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Origin and evolution of life

The creation–evolution controversy raised the issue of whether creationistic ideas may be legitimately called science. In the debate, both sides and even courts in their decisions have invoked Popper's criterion of falsifiability (see Daubert standard). In this context, passages written by Popper are frequently quoted in which he speaks about such issues himself. For example, he famously stated " Darwinism is not a testable scientific theory, but a metaphysical research program—a possible framework for testable scientific theories." He continued:

And yet, the theory is invaluable. I do not see how, without it, our knowledge could have grown as it has done since Darwin. In trying to explain experiments with bacteria which become adapted to, say, penicillin, it is quite clear that we are greatly helped by the theory of natural selection. Although it is metaphysical, it sheds much light upon very concrete and very practical researches. It allows us to study adaptation to a new environment (such as a penicillin-infested environment) in a rational way: it suggests the existence of a mechanism of adaptation, and it allows us even to study in detail the mechanism at work.

He noted that theism, presented as explaining adaptation, "was worse than an open admission of failure, for it created the impression that an ultimate explanation had been reached". Popper later said:

When speaking here of Darwinism...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 severe and varied tests. The Mendelian underpinning of modern Darwinism has been well tested, and so has the theory of evolution....

He explained that the difficulty of testing had led some people to describe natural selection as a tautology, and that he too had in the past described the theory as "almost tautological", and had tried to explain how the theory 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 in 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.

Popper summarised his new view as follows:

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. Thus not all phenomena of evolution are explained by natural selection alone. Yet in every particular case it is a challenging research program to show how far natural selection can possibly be held responsible for the evolution of a particular organ or behavioural program.

These frequently quoted passages are only a small part of what Popper wrote on evolution, however, and may give the wrong impression that he mainly discussed questions of its falsifiability. Popper never invented this criterion to give justifiable use of words like science. In fact, Popper stressed that "the last thing I wish to do, however, is to advocate another dogma" and that "what is to be called a 'science' and who is to be called a 'scientist' must always remain a matter of convention or decision." He quotes Menger's dictum that "Definitions are dogmas; only the conclusions drawn from them can afford us any new insight" and notes that different definitions of science can be rationally debated and compared:

I do not try to justify, however, by representing them as the true or the essential aims of science. This would only distort the issue, and it would mean a relapse into positivist dogmatism. There is only one way, as far as I can see, of arguing rationally in support of my proposals. This is to analyse their logical consequences: to point out their fertility—their power to elucidate the problems of the theory of knowledge.

Popper had his own sophisticated views on evolution that go much beyond what the frequently-quoted passages say. In effect, Popper agreed with some points of both creationists and naturalists, but disagreed with both on crucial aspects. Popper understood the universe as a creative entity that invents new things, including life, but without the necessity of something like a god, especially not one who is pulling strings from behind the curtain. He said that evolution of the genotype must, as the creationists say, work in a goal-directed way but disagreed with their view that it must necessarily be the hand of god that imposes these goals onto the stage of life.

Instead, he formulated the spearhead model of evolution, a version of genetic pluralism. According to this, living organisms have goals, and act according to these goals, each guided by a central control. In its most sophisticated form, this is the brain of humans, but controls also exist in much less sophisticated ways for species of lower complexity, such as the amoeba. This control organ plays a special role in evolution—it is the "spearhead of evolution". The goals bring the purpose into the world. Mutations in the genes that determine the structure of the control may then cause drastic changes in behaviour, preferences and goals, without having an impact on the organism's phenotype. Popper postulates that such purely behavioural changes are less likely to be lethal for the organism compared to drastic changes of the phenotype.

Popper contrasts his views with the notion of the "hopeful monster" that has large phenotype mutations and calls it the "hopeful behavioural monster". After behaviour has changed radically, small but quick changes of the phenotype follow to make the organism fitter to its changed goals. This way it looks as if the phenotype were changing guided by some invisible hand, while it is merely natural selection working in combination with the new behaviour. For example, according to this hypothesis, the eating habits of the giraffe must have changed before its elongated neck evolved. Popper contrasted this view as "evolution from within" or "active Darwinism" (the organism actively trying to discover new ways of life and being on a quest for conquering new ecological niches), with the naturalistic "evolution from without" (which has the picture of a hostile environment only trying to kill the mostly passive organism, or perhaps segregate some of its groups).

Popper was a key figure encouraging patent lawyer Günter Wächtershäuser to publish his iron–sulfur world hypothesis on abiogenesis and his criticism of "soup" theory.

On the creation-evolution controversy, Popper initially wrote that he considered it

a 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

with a footnote to the effect that he

agree with Professor C.E. Raven when...he calls this conflict 'a storm in a Victorian tea-cup'...

In his later work, however, when he had developed his own "spearhead model" and "active Darwinism" theories, Popper revised this view and found some validity in the controversy:

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.