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How does the cluster-of-features approach define membership in a species?

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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 philosophy of biology, the cluster-of-features approach—which has roots in the work of the French botanist Michel Adanson and was later advocated by William Whewell—defines membership in a species based on a collection of traits rather than a single defining characteristic.1

Under this framework, an organism is considered a member of a species if it possesses a sufficient number of features from a designated set. Crucially, no single feature within that cluster is considered strictly necessary for membership. This approach serves as a potential solution to the limitations of traditional morphological definitions, which often struggle to account for individuals that lack specific traits—such as rationality in humans—yet are still clearly recognized as belonging to the species.1