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Karl Popper: On the Scientific Status of Darwin’s theory of Evolution

When speaking here of Darwinism, I shall speak always of today’s theory–that is Darwin’s own theory of natural selection supported by the Mendelian theory of heredity, by the theory of the mutation and recombination of genes in a gene pool, and the decoded genetic code. This is an immensely impressive and powerful theory. The claim that it completely explains evolution is of course a bold claim, and very far from being established. All scientific theories are conjectures, even those that have successfully passed many and varied tests. 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.

However, Darwin’s own most important contribution to the theory of evolution, his theory of natural selection, is difficult to test. There are some tests, even some experimental tests; and in some cases, such as the famous phenomenom known as "industrial melanism", we can observe natural selection happening under our very eyes, as it were. Nevertheless, really severe tests of the theory of natural selection are hard to come by, much more so than tests of otherwise comparable theories in physics or chemistry.

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. A tautology like "All tables are tables" is not, of course, testable; nor has it any explanatory power. It is therefore most surprising to hear that some of the greatest contemporary Darwinists themselves formulate the theory in such a way that it amounts to the tautology that those organisms that leave the most offspring leave the most offspring. And C.H. Waddington even says somewhere (and he defends this view in other places) that "Natural selection … turns out … to be a tautology". However, he attributes at the same place to the theory an "enormous power … of explanation". Since the explanatory power of a tautology is obviously zero, something must be wrong here.

Yet similar passages can be found in the works of such great Darwinists as Ronald Fisher, J.B.S. Haldane, and George Gaylord Simpson; and others.

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. It raises detailed problems in many fields, and it tells us what we would expect of an acceptable solution of these problems.

I still believe that natural selection works this way as a research programme. Nevertheless, 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. My recantation may, I hope, contribute a little to the understanding of the status of natural selection.

From "Natural Selection and the Emergence of Mind", Dialectica, vol. 32, no. 3-4, 1978, pp. 339-355