Chapter 26: The Species Problem

The species problem arises from a tension between two facts. On one hand, species seem real: we can readily distinguish robins from sparrows, oak trees from maples, dogs from cats. Linnaeus’s system of binomial nomenclature, which assigns every organism a genus and species name, has been the backbone of biological classification for over 250 years. On the other hand, the boundaries between species are often fuzzy, contested, and interest-relative.

Darwin himself recognized the difficulty. In On the Origin of Species, he wrote:

“No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species.”— Charles Darwin, On the Origin of Species (1859), Chapter 2

Darwin’s theory of evolution actually deepens the species problem. If species evolve gradually from ancestral populations, then the boundaries between species should be blurry. Speciation is a process, not an event; there is no sharp moment at which one species becomes two. Ring species — like the Ensatina salamanders of California, where neighboring populations can interbreed but the terminal populations cannot — dramatically illustrate the continuity of variation that Darwinian theory predicts.

The philosophical challenge is to determine whether species are “real” categories in nature or merely convenient fictions imposed by human taxonomists. And if they are real, what makesthem real? What property or relation unifies the members of a species into a genuine natural group?

The most influential species concept of the twentieth century is the Biological Species Concept(BSC), developed by Ernst Mayr in his 1942 masterwork Systematics and the Origin of Species:

“Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.”— Ernst Mayr, Systematics and the Origin of Species (1942)

On the BSC, what makes a species a species is reproductive isolation: members of a species can interbreed with each other but not (under natural conditions) with members of other species. Reproductive isolation maintains the genetic integrity of species, allowing them to evolve as distinct lineages. Mayr argued that the BSC captures the biologically significant fact that species are “protected gene pools” — cohesive genetic systems maintained by gene flow within the species and reproductive barriers between species.

The BSC has several well-known limitations:

• •Asexual organisms: Bacteria, archaea, and many protists reproduce asexually. The concept of reproductive isolation simply does not apply to them, yet they are classified into species.
• •Hybridization: Many species that are clearly distinct — lions and tigers, for instance — can interbreed in captivity. More importantly, hybridization is common in nature, especially among plants. Over 25% of plant species are estimated to hybridize with at least one other species.
• •Allopatric populations: If two populations are geographically separated, we cannot test whether they would interbreed. The BSC must rely on counterfactuals (“they would interbreed if they met”), which are difficult to evaluate.
• •Fossil species: We cannot test reproductive isolation for extinct organisms, so the BSC is inapplicable to the paleontological record.

Despite these limitations, the BSC remains enormously influential, especially in zoology. Its defenders argue that the limitations are practical rather than principled, and that no alternative concept does better across all cases.

The Phylogenetic Species Concept (PSC) defines species in terms of evolutionary history rather than reproductive isolation. Several versions of the PSC have been proposed, but the most influential defines a species as the smallest monophyletic group — that is, the smallest group that includes an ancestor and all of its descendants.

Joel Cracraft (1983) defined a phylogenetic species as “the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent.” On this view, what matters is not whether populations can interbreed but whether they form a distinct evolutionary lineage that can be diagnosed by unique characteristics.

The PSC has several advantages over the BSC:

• •It applies to asexual as well as sexual organisms.
• •It applies to fossil species.
• •It does not depend on counterfactual claims about potential interbreeding.
• •It aligns with the goal of systematics, which is to reconstruct the tree of life.

However, the PSC also faces difficulties. It tends to “split” species more finely than the BSC, recognizing as distinct species populations that the BSC would treat as subspecies or varieties. This has practical consequences: adopting the PSC would dramatically increase the number of recognized species, with significant implications for conservation priorities and funding. The PSC also requires a prior phylogenetic analysis, and different genetic markers can yield different phylogenies, leading to different species boundaries.

The Ecological Species Concept (ESC) defines species in terms of their ecological niches. Leigh Van Valen (1976) proposed that a species is “a lineage (or a closely related set of lineages) which occupies an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all lineages outside its range.”

On this view, what makes a species a species is its unique ecological role. Members of a species share a common niche — they exploit similar resources, occupy similar habitats, and face similar selective pressures. The ecological species concept highlights the fact that species are not merely genealogical units but functional units in ecosystems.

The ESC has the advantage of applying to both sexual and asexual organisms and of connecting species concepts to ecological theory. However, it faces the problem that niches are difficult to define precisely and that conspecific organisms may occupy quite different niches (e.g., males and females, larvae and adults). The concept also struggles with “sibling species” — morphologically indistinguishable species that occupy the same niche.

The ecological perspective has been developed further by proponents of the cohesion species concept(Templeton, 1989), which defines a species as the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms. This concept attempts to unify reproductive, ecological, and genetic factors into a single framework.

The proliferation of species concepts has led some philosophers to embrace species pluralism— the view that there is no single correct species concept and that different concepts are appropriate for different purposes. Philip Kitcher (1984) and John Dupré (1993) are the most prominent defenders of this view.

“There is no unique relation among organisms that is privileged in the sense of being the species relation... Different legitimate interests of biologists are served by different species concepts.”— Philip Kitcher, “Species” (1984)

Kitcher argues that different biological projects — ecology, systematics, population genetics, conservation biology — have different needs, and no single species concept serves all of them equally well. The BSC serves the needs of population geneticists studying gene flow; the PSC serves the needs of systematists reconstructing phylogenies; the ESC serves the needs of ecologists studying community structure. Rather than seeking a single “true” species concept, we should acknowledge that species classification is a tool that can be adapted to different purposes.

Dupré’s “promiscuous realism” goes further, arguing that there are many legitimate ways to classify organisms, and no single system captures the one true structure of biological reality. On Dupré’s view, the search for the species concept is misguided because it presupposes a metaphysical monism about natural kinds that is unjustified.

Critics of pluralism argue that it leads to an unsatisfying relativism. If “species” means different things in different contexts, then claims about biodiversity, extinction rates, and conservation priorities become ambiguous. Marc Ereshefsky (1998) has argued for an even more radical position: species eliminativism, the view that “species” is not a legitimate scientific category at all and should be replaced with more precise concepts like “biospecies,” “phylospecies,” and “ecospecies.”

One of the most provocative theses in the philosophy of biology is the species-as-individuals(SAI) thesis, independently proposed by Michael Ghiselin (1974) and David Hull (1978). On this view, species are not classes or kinds (like “gold” or “electron”) but individuals (like “the United States” or “the Rocky Mountains”).

“Species are not classes of organisms but individuals, of which organisms are parts, not members.”— Michael Ghiselin, “A Radical Solution to the Species Problem” (1974)

The SAI thesis rests on several arguments:

• •Spatiotemporal locatedness: Species have beginnings (speciation events) and endings (extinction). They are located in space and time. Classes, by contrast, are abstract, atemporal entities. Gold existed before any gold atoms formed, and would continue to exist as a kind even if all gold atoms were destroyed. But Homo sapiens came into existence at a particular time and place, and will go extinct at some future time.
• •No essential properties: Natural kinds are traditionally defined by essential properties (gold is the element with atomic number 79). But species have no essential properties — no trait is shared by all and only members of a species. This is a consequence of population thinking: variation is the rule, not the exception.
• •Cohesion through causal relations: What unifies the members of a species is not shared properties but causal relations — gene flow, common descent, shared selective pressures. These are the kinds of relations that unify the parts of an individual, not the members of a class.
• •Role in evolutionary theory: In evolutionary theory, species evolve, speciate, and go extinct — they are the subjects of historical narratives. Classes do not evolve; individuals do.

If species are individuals, then organisms are parts of species, not members of species — just as cells are parts of organisms, not members of a class of cells. And species names (“Homo sapiens”) are proper names, not kind terms.

The SAI thesis has been enormously influential, but it also faces objections. Some philosophers argue that species lack the cohesion and integration required of genuine individuals. A geographically fragmented species, with populations on different continents that have no gene flow between them, does not seem like a unified individual. Others argue that the natural kinds framework can be modified to accommodate species — through the notion of homeostatic property clusters (Boyd, 1999) or historical natural kinds — without going to the extreme of treating them as individuals.

If the species category raises philosophical puzzles, higher taxa — genera, families, orders, classes, phyla — raise them in spades. Are higher taxa natural groups, or are they merely conventional divisions imposed on the continuous tree of life?

Cladists argue that only monophyletic groups — groups consisting of an ancestor and all of its descendants — are real biological entities. On this view, “Reptilia” as traditionally conceived is not a real group because it excludes birds, which are descended from reptilian ancestors. The only real group is “Sauropsida,” which includes both traditional reptiles and birds.

But even if we restrict ourselves to monophyletic groups, there is no objective way to assign ranks. Why is Mammalia a “class” rather than an “order” or a “phylum”? There is no principled criterion for assigning ranks to monophyletic groups, which has led some systematists to advocate for rankless taxonomy (the PhyloCode).

The philosophical upshot is that while the tree of life may be objectively real, the way we carve it into named groups involves conventional elements. This raises questions about the relationship between scientific classification and natural reality that echo broader debates about natural kinds and the structure of nature.

The species problem is not merely academic; it has direct consequences for conservation policy. Biodiversity is typically measured in terms of species richness — the number of species in a given area. Conservation legislation, such as the U.S. Endangered Species Act, protects species, not populations or lineages. If we cannot agree on what a species is, we cannot agree on how many species there are, which ones are endangered, or which ones deserve protection.

The choice of species concept directly affects species counts. The PSC, which “splits” species more finely than the BSC, tends to recognize more species. When the PSC is applied to birds, for example, the number of recognized species roughly doubles. This has profound implications: more species means more potential candidates for endangered status, but also means that each species has a smaller geographic range and population size, making them more vulnerable to extinction.

Some conservation biologists have argued that we should bypass the species problem entirely and focus on conserving evolutionary significant units (ESUs) or phylogenetic diversity rather than species counts. These approaches shift the focus from the question “How many species are there?” to the question “How much evolutionary history is at stake?” But even these alternatives presuppose some way of carving the tree of life into units, and the philosophical puzzles resurface at a different level.

• •Mayr, E. (1942). Systematics and the Origin of Species, Chapter 5.
• •Ghiselin, M. (1974). “A Radical Solution to the Species Problem,” Systematic Zoology 23(4).
• •Hull, D. (1978). “A Matter of Individuality,” Philosophy of Science 45(3).
• •Kitcher, P. (1984). “Species,” Philosophy of Science 51(2).
• •Dupré, J. (1993). The Disorder of Things, Chapter 2.
• •Ereshefsky, M. (1998). “Species Pluralism and Anti-Realism,” Philosophy of Science 65(1).
• •Richards, R.A. (2010). The Species Problem: A Philosophical Analysis. Cambridge University Press.