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Did Popper refute evolution?
Skeptical Inquirer, Sept-Oct, 2004 by Massimo Pigliucci
The neo-Darwinian theory of evolution is the currently accepted paradigm to explain the history and diversity of life on Earth. Yet ever since the publication of Darwin's Origin of Species in 1859, the theory has been under attack on a variety of grounds. Readers of SI are familiar with standard creationist nonsense, as well as with the slightly more sophisticated sophistry of "intelligent design" proponents. Some of the criticisms of evolutionary theory have been put forth in the professional philosophical arena, where serious scholars have often accused it of being incoherent or logically fallacious.
Perhaps the best-known philosophical criticism of evolution was put forth by Karl Popper, who once claimed that "Darwinism is not a testable scientific theory, but a metaphysical research program" (Unended Quest, 1976). Popper famously retracted his comments once it was explained to him that there was quite a bit more to the theory than he had understood from a cursory examination of the subject: "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" (Dialectica 32:344-346).
Besides being an impressive example of a famous scholar admitting in public that he was wrong (when was the last time a creationist has done that?), the episode is important for two reasons. First, it is interesting to understand where Popper's original criticism came from; second, it is crucial to realize exactly why he recanted. One of Popper's chief interests in philosophy of science was the so-called demarcation problem, something very dear to skeptics. The problem consists of identifying the criteria that separate science from pseudoscience. Popper took, not surprisingly, physics and astronomy to be quintessential examples of good science. On the other hand, he considered Marxist history, Freudian psychoanalysis, and astrology as examples of pseudoscience. The task, then, could be rephrased in terms of pinning down what distinguishes these two groups of theories. As it turns out, Popper's approach was vitiated by his failure to appreciate the heterogeneity intrinsic in both the categories of "science" and "pseudoscience," i.e., in the fact that not all science (or pseudoscience) is created equal, operates by the same criteria, and can therefore be defined in similar manner. But I will return to that topic in a future column.
As far as we are concerned here, Popper proposed his famous criterion of falsification to solve the demarcation problem: good science is done when hypotheses can be shown to be false (if they indeed are). That's where the philosopher's criticism of evolutionary theory originated from. Popper understood evolutionary biologists to say that their theory predicts that natural selection allows only the fittest organisms to survive; but, he countered, the "fittest" organisms are defined as those who survive, which makes the statement tautological. Now, in philosophy being a tantology isn't necessarily a bad thing: after all, tautologies are the bread and butter of logic and mathematics--in the sense that one logically deduces consequences from premises that are taken as given, like definitions--and consider how many interesting things have come out of both logic and mathematics. That is why Popper initially concluded that, though tautological, evolutionary theory was a useful "metaphysical" (in the philosophical, not religious, sense) program, i.e., an overarching idea that could provide a powerful framework to interpret the biological world. But, as in the case of Freudian psychoanalysis or Marxist history, it wasn't good science.
Why, then, did the Austrian philosopher change his mind? Because it turns out that while it is true that evolutionary biologists predict (i.e., deduce) the survival of the fittest (and the much more important fact that s/he is going to have the most offspring) by means of natural selection, they have independent ways to assess which members of a population of organisms actually are the "fittest." For example, biologists employ optimization analyses to predict which combinations of morphological, behavioral, or physiological traits are more likely to be advantageous (i.e., to increase "fitness") in the range of environments actually encountered by a given living form. They then sample natural populations of organisms, determine in which environments they actually live, measure those traits they hypothesize are more likely to make a difference, and obtain statistical predictions on where natural selection should push the population next. Finally, biologists wait until the next generation of organisms comes out and measure their characteristics again.
If the theory were correct (and given some other verifiable conditions, such as the presence of adequate genetic variation for the traits in question), the population's mean for the characters under selection should have shifted in the predicted direction. This is an eminently falsifiable hypothesis, in the same sense in which predictions made by astronomers or physicists are falsifiable, and very much unlike the explanations of human behavior put forth by psychoanalysts, which are notoriously so flexible that they can fit (a posterion) virtually any observed pattern.
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