|Hybrid domestic pig x wild boar|
The common understanding of a 'species' is that it consists of a group of animals that cannot interbreed with animals outside that species to create fertile offspring. This, however, is an oversimplification; lines blur, and not always in neat and tidy ways. Scientifically, there are multiple different ways of defining what a 'species' is, but the general idea is that it has to be a group of animals that can be readily identified as different from their relatives. Which tends to mean that, even if they can interbreed with other species, they generally don't, at least in the wild, and maintain a separate gene pool.
The problem is, of course, that they do interbreed - albeit not usually under natural conditions. This, naturally, is even more true for subspecies, which can be difficult to define at the best of times. This interbreeding can be a major driving force behind evolution - perhaps more so in plants than animals, although it does occur in both. But, when it comes to conservation, hybrids are generally seen as a nuisance, muddying the genetic waters, and perhaps even supplanting the pure populations over time.
Assuming we agree with that, it's clearly useful to conservationists to know how much hybridisation is occurring in the wild, and whether or not it really does pose a threat. One of the places in which hybridisation occurs a lot, even among relatively large mammals, is Europe. This is because of the continent's relatively dense population and long settlement, dramatically changing the landscape, especially over the last few hundred years. (Obviously, Europe isn't alone in this, with places such as Japan and India being similarly altered by human activity, but it's more so than, say, most of America).
So where do we stand on the presence of hybrid cross-breeds in Europe? A review of the last 18 years of reports in the scientific literature of hybrid hoofed animals on the continent gives us a picture.
Perhaps the most obvious cases of hybridisation are those that involve domestic animals cross-breeding with those in the wild. Domestic animals are sometimes considered species in their own right, so different are they from their forebears, but at the very least they are subspecies and cross-breeding can have significant effects on the genetics of the wild population, given that we have usually bred them for specific traits not seen in the wild.
Perhaps the most common such example in Europe is with pigs, where pig/wild boar hybrids are found across many parts of Europe, including most of Spain, France, Poland. One study on animals in northwestern Europe found genetic markers for domestic breeds in 10% of the wild boar they examined, indicating a high level of interbreeding from multiple different sources.
In many cases, this is likely the result of deliberate practices, at least to start with. Wild boar are widely farmed in parts of Europe because their meat has a stronger taste, and a common practice is to cross-breed them with domestic pigs, especially the ancient Tamworth breed. This results in pigs that can produce larger litters than wild boar, and with a meat said to be between the wild and domestic types in flavour. Such pigs may at times escape from their farms, or even be kept semi-wild, reducing the cost of looking after them.
Surprisingly, a study on the wild boar of Sardinia, which belong to a subspecies that's only otherwise found on Corsica, failed to find any evidence of cross-breeding with domestic pigs - but did find evidence of hybridisation with the mainland Italian subspecies of wild boar. So, even here, if we want to preserve the original, pure, subspecies, we want to concentrate on the wild animals of the western parts of the island, where hybrids are less common.
Speaking of Sardinia and Corsica, while wild sheep as such don't live in Europe, the islands are home to mouflon. These are more accurately described as feral animals, being descended from domestic sheep that were brought to the islands over six thousand years ago, and subsequently reverted to the wild. Because of this history, it's difficult to identify mouflon-sheep hybrids, since they're pretty much the same animal, but they're generally described as at least a subspecies (Ovis aries musimon), and some hybrids between mouflon and local breeds of (modern) domestic sheep have been identified.
Another domestic animal that we'd reasonably expect to hybridise with wild populations in Europe is the goat. Domestic goats are often left to roam relatively freely, and in some places they share their range with wild populations or (in the case of Crete and Mallorca) longstanding feral herds. There seems, however, to have been little research to prove this genetically, although the anecdotal evidence appears strong. In any event, it would be fairly surprising if it didn't happen.
What may be more surprising though is that genetic and biochemical studies have confirmed the presence of hybrids between domestic goats (Capra hircus) and the entirely separate Alpine ibex species (Capra ibex). In some respects, this might be a good thing for the ibex, since the recent decline in their population has left them with little native genetic variation, and the inclusion of some genes from outside might be helpful. The hybrids are also larger than their parents and apparently have stronger immune systems. On the other hand, while they can breed and even form their own herds, they seem to have more difficulty doing so than the pure species, which could be problematic.
Although they are native to North Africa, Nubian ibex (Capra nubiana) have been introduced for game hunting in parts of Slovakia, and there is at least some evidence that they, too, can cross-breed with wild goats - and, indeed, with the local Alpine ibex. Oddly, while they are technically capable of it, there seems no evidence for the same happening with Iberian ibex (Capra pyrenaica) in the wild, perhaps because their breeding habits are just too different for them to be attractive to domestic goats.
Chamois are also members of the goat subfamily but are too distantly related to the domestic animal to cross-breed with it. Nor do the various species cross-breed in the wild that we know of, although subspecies do where they can. For example, both the Balkan and nominate subspecies of Alpine chamois (Rupicapra rupicapra) have been introduced to Croatia as targets for hunters, and do interbreed now that they have the opportunity to do so.
Another instance where we have actual cross-species hybridisation, however, is provided by roe deer. The native species is called, appropriately enough, the European roe deer (Capreolus capreolus) and lives across most of the continent. Populations around Poland and Lithuania, however, show genetic markers indicating that they are partly descended from Siberian roe deer (Capreolus pygargus), native to Siberia, Central Asia, China, and Korea. Such hybrids are probably better able to withstand harsh winters than the pure European species, which may explain their survival, especially in this part of the continent.
Quite how this happened, however, is unclear - either somebody introduced some of the Asian sort for hunting without scientists being aware of it or they once lived a good deal further west than they do now. As it stands, they never meet... yet hybrids between the two apparently still exist.
One type of non-native deer that certainly has been introduced to Europe is the American elk or wapiti. These were introduced to the UK in relatively recent times, largely because they look impressive. These animals are so similar to the native red deer (Cervus elaphus) that they are usually considered to be the same species, although there are dissenting opinions on that point. Indeed, the introduction of the animals to England and Scotland was likely at least partly in the hope that they would crossbreed with the locals, in the hope of producing offspring with larger antlers. Funnily enough, though, this didn't work, and hybrids are rare... it's possible that they have difficulty breeding.
On the other hand, red deer definitely do crossbreed with sika deer (Cervus nippon), a more genetically and physically distinct species native to Japan and some neighbouring parts of East Asia. These were introduced into Europe in the 19th century, largely as ornamental animals for parks, and have become widespread across wilder woodlands since. Significant numbers of fertile hybrids of these otherwise distinct species are found in eastern Europe, and in parts of Britain. This, at least, is a well-known issue, perhaps spurred on by hunters, since sika deer hybrids have smaller antlers than pure red deer, making them less valuable as trophies. The sheer number of such hybrids may also, of course, be relevant, even if they aren't that easy to identify without mitochondrial DNA analysis.
Is all of this a bad thing? As noted at the start, that's perhaps debatable. None of these species are particularly endangered, after all, so they're not likely to be in any danger of imminent extinction. But the fact is that we don't really know, and it can often be hard to even identify hybrids if they're between sufficiently similar species (let alone subspecies). As our ability to perform genetic testing improves, we may find more evidence of hybridisation than we had previously expected, and may get an improved idea of whether it's something we really ought to be worried about or not.
In the meantime, what we can say is that species are by no means as hard-and-fast as we might think. You don't necessarily have to put animals in unnatural conditions such as a zoo in order to get them to breed with something you wouldn't expect them to. Sometimes, just being in the same part of the wild is enough.
[Photo by Miguel Tremblay, from Wikimedia Commons.]