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Why did Karl Popper question whether evolutionary theory could be falsified?

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Marble bust of Aristotle with a beard and draped robe, showing detailed facial features and wavy hair, set against a dark background. — Philosophy of biology Detail of a Roman copy (2nd century bce) of a Greek alabaster portrait bust of Aristotle, c. 325 bce; in the collection of the Roman National Museum.

Karl Popper questioned whether evolutionary theory could be falsified because he believed it lacked the necessary openness to empirical refutation required of a genuine science.1

Popper’s criterion of falsifiability posits that for a theory to be scientific, it must be possible to conceive of observations that would prove it false. Regarding natural selection, Popper observed that when features were found to lack adaptive value, supporters often dismissed these as areas requiring further study or as phenomena that would eventually support the theory in the future. Because he perceived that the theory could be shielded from counterevidence in this way, he famously characterized evolutionary theory as a "metaphysical research programme" rather than an empirical science.231

It is important to note that many scientists and philosophers have since challenged this assessment. Critics of Popper’s view argue that counterevidence to evolutionary hypotheses is indeed possible and that the theory is subject to rigorous testing through methods like the comparative method and the use of optimality models, which allow for the revision or rejection of hypotheses based on empirical data.1