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Why are species considered examples of universals in metaphysical arguments?

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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. — metaphysics Detail of a Roman copy (2nd century bce) of a Greek alabaster portrait bust of Aristotle (c. 325 bce); in the collection of the Museo Nazionale Romano, Rome.

In metaphysics, species are considered examples of universals because they are viewed as "general things" that exist over and above the individual organisms that belong to them. When philosophers argue that a species—such as Equus caballus—is a universal, they are asserting that it serves as a common nature or entity that all individual members of that species share or "exemplify".12

The classification of species as universals is central to the "problem of universals," which examines how multiple distinct particulars can fall under the same type. Historically, this has led to different metaphysical interpretations:13

Platonic Realism: Argues that the species exists as an eternal, changeless form independent of the physical world; individual organisms "participate" in this form.2
Aristotelian Realism: Contends that the species is a universal that inheres within the individual organisms themselves as a common constituent.2
Nominalism: Rejects the existence of species as real entities, viewing them instead as mere names or labels used to group similar particulars.132

Whether species are truly "real" features of nature or merely human-made categories remains a significant debate in the philosophy of biology.3