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Why did Herbert Spencer’s phrase “survival of the fittest” lead to accusations of tautology?

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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.

The phrase "survival of the fittest," coined by Herbert Spencer, led to accusations of tautology because of how the terms are defined in relation to one another.1

Critics argue that if "the fittest" are defined simply as those who survive, and the process of natural selection is described as the "survival of the fittest," the statement becomes circular: it effectively claims that "those who survive are those who survive". When interpreted this way, the phrase loses its status as a substantive scientific observation about the mechanisms of evolution and instead functions as a statement that is true only by the definition of its terms.1

Defenders of the theory respond that this is a misunderstanding of biological fitness. In science, fitness is not merely a label for survivors; it refers to a propensity for successful survival and reproduction based on specific adaptive traits. Furthermore, because random events like accidents or genetic drift can cause even well-adapted organisms to perish, the theory does not claim that the fittest always survive, but rather that they have a higher statistical probability of doing so over time.12