What Makes a Species?

What makes a species? We have seen that vagueness affects many of the qualities that we ascribe to people and things. But surely, this is not true for the central concepts that we use to classify the things around us? It is one thing to say that words like ‘tall’, ‘fat’, and so on, lack crisp boundaries (perhaps until a scientist comes around to impose such boundaries, as we have seen in the discussion of obesity), but it would be quite something else to claim that words like ‘man’, ‘chimpanzee’ and ‘tiger’ are also vague. Roughly speaking, the distinction appears to coincide with the distinction between English adjectives and nouns: adjectives (such as ‘tall’) denote subtly varying qualities and may therefore be vague, but nouns (such as ‘tiger’) denote natural classes of things and are therefore crisp, one might hope. To show that this hope is illfounded, we will now focus on a central building block of our thinking – at a level of common sense, but in biological taxonomy too – which is the concept of a species. ‘Species’, of course, is a somewhat abstract word: Fido may be a dog but that does not make Fido a species. The concept ‘dog’ itself is a species. Likewise, common chimpanzee is a species, and so is Homo sapiens (i.e., man). Species are the bedrocks of biology, far stabler than more inclusive biological groupings such as genus, class and order, and also stabler than less inclusive groupings, which subdivide the species. All of these other groupings are more difficult to justify scientifically; equally, none are as entrenched in everyday conversation as the names of (at least some) species. One might therefore expect that species- denoting terms have well-defined, crisp borderlines. Historically, biologists only started thinking systematically about these matters fairly recently. Even the famous Linnaeus who put biologists’ thinking about species on a solid footing (around the year 1750), gave every ap- pearance of believing that there is not much of a problem here, at least in principle. All species are different from each other, aren’t they, so it is just a matter of working hard to discover what the differences are. But in later years, particularly when it became plausible that species had developed grad- ually over time (rather than being created collectively in one mighty gesture), biologists realised that it would be rather nice to have a firm principle for de- ciding whether two animals belonged to the same species. Around 1940 this realisation had started to culminate in something approaching consensus. In what follows, let us sketch what this near-consensus amounts to. Simply put, a species was defined to be a group of organisms whose members interbreed with each other. The idea is essentially the following: when looking for boundaries between species, not just any boundary will do: we want a species to consist of organisms that are reasonably similar to each other in important respects. (What else is the point of grouping them together?) But how similar exactly? How do we prevent a situation in which 1 each individual biologist has his own idiosyncratic understanding of what it takes to be a lion? – Biologists came up with an elegant idea, namely to invoke interbreeding as a criterion: if two animals of different sexes can interbreed then they belong to the same species, otherwise they are not. The beauty of the idea lies in the fact that it uses the crisp concept of interbreeding to give sharpness to what would otherwise threaten to be a fuzzy boundary. This is done on the plausible assumption that if two animals are similar enough to interbreed then their offspring must once again be quite similar to the parents. (Nature could conceivably have worked differently, for example by making offspring as different form their parents as possible, but this is clearly not what we see around us.) But is interbreeding a well defined concept, and is it as crisp as one would like it to be? On reflection, some uncomfortable questions may be asked. For example, • The notion of inter-breeding only applies to organisms that reproduce sexually, so the standard definition does not apply to other species. – For our purposes, I propose not to worry too much over this objection. Let’s leave single-celled organisms and other celibate life forms aside and concentrate on the rest of us. • Horses and donkeys (and reputedly even lions and tigers) can produce offspring together, but none that is fertile, so their mating does not have any long-term effects. Presumably, fertility of offspring should be taken into account in the definition of a species. • Some types of animals that do not interbreed under normal circumstances can be induced to interbreed. Not much encouragement is needed, in some cases. Should these animals be counted as interbreeding with each other or not? • Animals of the same sex tend not to interbreed with each other. Yet, we count all people as part of the same species. – Clearly, the notion of interbreeding needs to be taken with a pinch of salt. One way of doing this is to say that two same-sex animals belong to the same species if they could interbreed, if only they had different genders. Differences in age will be regarded in the same manner. • Chihuahuas and Great Danes do not produce puppies together, but this is arguably for no deeper reason than their difference in size (although a lack of inclination might play a subsidiary role). Does this justify regarding them as different species? – The standard view would answer this in the negative. • Some groups of animals fail to interbreed solely because they are ge- ographically apart from each other. If only that waterfall, mountain range, or stretch of desert didn’t exist, they would happily interbreed. 2 (I’m reminded of school trips during which our teachers tried to keep us in our own sleeping quarters, separate from the girls.) – It seems reasonable to disregard geographical separation, and focus on whether two animals could interbreed, if given a reasonable chance. • Temporal separation can have the same effect as geographical separation: the fact that you are unlikely to have children with any of your great grandparents does not mean you belong to a different species. Taking complications of this kind into account, and disregarding organisms that do not reproduce sexually, a species is usually thought to be something like the following: Species: a maximally large group of animals, such that healthy young adult specimens of the right sex are able in principle to produce fertile offspring under favourable circumstances such as occur naturally. One might think that all the obvious wrinkles in the notion of a species have now been ironed out. Enter the Ensatina salamander. Ensatina salamanders live along the hilly edge of California’s Central Valley. They tend to avoid the centre of the Valley, presumably because of the heat there. Ensatina comes in six-or-so forms, which are usually viewed as its sub-species. Two of these, Ensatina eschscholtzii and Ensatina klauberi (both of which live in the south of the valley, with eschscoltzii dominating one side and klauberi the other) do not interbreed with each other, but a reasonable case can nevertheless be made that they belong to the same species: eschscholtzii does mate with a third subspecies living just north of it; these mate with a fourth subspecies, these mate with a fifth; and these, in turn, mate with our old friends Ensatina klauberi. The reasons why two salamanders can or cannot get fertile offspring are buried somewhere in their biology. For us, however, these reasons are not what matters: we are only interested in the facts on the ground, so to speak. FIG (perhaps plate 21 from Dawkins, or the original from Stebbins 2003, or some abstraction) Let us now schematise the story a bit, focussing on those aspects that matter most to us. In doing so we will use a broad brush. A more precise account would talk about individual animals. It will be convenient, however, to simplify a little, by grouping all the salamanders in a particular sub-species (e.g., all the members of Ensatina eschscholtzii) together, pretending that they are all alike. (Having read the next page, you probably won’t have difficulty reconstructing the story in a more precise style, focussing on individuals rather than subspecies.) We arrange the six kinds of Ensatina in 3 a sequence from E1 (this is eschscholtzii) to E6 (this is klauberi), in such a way that each member of the sequence interbreeds with the next one, but the first one (E1) does not interbreed with the last (E6). Writing I to abbreviate ‘Interbreed’, the situation can be depicted schematically as follows: E1 IE2 IE3 IE4 IE5 IE6 For concreteness – and in order not to make the creatures look more promis- cuous than necessary – let us assume that I shows all the interbreeding that goes on. E1 does not interbreed with E3, .., E6; similarly, E2 does not interbreed with E4, .., E6, and so on. Each sub-species interbreeds with the previous and next sub-species in the series only, in other words. If these are the facts about breeding, what does this mean for the definition of Ensatina? Do all six types of salamanders count as Ensatina? The answer, based on the standard definition of ‘species’, is No! Sure enough, E1 and E2 form a species together, and so do E2 and E3. But E1 and E3 do not, hence they do not belong to the same species.

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