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Michael Ghiselin (1939–)

Ghiselin proposed that biological species are not natural kinds — not classes defined by shared properties — but historical individuals: particular, spatiotemporally bounded entities, each with a unique origin, history, and eventual extinction. The thesis, first argued in “A Radical Solution to the Species Problem” (1974), was developed independently of and simultaneously with David Hull’s parallel argument (1976, 1978), and between them the two transformed the metaphysics of biology. If species are individuals rather than kinds, then there are no laws about particular species (“all swans are white” is an empirical generalisation, not a law), biological taxonomy is a historical rather than a classificatory enterprise, and the ontological structure of biology differs fundamentally from that of physics and chemistry. Ghiselin is also the author of The Economy of Nature and the Evolution of Sex (1974), a wide-ranging theoretical work on sexual selection, and an authority on the systematics and natural history of opisthobranch molluscs (sea slugs).

Life

Born 13 March 1939 in Salt Lake City, Utah. Educated at the University of Utah (BS in zoology, 1960) and Stanford University (PhD in zoology, 1965). His doctoral work was on the comparative functional morphology of opisthobranch gastropods — a research programme he maintained alongside his philosophical work for his entire career.

Research fellow at the Marine Biological Laboratory, Woods Hole, Massachusetts (1967–68). Joined the California Academy of Sciences in San Francisco (1983), where he served as a research fellow and later as chair of the Department of Invertebrate Zoology. The California Academy appointment reflected Ghiselin’s dual identity: he was simultaneously a practising systematist (describing new species, revising taxa, conducting fieldwork on Pacific and Caribbean reefs) and a philosopher of biology working on the foundations of taxonomy, individuality, and evolutionary theory.

Ghiselin’s philosophical work was initially met with resistance. The species-as-individuals thesis struck many biologists as counterintuitive — the idea that Homo sapiens is an individual (like a particular organism) rather than a kind (like gold or water) challenged deeply embedded habits of thought. The thesis gained traction through Hull’s independent advocacy and its adoption by Ernst Mayr (who noted its compatibility with the biological species concept) and others. It is now widely, though not universally, accepted in the philosophy of biology.

The species-as-individuals thesis

“A Radical Solution to the Species Problem,” Systematic Zoology 23 (1974). The argument proceeds from an analysis of what it means for something to be a natural kind versus an individual.

The thesis grows directly out of Mayr’s distinction between typological and population thinking — the argument Mayr had been pressing since the late 1950s that species are not Platonic types defined by a shared essence but varying populations for which the “type” is an abstraction. Ghiselin’s move takes Mayr’s anti-typology one step further: the population is not merely variable, it is an individual — a particular entity, not a kind at all. Mayr later adopted the individuality thesis and noted its compatibility with the biological species concept, but the conceptual precursor was his own population thinking.

Natural kinds are classes defined by intrinsic properties shared by all members. Gold is a natural kind: every sample of gold has atomic number 79, and this property is necessary and sufficient for membership. Natural kinds support laws: “All gold conducts electricity” is a law because it follows from the intrinsic properties that define the kind.

Individuals are particular, spatiotemporally bounded entities. An organism is an individual: it has a specific origin (birth), a specific history, and a specific end (death). Its identity is determined by its spatiotemporal continuity, not by any set of intrinsic properties — an organism can change every one of its properties over its lifetime and remain the same individual.

Species as individuals. Species, Ghiselin argued, have the ontological structure of individuals, not kinds. A species has a specific origin (speciation), a specific history (its evolutionary trajectory), and a specific end (extinction). Its members are not defined by shared intrinsic properties — variation is the norm, and no trait is universal within a species. What holds a species together is not a shared essence but historical continuity: gene flow, common descent, and the cohesive mechanisms that maintain the lineage. “Homo sapiens” is a proper name, not a class term; Homo sapiens is a thing, not a set.

Consequences. If species are individuals, then generalisations about them are not laws but historical summaries. “All swans are white” is not a law (it is falsified by black swans) because “swan” does not name a natural kind with a definable essence; it names an individual whose members vary. Biological taxonomy is not classification (assigning members to classes by shared properties) but a historical enterprise (tracing the genealogical relationships among lineages). The philosophy of biology cannot be modelled on the philosophy of physics, where laws about natural kinds are the paradigm; it requires its own ontology.

The Economy of Nature and the Evolution of Sex

The Economy of Nature and the Evolution of Sex (California, 1974) is Ghiselin’s other major contribution — a wide-ranging theoretical work on why sexual reproduction exists, given its apparent twofold cost (a sexual female passes on only half her genes to each offspring, while an asexual female passes on all of them).

Ghiselin’s argument draws on the diversity of sexual systems in nature (hermaphroditism, separate sexes, sex change, parthenogenesis) and proposes that the advantage of sex lies in the production of genetically variable offspring — the “tangled bank” hypothesis (named after Darwin’s closing passage in the Origin). In a heterogeneous environment, genetically diverse offspring are more likely to find niches they can exploit than genetically uniform clones. The argument predates and partly overlaps with Hamilton’s parasite-driven Red Queen hypothesis and Maynard Smith’s analysis of the cost of sex.

Where Ghiselin stops

The species-as-individuals thesis is widely accepted as a philosophical clarification but its biological consequences are debated. If species are individuals, then the mechanism of individuation — what holds a species together as a cohesive entity — becomes the central question. For sexual species, gene flow and interbreeding provide cohesion. For asexual species, the question is harder: what makes a clonal lineage a species rather than an arbitrary collection of similar organisms? The thesis clarifies the ontology but defers the mechanism, and the problem of asexual species individuation remains open.

The thesis also raises questions about higher taxa. If species are individuals, are genera, families, and orders also individuals? Ghiselin and Hull both answered yes — monophyletic higher taxa are individuals defined by common descent. But the higher taxa are less cohesive than species (no gene flow holds a genus together), and whether the individual concept extends cleanly to these levels is contested. Kevin de Queiroz and others have developed species concepts that build on the individuality thesis while addressing its unresolved cases.

The individuality thesis is usually motivated by freeing biology from a pre-Darwinian essentialist tradition — the assumption that species have fixed essences. But historians of science — Mary Winsor, John Wilkins, and others — have argued that the “essentialism story” is substantially a twentieth-century construction by Mayr, Hull, and Popper, not an accurate account of what pre-Darwinian taxonomists actually believed. If the essentialist tradition the thesis opposes is partly a myth, the contrast that motivates the individuality claim weakens — though the thesis’s logical content (species have the structure of individuals, not kinds) stands independently of whether its historical framing is accurate.

The relationship between the individuality thesis and the species concepts used by practising systematists (biological species concept, phylogenetic species concept, ecological species concept) is not fully resolved. The thesis is a claim about what kind of entity a species is; the species concepts are criteria for recognising and delimiting species in practice. Whether the ontological claim constrains the operational criteria — or whether the two are independent — is a question the thesis poses but does not settle.

Key works

See also: Hull · Mayr · Darwin · Darwinism