Did Karl Popper Doubt Evolution?

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"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."

To biologists, these words, spoken by eminent philosopher of science Karl Popper, might seem like a betrayal. To creationists, a victory. Popper, the ardent empiricist and architect of falsifiability – the notion that for something to be scientific it must be testable – seemed to have doubts about evolution by means of natural selection.

It's partially true, but only in a philosophical, some might say "semantic", sense. Popper particularly took umbrage with evolutionary theory's notion of "survival of the fittest," believing it to be a tautology – it assumes the very thing that is meant to be proven through testing the theory. In 1969 lectures at Emory University, Popper said:

Biologists (especially Fisher) felt compelled to define as “more fit” those which more often survive. Thus, what once looked like a promising explanatory theory becomes quite empty. The statement “Evolution tends to produce higher forms because only the fittest survive” may sound like an explanation. But if we substitute here for “the fittest” its defining phrase, we get: “Evolution tends to produce higher forms because those forms which more often survive more often survive.” So our “because” phrase has degenerated into a tautology. But tautology cannot explain anything. All tautologies are equivalent to “All tables are tables” or “Those who live long are those who live long.”

Yet, at the same time he was irked by this facet of evolutionary theory, Popper noted "I see in modern Darwinism the most successful explanation of the relevant facts." He also was no fan of theism, calling it "worse than an open admission of failure, for it created the impression that an ultimate explanation had been reached."

But Popper, flexing the intellectual muscle of any true empiricist, later retracted his philosophical critique of Darwinism.

“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 because I too belong among the culprits. . . . My solution was that the doctrine of natural selection is a most successful metaphysical research programme. . . . I still believe that natural selection works in this way as a research programme. Nevertheless I have changed my mind about the testability and the logical status of the theory of natural selection; and I am glad to have an opportunity to make a recantation...”

Frank J. Sonleitner, an Associate Professor Emeritus of Biology at the University of Oklahoma, explained why Popper changed his mind:

"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."

"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."

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