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Why is the biological species concept difficult to apply to asexual organisms?

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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 biological species concept defines a species as a group of natural populations that are capable of interbreeding and are reproductively isolated from other such groups. This definition is difficult to apply to asexual organisms because they do not reproduce through the union of gametes, rendering the criterion of interbreeding inapplicable.123

Because asexual organisms, such as bacteria and certain other microorganisms, reproduce through processes like fission or budding, they do not form the interbreeding populations that the biological species concept requires. Consequently, biologists must rely on alternative criteria to classify these organisms, including:13

External morphology: Examining physical form and structure.12
Chemical and physiological properties: Analyzing metabolic or functional traits.1
Genetic constitution: Comparing DNA and hereditary material.1

The reliance on these diverse methods highlights the broader "species problem" in the philosophy of biology, where no single definition of a species is universally applicable across all forms of life.2