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Why does natural selection claim that fitter organisms succeed on average rather than always survive?

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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.

In the context of the philosophy of biology, it is a common misconception that natural selection implies the fittest organisms will always survive. In reality, no Darwinian theory claims that the fittest always survive, as the world is influenced by far too many random events for such a deterministic outcome.1

Even a highly fit organism can be struck down by accidents, such as lightning, disease, or a failure to find a mate, which prevents its adaptive features from being passed to the next generation. Furthermore, in small populations, genetic drift—a process driven by pure chance—can lead to less fit individuals being more successful than those who are more fit.12

Therefore, natural selection is understood as a probabilistic claim: on average, organisms with higher fitness possess a greater propensity for successful survival and reproduction compared to those that are less fit. It describes a statistical trend across populations rather than a guaranteed outcome for every individual.1