|This one really is a golden jackal|
Its also not at all obvious how you're supposed to apply this definition to animals that don't reproduce sexually (though that's obviously not a problem with mammals). And what about those cases where two species have separated so recently in the evolutionary past that some individuals can crossbreed, but others can't? The classic example here are the herring gulls and lesser black-backed gulls, where the Siberian populations of these two species can interbreed, but the British ones can't. It really does get messy very quickly.
Even if it wasn't for all this, if we really did use the fertile interbreeding definition, how useful would it be? Well, we would have to accept that polar bears are really just odd-looking brown bears, for a start. But more than that, what about other species? Are we really going to try and cross-breed every plausible combination to see what happens? It can be hard enough to get some animals to breed in captivity at the best of times, making the whole idea completely impractical.
Inevitably, this means you sometimes have to end up applying some common sense, and describing something as a separate species largely because it's useful to do so. So polar bears, for example, are considered a species, because they are different in so many ways from brown bears. At the opposite end of the scale, there are many species that don't generally interbreed, but are so closely related that they look almost exactly the same, making it really hard to tell where one species starts and another ends. A lot of these are mice of various kinds, but it's also true of dogs.
The wolf (Canis lupus) is a very widespread species. Before humans drove it off from many parts of its range, it was found throughout Europe, and in most parts of Asia and North America. Human intervention brought them to Australia, in the form of dingos, and the domestic dog is also considered a subspecies of wolf. They once used to inhabit the southern coast of the Mediterranean, but now wild wolves are completely absent from Africa. Or are they?
The closest living relative of the wolf is the golden jackal (Canis aureus). Golden jackals are found in the Balkans, and across Arabia and southern Asia, but they are also found across northern and eastern Africa as far south as Nigeria. It would seem, therefore, that the golden jackal is more suited to these hot climates than its more northerly cousin, the wolf.
Wolf Golden Coyote Ethiopian
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| | | | Other
------------ ------------- Jackals
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----------------------- | Dhole
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Like the wolves, golden jackals are a varied bunch, with several different subspecies recognised. Indeed, one subspecies, found in Egypt, is quite hard to tell apart from a wolf - were it not for the fact that, officially, there are no wolves in Egypt. There had been some dispute in the past as to the exact status of these Egyptian animals, but, more recently, some very wolf-like jackals were also spotted much further south, in Ethiopia. Were they really jackals, as the textbooks said they had to be, or were they something else?
You can't just capture them, breed them with wolves, and see what happens. Not least because we know that wolves can breed with coyotes without any difficulty, and they're certainly a different species, so fertile offspring produced in captivity wouldn't prove anything either way. To see what they really are, we need to look at their genetics. If two groups of animals regularly crossbreed in the wild, their genetics should be very similar, and we have as good a reason as any we can come up with to say that they're really the same species.
So that's what the researchers did. They collected samples of the animals' dung, sequenced their DNA, and compared it with samples taken from jackals in Egypt, with wolves and golden jackals from a range of locations, and, for good measure, from seven other species of the dog family. (Including foxes - when you're doing this sort of test, you need to include something that's sort-of-similar yet manifestly not the same, just to make sure it's working properly).
Of course, they didn't analyse the entire genome, since modern technology isn't up to making that at all routine. Instead, they looked at a particular gene, cytochrome b, which is used as a standard in these sorts of studies. It's presumably not perfect, but with everyone using the same gene, at least it is possible to compare results with those from other workers, and this particular gene has done a good job so far at figuring which mammals are related to which other mammals.
The results showed, firstly, that these Ethiopian jackals were almost identical, genetically, to the ones seen in Egypt. This alone is significant, since it means that this subspecies is found much further south than previously thought. But it also showed that these animals were more closely related to wolves than to other golden jackals. Indeed, in some respects, they were more similar to wolves from places like Canada and Sweden than they were to wolves from the Himalayas.
So, whatever these animals are, they can't possibly be golden jackals. It appears that they diverged from the main group of wolves after the Himalayan wolves did, which means they have to be wolves, too. All the textbooks say there are no wild wolves in Africa, but we now know that there are - at least in Egypt and Ethiopia, where they have been living alongside golden jackals, and looking so similar that few people could tell them apart.
This is exactly the sort of problem that occurs when you have very similar looking species living side by side. Indeed, this sort of finding crops up quite often with mice, with nobody much noticing (or caring, frankly). But even then, is this animal actually a subspecies of wolf, or something else entirely? Just a few years ago, it was argued that Himalayan wolves were themselves a separate species. If that's right, then the Ethiopian animals might well be a species of their own, too - one we didn't previously know about. It's debatable either way, since they do seem to be much closer to northern wolves genetically than we'd expect a new species to be, yet different enough that a blood test will tell them apart.
Does it matter, though? Do we care whether this is a species or a subspecies? Well, funnily, enough, yes it does make quite a difference. Wolves are not an endangered species (and neither are golden jackals), so if these animals are a special sort of wolf, they will get relatively little protection from the various international conventions about such things. But if they're a species, then there's pretty good reason to suppose they might be endangered - there certainly aren't very many of them, and the local farmers do have a habit of killing them to protect their livestock. That's not a problem (from a species conservation perspective) if they're also found elsewhere in the world, but if this is the only place the species is found, then they can get protection under international law.
Considering that we can't come up with a good definition of what a species is, it makes a heck of a lot of difference whether a group of animals is one or not.
[Picture from Wikimedia Commons]