The species problem

THE THREE SPECIES PROBLEMS

The species problem is actually a number of problems that biologists have dealt with since the term was first applied to biological organisms by Aristotle. I call the three main problems the grouping problem, the ranking problem, and the commensurability problem. It will benefit us to clearly distinguish these at the beginning of our discussion and to bear them in mind as we consider the philosophy of species.

THE GROUPING PROBLEM

Our first problem is, How are members of species to be grouped together and excluded from other groups? No matter whether species are constructs and arbitrary, or natural and objective, the issue remains how we can include some and exclude other organisms from a species. During the Middle Ages, the notions of differentiae and relata were brought to bear. A species was defined by characters, and those that differentiated organisms in some relevant way enabled classifiers to exclude some character bearers from a species. Likewise, characters that related organisms were thought to be sufficient to include organisms within species despite their individual differences. In short, the grouping problem is one primarily of operationality. But not entirely—organisms can be seen as self-classifiers, especially in the reproductive isolation concepts of species. So long as organisms are able, it is said, to differentiate between themselves and find related enough organisms to mate with, then they form species even if we humans cannot identify what it is about them that the organisms themselves find salient in the process. Maynard Smith, for example,¹⁰ notes that one reason we have so little trouble identifying some kinds of species (e.g., birds) is that they are of the same general size as we are, and use much the same criteria in self-differentiating: sound, appearance, and behavior. It follows, some think, that other groups equally salient to us must also have some causally significant differentiae and relata. Likewise, it follows that organisms that are (reproductively) distinct but are otherwise indistinguishable by us must have some differentiae and relata that we cannot recognize (e.g., biochemical differences) without specialist equipment. So our grouping problem reduces to the issue: what is appropriately seen (by us or by organisms themselves) as grounds for distinguishing some and grouping others into species?

THE RANKING PROBLEM

Is species a fixed—that is to say, absolute—rank in taxonomy (in contrast to the so-called "artificial" higher taxa)? Arguments about whether some or all taxonomic ranks are natural or not go back a very long way. Linnaeus clearly thought that species and genera were natural ranks, while Buffon felt that they were not (at one time), and even today some present alternative views that species themselves are not natural, but only individuals, or presently terminal taxa, are. Much of the species debate has been over what it is that species are that makes them a natural (that is, real) rank. Discussions of reproductive isolating mechanisms, for example, or ecological niches, geographic replaceability, and so forth take it for granted that there does exist a real rank of taxa, that it is appropriate to call that rank the species-level, and that what we are all disagreeing about is what to use to define that level, or rather what it is that makes some taxon that level. Other problematic cases, such as ring species, well-defined geographic races, facultatively interfertile species, superspecies, sympatric species swarms, and of course asexuals, both quasispecies and secondarily asexual taxa, lead some to think that the notion of species is a homonym for many distinct concepts. Moreover, species seem to be constituted in different ways in different groups of organisms. Many plant species are formed, for example, by hybridization, a process that is often ignored or glossed over when discussing animal species. Some fungi have multiple sexual morphs instead of the regulation two, and so reproductive isolation becomes a much harder concept to qualify in their case than in animals. Consequently, if we are trying to compose a general concept of species that applies to all living things, or to compare and contrast the concepts that are relevant only in particular cases, two questions arise: one is whether there is warrant for thinking that there is indeed a rank of species in all taxonomic hierarchies or whether it is relative to the discriminating criteria used in a given group of taxa; the other is whether a species taxonomic level is required at all.

THE COMMENSURABILITY PROBLEM

The most common and popular species concept is, of course, the biological species concept championed by Mayr since 1940. It is typically the definition taught to undergraduate zoology students and often the one taught to botany students. It is not, however, taught to bacteriology students, nor is it the preferred definition of lichenologists, as the taxa that are formed by most bacteria do not rely on constant sexual reproduction if any, and lichens are obligate symbionts formed by a mutualistic association between blue-green algae and fungi.¹¹ Immediately the commensurability problem rears its head. Species among animals are not commensurate with species among many single-celled organisms or species among symbionts. Some, such as Dobzhansky,¹² simply denied, as we saw, that asexual organisms can form species (since by definition reproductive isolation is what makes species, and every asexual individual is reproductively isolated). This is surely putting the definitional cart before the evolutionary horse. If species are outcomes of evolution, and asexual taxa—forms, morphs, types, niche occupiers, whatever—are the outcome of evolution, then we need to be able to justify the special status of taxonomic units like species over other taxonomic units. Simple familiarity through tradition and acquaintance in one domain of biology is insufficient—zoological hegemony has been a tendency in evolutionary and taxonomic theory for most of the twentieth century. In part, the species problem arises because categorical concepts that applied well in zoology failed to generalize outside it, or even those that applied well to some particular group such as birds, mammals, or insects failed to generalize even to other animals, let alone plants, fungi, lichens, algae, and so on. Perhaps, some commentators—both biologists and philosophers—suggest, the term "species" is a trashcan categorical and should be replaced altogether. At least one major group of polychaete worms has been described recently without mention of species, except to explain why species are not mentioned. Perhaps "taxon" is sufficient.

(Wilkins, John S. "Philosophy and Species". Species: A History of the Idea (2nd ed.).)