In the September/October issue of Skeptical Inquirer, Massimo Pigliucci writes:
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 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.
Some creationists make similar criticisms today, but they fail to appreciate the fact that Popper retracted his criticism:
“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).
Why the change? Because there are independent means for determining which members of species are “fittest.”
[B]iologists 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.
Karl Popper was a philosopher rather than a scientist, but he demonstrated the scientific mindset: he changed his mind about a conclusion he reached once he was show new information which contradicted his beliefs. This often isn't easy to do because no one likes to be wrong; with some conscious work, though, a person can make it a bit easier and learn to accept their own fallibility.
We can contrast this with the creationists who so love to cite Popper or at least make the same arguments he did without acknowledging their debt to him. When confronted with information that contradicts their beliefs, they refuse to change their minds. Instead, they deny the evidence or rationalize ways to ignore anything they don't like. Creationists want to benefit from the thinking of Karl Popper without committing themselves the consequences of his skeptical, philosophical, and critical mindset. Even worse, it's seems unlikely that they have any idea what this means.