more than six hundred theories—about five hundred from biology and the remainder from other disciplines. Over one hundred lists of these postulates, which were taken from recent published papers, were sent to their authors for corrections and comments. From the more than 80 percent who replied, I gathered that my view of theory was acceptable to these authors.
In light of the above definitions of fact and theory, now examine The Origin of Species, look at most textbook discussions of Darwinian evolution, and then review recent research papers on the subdisciplines of biology.
In the first edition of the Origin, "theory of descent with modification" occurs twice in the table of contents, seven times in the concluding chapter, and a number of times in the remainder of the book. "Theory of natural selection" occurs three times in the contents, five times in the final chapter, and many times in the rest of the book. In a few places, Darwin says, "Theory of descent with modification through natural selection" and many times he says "my theory." In almost every place where he used these terms, the discussion that follows refers to the descent theory or to the natural selection theory. A rereading of the Origin with the above terms in mind as one follows Darwin's arguments will convince readers that Darwin gave us two major theories—the kinematic theory of descent with modification and the dynamic theory of natural selection. (A kinematic theory deals with noncausal relations between things and/or events. A dynamic theory deals with mechanisms and causes of things and/or events.)
In the present era of overt hypothetico-deductive biology, the descriptions of Darwinian evolution ought to make clear that Darwin gave us two major theories. Most textbook authors treat natural selection well, but they fail to treat the theory of descent with modification as an active theory. If this failure stems from the acceptance of the statement "evolution is a fact," then the statement needs careful qualifications about what is being accepted as fact and what should be accepted as theory.
The descent theory is active in two ways: some of its postulates are being tested, and it is spawning hundreds of subtheories. (A subtheory is a theory whose postulates are consistent with those of its over-theory and whose postulates make it possible to apply the over-theory in a special limited range.) The study of punctuated equilibrium may require the clarification of two of the descent theory postulates: (1) evolutionary changes were gradual and of long duration (however, see the quotation from Origin in Sonleitner, 1987), and (2) the geologic record is very incomplete.
Most theories in paleontology, comparative anatomy, taxonomy, geographic distribution, and even many in molecular biology are subtheories of the descent theory. These subtheories say nothing about the mechanism of evolution, so they are clearly a part of the descent theory system of theories. (For views of the mechanism system of theories, see, Caplan, 1978; Lewis, 1980; Tuomi, 1981.)
Leaders in the study of evolution were well aware of Darwin's two theories (see, Lewis, 1980, p. 555), but, since the time of Fisher, they chose to concentrate on the mechanism of evolution, thus accounting in part for the neglect of the descent theory in most textbooks. One textbook whose authors contained a leader was, however, very clear: "First, there is the theory of evolution in the strict sense. . . . Second, there is the theory of natural selection" (Simpson, Pittendrigh, and Tiffany, 1957).
To say that "evolution is a fact" and to imply that the theory of descent with modification is complete and finished as a theory would misrepresent evolutionary biology. Since most textbooks today do not explicitly discuss the descent theory as an active theory, a large misrepresentation is being passed on to students. If this stems in part from saying that "evolution is a fact," then care must be taken to make sure exactly what is meant by "evolution is a fact."
To say that "evolution is just a theory" displays a great ignorance of the meaning of theory. It implies that theories are flighty somethings that are of little import. Yet, those who know science properly know that theories are the most powerful intellectual tools for the discovery of knowledge. They know that there are highly tentative theories, very strongly supported theories, and a range of in-between theories. They know that strongly supported theories that have been tested for years will probably remain in established knowledge forever and that theories of this sort might be called facts.
I wish to thank Alain F Corcos and Floyd Monaghan for criticisms and suggestions.