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 geographically 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. Note, however, that the two species that we have just found ({E1,E2} and {E2,E3}) overlap, because E2 occurs in each of the two. Instead of being nicely separate chunks of reality, these two species end up all intertwined. This, surely, is not how biologists (let alone us regular folk) like to think about animals. A director of a large multinational company once remarked that he wanted to think of his company as a plate of asparagus, with all divisions and components neatly defined and separate, not as a plate of spaghetti, on which everything was all mixed up and intertwined. I suppose we’re essentially all a bit like him when we try to make sense of the world around us.

PICTURE

How have biologists responded to this problem? Their dominant reaction appears to be to let common sense prevail over formal definitions. The standard view of the situation goes roughly as follows: E2 belongs to the same species as E1. But E3 belongs to the same species as E2, therefore E3 must belong to the same species as E1. The argument can be repeated for E4: it belongs to the same species as E3, therefore it too must belong to the same species as E1. Similary, the argument can be repeated for E4, for E5, and for E6, showing that all six types of salamanders belong to one big species. This standard view, however, lumps together all the different types of salamanders into what is sometimes called a ring species. This includes E1 and E6, which do not interbreed together and which would therefore not belong to the same species, if the standard definition of ‘species’ had anything to do with it. Fortunately, E1 and E6 are still a bit similar – they are all ordinary

4 salamanders. If there happened to be lots of other animals in the world, then perhaps there would exist animals E7, E8, and so on, each of which a little different from the previous one, including some En (where n is some seriously large number) which is entirely unlike any salamander ... and a biological version of the sorites paradox would arise. How should we view the story of the Ensatina salamander? Is it indicative of an important flaw in the concept of a species, or just a little anomaly about which we should not worry? We shall use the remainder of this chapter to answer this question. Admittedly, ring species are unusual. Before concluding that there is no reason for worry, let us follow the biologist Richard Dawkins, who wrote engagingly about Ensatina, by thinking about more ordinary situations, when a species develops over time. Our own species will provide as good an example as any. Suppose you were to draw up a huge list of your ancestors, always choosing a parent whose sex is opposite your own. So if you are a woman, you list your father, parental grandfather, parental great grandfather, and so on, in a huge sequence. Suppose you had super-human amounts of time and patience, going back to the person who lived 250.000 generations or so ago (something like 6 million years BC). Now we need to ask some rather personal questions. – Would you be able to interbreed with the first ancestor in the sequence (your mother or father, that is)? Unpalatable though the thought is to most of us, I take it that the answer is “probably yes”. How about the second ancestor (one of your grand parents)? Well, if we gloss over some minor problems posed by time and emotion, then probably, the answer must be yes. But at some time t in the past, so many biological differences will have accumulated between you and the ancestor in the list who lived at that time t that it would be impossible for the two of you to have fertile offspring even if you had been the fondest of contemporaries. I do not know precisely how far back in time we need to go to find a suitable t, but 6 million years ago is almost certainly long enough ago, since this is estimated to be the time when your ancestor was also an ancestor of today’s chimpanzees: probably an ape who walked on four legs. For future reference, let’s call him Richard. Recall the story of the Ensatina salamaner: Ensatina is often treated as just one species of salamander. This seems sensible. What do all these tiny differences between six types of animals matter, given that each interbreeds with the next one? But suppose we applied the same logic to the human species, based on the story involving Richard? We would start reasoning that you are the same species as your parents, who are the same species as their parents, and so on. After 250.000 or so reasoning steps, we would be concluding that your ancestors 6 million years ago – walking on four legs, having no language and hardly using any tools – were human too. If this is not bad enough for your taste, then let’s reverse direction: starting from Richard, we keep choosing a child instead of a parent: first we choose one of

5 Richard’s children, then one of its children, and so on. By making the right choices often enough, we arrive at a chimpanzee now living in the London zoo. Using the same reasoning as before, we conclude that this chimp is of the same species as Richard, who is of the same species as you. In other words: chimps are human. The conclusion is that people and chimps belong to the same species. Let us reflect a bit on the situation. Firstly, the reasoning is not limited in any way to chimps and people: if we follow current biological orthodoxy and accept that all todays’ life evolved from one and the same source, then all living beings are literally cousins of each other. Therefore, the reasoning above can be applied with equal force to, say, trouts and tigers: the only difference is that the required number of reasoning steps would be far larger in that case, because the latest common ancestor shared by trouts and tigers is a long time ago indeed (slightly less than half a billion years ago, according to current estimates). So, if you buy into the standard way of reasoning about species then all living beings are one and the same species. This conclusion may be morally pleasing in some sense, but it makes a mockery of our attempts to classify the world. Secondly, the reasoning seems farfetched primarily because we are not acquainted with all the intermediate types of animals that link you to the ape called Richard, and Richard with today’s chimpanzee. It is not only that most of them are dead, for the reasoning could also be applied to fossils; but far too few fossils have been found to perform this reasoning on the basis of them: we simply don’t have enough of them (and of the right varities) to be able to arrange them in the kind of ‘n interbreeds with n + 1’ sequence that gave rise to the argument. This is where Ensatina was different, since so many intermediate forms are alive and well, geographically close enough to each other to rub our noses into their breach of biological principles. The story of the Ensatina salamander was popularised in Dawkins’s book The Ancestor’s Tale, a delightful account of evolution organised as a journey back in time. It will be useful to examine Dawkins’ conclusions in some de- tail. Reflecting on people’s tendency to think in terms of discrete categories, honouring them with separate names (e.g., the names of species), he rejects what he calls the ‘tyranny of the discontinuous mind’. Following the biologist Ernst Mayr, he traces this ’tyranny’ back to Plato, who famously believed that words reflected a fixed and unchangeable pattern that underlies reality, independent of human thought. In Plato’s philosophy, these patterns include not only such basic concepts as a circle or the number 2 – for which this ’Platonic’ position has undeniable appeal – but also words like ‘man’, ‘dog’, ‘table’, and so on. Dawkins rightly has little time for this outdated manner of thinking. But if Platonism is the wrong way of thinking about biology, then what is the ‘correct’ way of thinking about, say, species? Rather than rejecting the notion of a species, Dawkins decides to keep using it, but always with a pinch of salt:

6 ’Let us use names as if they really reflected a discontinuous reality, but let’s privately remember that, at least in the world of evolution, it is no more than a convenient fiction, a pandering to our own limitations’. (Dawkins 2004, p.320) Dawkins observes that our minds work in a ‘discontinuous’ fashion. When exactly this is acceptable (rather than a tyrannical abberation) is a question that he leaves aside. Interestingly, he seems to tacitly accept that Ensatina is just one species, thereby contradicting the standard definition which dictates, as we have seen, that there are five (i.e., one consisting of E1 and E2, one consisting of E2 and E3, and so on). If the same principle was applied through time, then the total numer of species that ever existed in the world would have to be just one. In the presence of inconsistency – the inconsistency between entrenched species terms on the one hand and the interbreeding principle on the other – it’s easy to stumble.

What exactly is it about the biological examples discussed in this chapter that causes them to be problematic for the notion of a species? To answer this, let us return to the Ensatina salamander. Suppose you are a biologist in the year 3000, and suppose you are studying Ensatina. Progress being what it is, it seems likely that several of the six types of Ensatina salamanders will be extinct and, for simplicity, let us assume that the ones that are still around have not changed very much. Suppose, concretely, that E2, E4 and E6 have gone extinct and that, somehow, all traces of them (including the writings of such biologists as Dawkins) have sadly disappeared off the face of the earth. The schematic picture that we drew earlier in this chapter can now be simplified substantially. All you find are three types of salamanders, E1,E3,E5 none of which interbreed with each other:

E1 E3 E5

How many species do you see here? Three of course, because there are three groups here which do not interbreed. The problem has disappeared (and the number of salamander species has miraculously increased, even in the face of mass extinction – but let us leave this miracle aside). What this shows us is that, in our day and age, Ensatina salamanders caused us trouble (or taught us a lesson, depending on your point of view) because not enough of its sub-species had the decency to go extinct. Biological taxonomy is often unproblematic in practice precisely because all manner of intermediate cases have died out, leaving us mostly with species all of which are neatly distinct from each other. Ensatina is an exception that opens our eyes to how things might have been. There is a much more general lesson here, namely that the suitability of a concept depends on the abstract structure of the reality described by it: If the facts on the ground are as they were around the year 2000 then

7 ‘Ensatina’ is a problematic concept, but if they are as we imagined them to be in the year 3000 then it is unproblematic. The key difference between the two situations is this. In the year 2000, there are groups of salamanders A, B and C such that A interbreeds with B, B interbreeds with C, but A does not interbreed with C. In a mathematician’s jargon, the relation ‘interbreeds’ is not transitive. In the year 3000, the relation is neatly transitive, as a result of which it leads to a neat division into non-overlapping sets of organisms (essentially a partitioning, as mathematicians say.) In the chapters about logic, we shall see that non-transitive relations of this kind are the root of many problems. The rather formal considerations that were examined in this chapter are not the only ones driving a field biologist’s decisions. Interbreeding, after all, is only used as a criterion for specieshood because it tends to lead to a grouping of animals that are similar in important respects. Furthermore, in- formation about an animal’s breeding pattern can sometimes be hard to come by, making the standard definition difficult to apply. In practice, therefore, biologists let genetic, organic and behavioural similarities (including habi- tat) play a role alongside interbreeding. None of this, however, invalidates the claim that species-denoting terms are vague. Quite the contrary, it com- plicates the situation even further, by allowing different biologists to make subtly different decisions from case to case.

We have focussed on the notion of a species, but other concepts could be analysed along similar lines. Consider the concept of a language. American and British English, for example, are commonly grouped together as dialects of one language. Why do we not count them as separate? A principle that is sometimes invoked is that two dialects are part of one language if and only if their speakers can understand each other without specific training. One problem with this definition is that to ‘understand each other’ is a vague concept. Clearly, one vague concept (the concept of understanding) will now lead to another (the concept of a language). But suppose we somehow managed to define understanding in some crisp fashion, so that any two people would either understand each other (in which case they speak the same language) or they do not understand each other (in which case they speak different languages). The result would be almost exactly analogous to what happened when the notion of a species was defined using the crisp notion of interbreeding: understanding is not a transitive relation, since one can have people A, B, C such that A understands B, and B understands C, but A fails to understand C. The result, of course, would be a huge number of mutually overlapping languages, very different from the ones that we are all familar with. This may not be so obvious when speaking about island nations, but consider my country of birth, The Netherlands. Its language, Dutch, is as well defined a language as you might wish to find, with its own grammar and lexicon, with its own literature and so on. Yet Dutch speakers who grow

8 up close to the border with Germany have no diﬃculty understanding the dialect spoken across the border with Germany, which is commonly regarded as a dialect of German. As in the case of species, a concept that we regard as crisp turns out to be diﬃcult to pin down.

Issues to remember

• The classical notion of a species, which is based on interbreeding, is an attempt by biologists to give all species crisp (i.e., non-vague) and objective boundaries while ensuring, at the same time, that species are held together by physiological similarities.

• So-called ring species cause problems for this classical notion of a species because interbreeding is not a transitive relation.

• The classical concept of a species is not consistent with the way in which species-denoting words are actually used. Rather than a properly shaped ‘tree of life’, the criterion of interbreeding leads to a multitude of mutually overlapping species which would be very diﬃcult to make sense of by either lay people or biologists.

• The problems discussed in this chapter could make ordinary words like ‘chimpanzee’ and ‘tiger’ vague to the point of uselessness. In practise, these words are nevertheless useful because organisms that lie in between the diﬀerent species have often gone extinct. They loose much of their appeal when they are applied across a long period of time, during which the organisms in question have undergone important changes.

• Similar problems affect the notion of a language, as defined by the criterion of mutual understandability, according to which two dialects are part of the same language if their speakers can understand each other. Other definitions that are sometimes heard – e.g., “A language is a dialect with a government and an army” – suffer from equally severe flaws.

Footnotes associated with this chapter

(All footnotes to be presented as endnotes)

In addition to the cited literature, various Wikipedia websites (including the entries Species and Species Problem at http://en.wikipedia.org/wiki in August 2008) have been useful in preparing this chapter.

The modern concept of species, based on interbreeding, appears to stem from the works of Dobzhansky (1937) and Mayr (1942). The types of

9 problems with this type of deﬁnition that were listed at the start of this chapter have caused some researchers to propose alternative deﬁnitions in which morphological or genetic properties play a role, but these appear not to have gained much currency at the time of writing.

Dawkins uses the terms continuous and discontinous informally. He does not deﬁne these terms explicitly, but it appears that, to him, a concept is discontinuous if it must always either apply or fail to apply; this makes discontinuous synonymous with ‘crisp’. Continuous, for Dawkins, means what we call vague. The examples discussed in this chapter demonstrate that a situation can be essentially discrete while still giving rise to vague concepts. The mathematical concept of continuity will be discussed in chapter 7.

Plato’s Theory of Ideas (attacked by Mayr and others) was made famous by his eloquent parable of the cave. The bottomline of this parable is that the things around us are nothing but feeble shadows of a world that is much more real and important than they themselves are. According to this most extreme version of Platonism, individual dogs such as Fido and Towser are only half real: the real deal is the immutable concept ‘dog’ which they both mirror but imperfectly. Needless to say, latter-day Platonists did not necessarily hold such extreme views.

Relations can fail to be transitive for more than one reason. Given the shape of the earth, for example, the relation ‘is to the East of’ is non-transitive for a diﬀerent kind of reason than the ones that have occupied us in this chapter: Tokyo is to the East of London, San Francisco is to the East of Tokyo, yet San Francisco is normally thought of as West (not East) of London. This is not because of any vagueness inherent in the relation in question, but because of what might be called its circularity. ADD PICTURE: London EAST Tokyo EAST San Francisco NOT London EAST SAN FRANCISCO It may be worth observing in this connection that the term ‘ring species’ has nothing to do with circularities of this kind. (The name ‘ring species’ appears to originate from Stebbins 1949; See also Wake 1997, and Dawkins 2004).) The name was probably inﬂuenced by the fact that the habit of the California Ensatina sub-species has a roughly circular shape. The analysis in this chapter, however, suggests that this shape is accidental, and that the expression ‘string species’ might have been a better name for the theoretical challenge posed by species of this kind to the concept of species.

10 References associated with this chapter

Dawkins 2004. R.Dawkins. The Ancestor’s Tale: A Pilgrimage to the Dawn of Life. Weidenfeld & Nicolson. (Paperback edition by Phoenix, Orion Books Ltd., 2005.) Dobzhansky 1937. T.Dobzhansky. Genetics and the origin of species. Columbia University Press, New York. Mayr 1942. E.Mayr. Systematics and the origin of species. Columbia University Press, New York. Plato Wake 1997. D.B.Wake. Incipient species formation in salamanders of the Ensatina complex. Proceedings of the National Academy of Sciences of the USA 94:7761-7767. Stebbins 1949. R.C.Stebbins. Speciation in salamanders of the plethod- ontic genus Ensatina. University of Califronia Publications in Zoology 48, pp.377-526.