In a 1981 article in Science Digest, Duane Gish, the master debater among creationists, said:

There were no human witnesses to the origin of the Universe, the origin of life or the origin of a single living thing. These were unique, unrepeatable events of the past that cannot be observed in nature or repeated in the laboratory. Thus neither creation nor evolution qualifies as a scientific theory and each is equally religious. As the scientific philosopher Sir Karl Popper has stated, evolution is not a testable scientific theory but a metaphysical research program. [Asimov and Gish, p. 82]

The most direct rebuttal one can give to these charges is that Gish and other creationists really don't believe them! The underlying point of the above quotation is that evolution is unscientific because it is not falsifiable (testable), yet creationists are always producing arguments and "evidences" that they say refute evolution. Gish does it in the article quoted above. In spite of that obvious contradiction, the argument impresses laypeople and legislators. But it completely distorts what Popper calls the logic of scientific discovery.

So what does Popper really say about evolution?

Indeed, the recent vogue of historicism might be regarded as merely part of the vogue of evolutionism—a philosophy that owes its influence largely to the somewhat sensational clash between a brilliant scientific hypothesis concerning the history of the various species of animals and plants on earth, and an older metaphysical theory which, incidentally, happened to be part of an established religious belief.

What we call the evolutionary hypothesis is an explanation of a host of biological and paleontological observations—for instance, of certain similarities between various species and genera—by the assumption of common ancestry of related forms.

. . . I see in modern Darwinism the most successful explanation of the relevant facts. [Popper, 1957, p. 106; emphasis added]

There exists no law of evolution, only the historical fact that plants and animals change, or more precisely, that they have changed. [Popper, 1963b, p. 340; emphasis added]

I have always been extremely interested in the theory of evolution and very ready to accept evolution as a fact. [Popper, 1976, p. 167; emphasis added]

The Mendelian underpinning of modern Darwinism has been well tested and so has the theory of evolution which says that all terrestrial life has evolved from a few primitive unicellular organisms, possibly even from one single organism. [Popper, 1978, p. 344; emphasis added]

Furthermore, in his book, Objective Knowledge, where he uses the Darwinian paradigm as a basis for his own theory of knowledge, Popper not only discusses Darwinism at length as a scientific explanation but offers as an additional component a scientific hypothesis of his own—genetic dualism—which is intended to strengthen the orthodox neo-Darwinian framework (Popper, 1972, p. 242 ff). Popper's genetic dualism is similar to the ideas of Wilson and Stebbins (Stebbins, 1977, p. 125) and Mayr (1963, p. 604 ff.; 1970, p. 363 ff.) concerning the role of behavior in evolution.

But he did make one mistake—for which we should forgive him; some well-known biologists (who should know better) have made the same mistake. Popper takes "survival of the fittest" as the definition of natural selection (Popper, 1972, p. 241). This catchy phrase was an invention of Herbert Spencer, which Darwin, in a rare example of bad judgment, interpolated into later editions of On the Origin of Species: "This preservation of favorable individual differences and variations and the destruction of those which are injurious I have called Natural Selection or the Survival of the Fittest" (p. 64). Clearly it is an alternate name (and not a very apt one) for the process in question but not a definition.

The argument regarding "survival of the fittest" is that the only way one can usually tell who the fittest are is to see who survives. But then survival of the fittest becomes "almost a tautology" and hence untestable (Popper, 1972, p. 69; 1963a, p. 964).

I have come to the conclusion that Darwinism is not a testable scientific theory, but a metaphysical research programme—a possible framework for testable scientific theories. [Popper, 1976, p. 168]

It is clear that here Darwinism means natural selection, not evolution. Popper states this explicitly earlier in the same work:

. . . because I intend to argue that the theory of natural selection is not a testable scientific theory, but a metaphysical research programme; . . . [Popper, 1976, p. 151]

There are two points to be made here:

First, natural selection being untestable is not the same as evolution being untestable. Evolution, to the creationist, is any hypothesis about origins. Astrophysical theories about stellar evolution or the "Big Bang" cosmology or scientific geology or, for that matter, many facets of biological evolution are not based upon Darwinian natural selection.

Second, Popper later admitted that he was wrong!

The fact that the theory of natural selection is difficult to test has led some people, anti-Darwinists and even some great Darwinists, to claim that it is a tautology. . . . I mention this problem because I too belong among the culprits. Influenced by what these authorities say, I have in the past described the theory as "almost tautological," and I have tried to explain how the theory of natural selection could be untestable (as is a tautology) and yet of great scientific interest. My solution was that the doctrine of natural selection is a most successful metaphysical research programme. . . . [Popper, 1978, p. 344]

I have changed my mind about the testability and logical status of the theory of natural selection; and I am glad to have an opportunity to make a recantation. . . . [p. 345]

The theory of natural selection may be so formulated that it is far from tautological. In this case it is not only testable, but it turns out to be not strictly universally true. There seem to be exceptions, as with so many biological theories; and considering the random character of the variations on which natural selection operates, the occurrence of exceptions is not surprising. [p. 346]

Thus the creationists were never correct in stating that Popper believed that evolution was not falsifiable (and hence not scientific), nor are they now correct in citing him as an authority for the claim that natural selection is tautological and not falsifiable!

Some might challenge my point that Popper never doubted the testability of evolution by citing the following:

I blush when I have to make this confession; for when I was younger, I used to say very contemptuous things about evolutionary philosophies. When twenty-two years ago Canon Charles E. Raven, in his Science, Religion, and the Future, described the Darwinian controversy as "a storm in a Victorian teacup," I agreed, but criticized him for paying too much attention "to the vapors still emerging from the cup," by which I meant the hot air of the evolutionary philosophies (especially those which told us that there were inexorable laws of evolution). But now I have to confess that this cup of tea has become, after all, my cup of tea; and with it I have to eat humble pie. [Popper, 1972, p. 241]

But in an earlier work, he explicitly identified these "vapors" as "the Great Systems of Evolutionist philosophy, produced by Bergson, Whitehead, Smuts and others" (Popper, 1957, p. 106). He was not speaking, then, of the scientific theory of evolution but of various metaphysical theories. He made a clear distinction between the two.

And his current support for the Darwinian idea of natural selection is expressed in equally plain language.

What Darwin showed us was that the mechanism of natural selection can, in principle, simulate the actions of the Creator and His purpose and design, and that it can also simulate rational human action directed towards a purpose or aim. [Popper, 1972, p. 267; see also Popper, 1978, pp. 342-343]

As for the notion of design as a useful hypothesis:

His theory of adaptation was the first nontheistic one that was convincing; and theism was worse than an open admission of failure, for it created the impression that an ultimate explanation had been reached. [Popper 1976, p. 172]

There are scientists who are unfamiliar with or misinterpret Popper. For example, Colin Patterson holds that, if we accept Popper's distinction between science and nonscience, evolution is not science because it deals with unique historical events. Popper, however, doesn't agree with this.

It does appear that some people think that I denied scientific character to the historical sciences, such as palaeontology, or the history of the evolution of life on Earth. This is a mistake, and I here wish to affirm that these and other historical sciences have in my opinion scientific character; their hypotheses can in many cases be tested. [Popper, 1981, p. 611]

In an earlier work, Popper discussed the historical sciences in which the scientific method of theoretical sciences is used:

This view is perfectly compatible with the analysis of scientific method, and especially of causal explanation given in the preceding section. The situation is simply this: while the theoretical sciences are mainly interested in finding and testing universal laws, the historical sciences take all kinds of universal laws for granted and are mainly interested in finding and testing singular statements. [Popper, 1957, p. 143ff]

What Popper calls the historical sciences do not make predictions about long past unique events (postdictions), which obviously would not be testable. (Several recent authors—including Stephen Jay Gould in Discover, July 1982—make this mistake.) These sciences make hypotheses involving past events which must predict (that is, have logical consequences) for the present state of the system in question. Here the testing procedure takes for granted the general laws and theories and is testing the specific conditions (or initial conditions, as Popper usually calls them) that held for the system.

A scientist, on the basis of much comparative anatomy and physiology, might hypothesize that, in the distant past, mammals evolved from reptiles. This would have testable consequences for the present state of the system (earth's surface with the geological strata in it and the animal and plant species living on it) in the form of reptile-mammal transition fossils that should exist, in addition to other necessary features of the DNA, developmental systems, and so forth, of the present-day reptiles and mammals.

What about repeatability? It is the observations that must be repeatable, if only to establish their validity independently of any one person's authority. This does not mean that the hypothetical mechanism or the phenomenon concerned must be repeatable or reproducible. In the experimental laboratory where the phenomena being studied are short-lived and transient, it is usually necessary to reproduce them in order to repeat the observations. But scientists must wait for the recurrence of natural phenomena—such as eclipses, earthquakes, seasonally recurring biological phenomena, and so forth. Yet, if a phenomenon is a stable, more or less permanent long-term condition, observations may be repeated anytime. A geologist may return to a geological formation to repeat or make new observations, or an anatomist or paleontologist may reexamine a museum specimen, either corroborating or refuting someone else's previous observations. Clearly, then, a hypothesis postulating a unique past event is scientific—as long as it has observable consequences for the present that can be repeatedly verified by any observer.

Thus we may conclude (as Popper did) that evolutionary theories or historical hypotheses about origins are no different than other scientific theories as far as their logical features are concerned and are just as falsifiable as hypotheses in the form of general laws and theories.