Th e interbreeding criterion

Like phyla and classes, species are monophyletic groups. Th e species ranking has its place in the hierarchy in that a species is a junior rank to a genus, a genus to a family and so on. But in saying that two groups are distinct species, we are saying more than just that they are distinct monophyletic groups with less divergence than two genera. Th ere is a further criterion that is usually used to decide if a group is a species. A common way to deal with this is to say that two individuals are the same species if they can interbreed with each other (or, if

they are the same sex, at least if their opposite-sex parents could have interbred with each other). Th is is called the biological species concept, and has its origin in the work of Ernst Mayr (1942). Th is is why dogs are considered a single species despite the fact that they come in a wide variety of shapes and sizes. Th ere are divisions of species elsewhere in the animal kingdom between creatures that look much more alike than an Irish wolfh ound and a Chihuahua: common chimpanzees and bonobos, for example. A biologist who did not already know, and did not have access to genetic data, might well look at our various breeds of dogs and decide that there were several diff erent species. But the criterion of interbreeding appears to settle the question. Calling a group a species, then, is not as subject to preference as calling a group a phylum. Unfortunately, things are not quite as clean-cut as this suggests, for several reasons.

9.1.1 Can they actually interbreed?

First, consider the question of whether an Irish wolfh ound and a Chihuahua could actually interbreed. Th ey certainly could not without artifi cial assistance. A male Chihuahua could only impregnate a female wolfh ound by artifi cial insemination, and a female Chihuahua could not carry puppies as large as the hybrids would probably be in her womb. Nonetheless, we can say that in prin-

ciple they could interbreed. Th is can be supported by a more robust criterion, that of genetic compatibility. A dog that had half Chihuahua genes and half wolfh ound genes would be perfectly viable, and moreover would be able to produce off spring itself. Th is is in contrast to a mule, which is sterile, and so we say that horses and donkeys cannot interbreed in the proper sense of the word. In actuality, groups are sometimes considered distinct species even though they can in principle interbreed, that is, even though they are genetically compatible in the sense just explained. Dogs and wolves can interbreed, but they are con- sidered diff erent species. Th is happens, for example, in cases where two groups cannot actually interbreed. Sometimes this is because of logistic considerations similar to the Chihuahua/wolfh ound case, but sometimes it is simply because of geographical separation. Th is last reason seems clearly insuffi cient to consider two groups to be distinct species. Would we consider Native Americans before 1492 to be a separate species from Europeans?

But the reason why the interbreeding criterion is taken to be important is that it allows genes to spread around the group, thus making it the group that evolves. Th is suggests that actual ability to interbreed is required. Moreover, if the two creatures in question are geographically separated and morphologically very diff erent, then we cannot say that an environmental change will aff ect them in the same way. Such considerations have led some theorists to propose an extreme “splitter” approach (e.g. Rosen 1978). At the other extreme are extreme “lumpers”, who argue that only groups that are completely unable to

interbreed should be considered diff erent species (e.g. Grant 1981). Th e super- splitter approach seems highly impracticable. It is always possible that some, even very small, geographical variation, or morphological diff erence, will mean that some environmental change will not aff ect the whole group in the same way. Moreover, it seems clear enough that Native Americans and Europeans

were the same species before 1492. Suppose, then, that we accept the genetic

compatibility criterion as suffi cient for single-specieshood, and accordingly rule that species classifi cations that fail to respect this are just wrong, and that the perpetrators should be reprimanded for indulging in unwarranted splitting. Would we then have a clear answer to whether any group is a species?

9.1.2 Ring species

Unfortunately, we would not have a clear answer to whether any group is a species. A further problem arises with what are known as ring species. Dawkins (2004: 252–61) drives this point home with the example of the salamanders of the western USA. In southern California there are two distinct species of salamander by the criterion of genetic compatibility: they cannot interbreed and that is that. So far so good. But if we take a group of one species – call it A – and compare it with its group of neighbours a little further north – call it B – we fi nd that they can interbreed. Go a little further north again, and we fi nd another group, C, which can interbreed with B, and so on around the ring, until we get to (let us say) group

F, which cannot interbreed with A. So we have: A can interbreed with B, B with C, C with D, D with E and E with F. But F cannot interbreed with A. So, which

species do we put the intermediate groups into? A, or F, or some other species? By the genetic criterion we have good reason to consider A the same species as B, B the same species as C and so on. But by the same criterion we have good reason to consider A and F diff erent species. So the logical outcome seems to be that A and

F both belong and do not belong to the same species. But this is a contradiction,

so something seems to have gone wrong somewhere.1

What it suggests is that we should classify creatures that are extremely unlikely to interbreed as separate species, rather than requiring that they be completely unable to. Th is would mean that whether a group is a species is a matter of degree, as Darwin had said. Th at is, sometimes it is clear that two groups are diff erent species, but not always.

9.1.3 Asexually reproducing organisms

But there is still more bad news. Ring species, it might be suggested, are not all that common, so perhaps we should not abandon a criterion that works well in most cases. Maybe so, but unfortunately asexually reproducing organisms are

the majority of living things. Th ere are far more individual single-celled organ- isms than anything else, and just in case you think it is a bit unfair to count one single-celled organism as equal to one multi-celled organism, the actual mass

of living matter (biomass) composed of single-celled organisms is believed by

many biologists to be greater than the rest as well. Th ere are also multi-celled organisms that reproduce asexually, such as the already mentioned rotifers. But when organisms reproduce asexually, there is no sense in which they can be said to interbreed, either actually or in principle. But how, then, can we speak of groups of these organisms as constituting species, as we do? Th ere is still genetic similarity, of course, to guide our decisions. But within the asexually reproducing world there is a continuous spectrum running from clone groups that are genetically identical, to diff erences as great as those that separate whole kingdoms in the sexually reproducing world. Clearly, being a clone group is too strong a requirement for single-specieshood. Imagine if we tried to apply it to human beings: only identical twins and the clones that science-fi ction writers promise us are coming would qualify. We seem to now have a situation where deciding if a group is a species is a matter of preference. In the case of ring species, we might say that the genetic compatibility criterion just happens not to work. At least we know what it would mean for it to work. But in the case of asexually reproducing organisms, it makes no sense at all.

Similar problems arise with many plants, which can form hybrids much more readily than animals. In many cases those hybrids, unlike mules, can breed. Once again, the criterion of genetic compatibility just does not work. But, it might be said, the fact that the genetic compatibility criterion does not work for very many, even the vast majority, of living things, does not imply that it never works. Perhaps the concept of species should only be applied to sexually reproducing organisms that do not form ring species and that do not produce fertile hybrids. As a further alternative, a pluralistic approach to deciding if some group is a species or not has been suggested, where the word “species” has a diff erent meaning in diff erent parts of the living world. I shall return to this point in §9.3. For the moment let us assume that we can live with at least one of these possibilities. Surely, it might be suggested, a criterion that works for some cases is better than no criterion at all? At least you can be sure that you are the same species as a person on the other side of the world, and that you are both a diff erent species from a chimpanzee.

9.1.4 Vagueness over time

But the problems do not stop there. You can be sure that you are the same species as Alfred the Great as well. We only need to stretch a little the sense in which you can in principle interbreed to show this. If you had a time machine, or if Alfred’s

parent) could produce a fertile off spring with him. So far, so good. Go further back in time, however, and things are not so good. It is beyond dispute that the people of one generation are the same species as those of the generation before it. Common sense and the genetic compatibility criterion both say the same thing. But trace your ancestry back a few hundred thousand generations, and you will eventually get to creatures that are indisputably a diff erent species from you. Th ey neither look like Homo sapiens, nor are they genetically compatible with us. But every successive generation in that journey is the same species as the one before. Th ere was no point at which a non-Homo sapiens gave birth to a Homo sapiens. We are faced with exactly the same paradox as with ring species. It can be shown by perfectly logical reasoning that the intermediaries are both the same species as us, and the same species as something that is not the same species as us.

But, unlike the case of ring species, this paradox arises not just in a few unusual cases, but throughout the living world. As we saw in Chapter 1, it is a central pillar of Darwin’s theory of evolution that new species arise by the accumulation of tiny steps. And, as we saw in Chapter 4, the reasonable version of punctuated equilibrium does not challenge that. So, even if we restrict it to sexually reproducing organisms that are not ring species, and do not produce fertile hybrids, the interbreeding criterion for single-specieshood does not work over time. Just as no answer can be given to the question “Are the salamanders of the western USA one species or two?”, it appears that no answer can be given to the question “When did such-and-such a species come into existence?” It would be like asking: on what day did John stop being an adolescent and start being an adult? For legal purposes such as being allowed to vote or buy alcohol, we do provide exact answers to this question. But we do this because we have to, not because we really think that adulthood is such a precise concept that it begins on a particular day. We would perhaps be comfortable with giving an approximate date to when John became an adult: perhaps when his behaviour started to show suffi cient signs of maturity. Palaeontologists do, when they have enough information, give approximate dates for the appearance of new species, and they do not appear to be talking nonsense when they do so. Several species of dinosaur really did arise and die out long before Homo sapiens arrived. Despite the paradoxes that arise when we allow our categories to be vague, biologists are perfectly right to allow them to be vague. And despite the paradoxes, biologists are not reduced to talking nonsense.