Darwin, evolution, and Popper
Popper originally said that evolution (by which he meant natural selection) was a "metaphysical research programme". Popper, unlike the logical positivists he opposed, held that metaphysical programmes were an essential element of science, and that without them, theories were effectively dead in the water.
The typical metaphysical research programme Popper gives in his Unended Quest sect 33, is metaphysical realism. He says that it, "the view that there is a physical world to be discovered" , is "a faith ... without which practical action is hardly conceivable" [150, quoting his own Logic of scientific discovery, sect 79]. This is the very basis of scientific research. So being a metaphysical research programme is not a bad thing for him. He then says that he introduced this because
I intend to argue that the theory of natural selection is not a testable scientific theory, but a metaphysical research program; and although it is no doubt the best at present available, it can perhaps be slightly improved .Now let's look at what he did then say in sect 37. First he outlines what the New Synthesis as he understands it consists of claiming: (1) An evolutionary tree and history, (2) an evolutionary theory which explains this, consisting of (a) heredity, (b) variation, (c) natural selection (NS), (d) variability (which can be controlled by NS). He is confused here, I think, but it is clear that NS is one aspect of the theory that underlies explanation of evolution itself . He is using the term "Darwinism" for this set of explanatory schemes.
Then he says why he thinks "Darwinism" is metaphysical and a research programme. "It is metaphysical because it is not testable." Darwinism does not predict the evolution of variety, he says. Therefore it cannot explain it. "At best it can predict the evolution of variety under "favourable conditions". But it is hardly possible to describe in general terms what favourable conditions are---except that, in their presence, a variety of forms will emerge." Then he raises the tautology claim, saying "To say that a species now living is adapted to its environment is, in fact, almost tautological." Almost, note. Then he says that "Adaptation or fitness is defined by modern evolutionists as survival value, and can be measured by actual success in survival: there is hardly any possibility of testing a theory as feeble as this." 
Note that Popper allows there is a possibility of testing NS, and that it is almost a tautology, not an actual one. We mustn't make Popper say more than he did.
Then he says this:
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. And it is the only theory so far which does all that. [171-172].So it is a theory of science, it does help research, and it is to be preferred, says Popper, even before his recantation.
Moreover, he notes that theism as an explanation of adaptation "was worse than an open admission of failure, for it created the impression that an ultimate explanation had been reached" . He also continues to outline what he sees are the other virtues and predictions of Darwin's' theory (again, he means natural selection). It "suggests" variety of forms of life; it "predicts" gradualness of change, accidental mutations and that [friends of Gould will like this] "we should expect evolutionary sequences of the random walk type" . Thereafter Popper discusses his own view or elaboration of "Darwinism", which is, in my view, rather confused.
Popper's claims were pretty mild. He most certainly did not think Darwinism was false or useless in science, as we have seen. He was attempting to make of Natural Selection (and NS only) something like an explanatory scheme that directs and suggests further research. I think, in that regard, he was correct. NS is an explanatory scheme that may or may not apply to a given case of evolution. Whether the scheme works depends on the individual facts of the matter. You can't disprove an explanatory scheme except to show that it is logically inconsistent, which NS isn't, by creationists' own admission.
Note that he claimed that adaptation or fitness equalled survival value. This is not true. Fisher in 1930, revised in 1958 said that fitness (he didn't use this word) was "reproductive investment". That is a rather different claim - it means that what counts is the number of progeny over time, not the survival of the individual organism. A shortlived organism might still have a major success in number of progeny. Also, Popper didn't really deal with selection taking place between members of the same species, but used the older confused terminology of selection taking place of species, or "for" the species, rather than individual organisms or genetic variations.
So even before the recantation, where Popper said:
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 recantationhe had not said what creationists claim he said.
Popper's influence on biologists is arguable. It seems to me that he was immediately employed by biologists to validate what they were doing anyway. One of the ironies of science and philosophy is that those who employed him the most - taxonomists - did so in support of an activity that Popper almost never talks about and clearly thinks with Rutherford is a form of stamp collecting - classification. Anyway, here's an interesting article by a former Popperian taxonomist.