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Why might the species problem matter less for microorganisms than for animals?

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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 species problem—the philosophical and scientific challenge of defining what constitutes a species—is often considered less applicable to microorganisms than to animals because the biological criteria used for macroorganisms frequently fail in the microbial world.1

In animal biology, the most prominent definition of a species relies on reproduction, specifically the ability of members to interbreed and remain reproductively isolated from others. This framework encounters significant difficulties with microorganisms for several reasons:1

Asexual Reproduction: Bacteria and other microorganisms generally reproduce asexually, rendering definitions based on interbreeding inapplicable.2
Horizontal Gene Transfer: Unlike animals, microorganisms frequently exchange genetic material across different lineages through horizontal gene transfer. This process blurs the boundaries between groups, making it difficult to establish the clear, distinct limits required by traditional species concepts.23
Ecological Adaptation: Because of these factors, some researchers argue that speciation in bacteria occurs at the level of ecotypes—genetically distinct populations adapted to specific niches—rather than traditional species.2

Consequently, some scientists suggest that the species level in bacterial classification is more comparable to the genus level in other organisms, leading some philosophers to conclude that the species problem may be less relevant or even unsolvable for the microbial world.12