A new report, Hybridisation in British Mammals, compiled for the Joint Nature Conservation Committee, assesses the implications of some of the closest liaisons taking place in the wild. Not all are bothersome. Take the mountain hare and the brown hare, two closely related mammals. Mountain hares, which often turn white in winter, don't usually inhabit the lowlands, where the brown hare thrives.
In parts of Scotland, though, their ranges overlap. Nevertheless, wild hybrids between the two occur only rarely. So the two species obviously remain faithful to their own kind. But conservationists are concerned that if red deer do not resist the advances of sika deer - first introduced to Britain in 1860 - the famous monarch of the glen will disappear.
Sika populations have expanded rapidly in the past few decades mostly because of their liking for the dense forests that young plantations of conifers grown for timber have provided par excellence. But there are reports, too, of sika developing interest in the more open hill country, traditionally the preserve of red deer. The first record of a hybrid between these similar-looking species was in 1940. Since then they have occurred in reasonable numbers in northern England and in several areas of Scotland. It is quite possible that hybrids more akin to sika will eventually replace red deer in woodland habitats.
Whether they make substantial inroads into the hill and moorland populations of red deer may depend on whether more conifer plantations are planted on land adjacent to such areas. Locally, sika deer and hybrid red/sikas are shot, but eradication of sika from Britain would now be an impossibility. Their spread is inevitable.
But our native red deer is a bit of a mixer to begin with. American subspecies, and continental races, have long been introduced to British parks and estates, supposedly to 'improve' the stock. Some of these animals, and their derivatives, have escaped. Apart from pure, unadulterated reds in North-west England, all English reds are thought to be mixed genetically. In Scotland, park deer have been introduced to most, if not all, of the red deer's range. Probably only on some Scottish islands do examples of racially pure red deer still exist.
Perhaps of more concern is the impact of domestic cats on Scotland's remaining population of wildcats. As wildcat populations have expanded from a low point early this century, they have increasingly inhabited less remote areas. So their contacts with feral cats - domestic cats living wild - have increased. Originally, our domestic cats had a truly wild ancestor. It may well have been the African wildcat, a species closely related to the wildcat of Scotland. Some experts think the two may even be the same species but exhibiting slight differences.
Scottish Natural Heritage is checking wildcats and feral cats across Scotland to discover if hybridisation is more prevalent in certain areas and whether unadulterated wildcats still occur. It is focusing not just on differences in appearance, including fur colour and pattern, but on genetic differences using DNA fingerprinting.
But does all this really matter? It all depends, perhaps, on what we understand a species to be. To most biologists, species are groups of interbreeding natural populations that are reproductively isolated from other such groups. Each species has a distinctive appearance, size, shape and habits. One can always distinguish a song thrush from a blackbird, and both from a mistle thrush. All three are related, but each is a distinct species. The problem is that we have defined each one in a rather simplistic way. Their complete genetic make-up is not known. So trying to understand how closely related two species are is fiendishly difficult.
The most important property of a species is that it is a separate line of evolution with little or no exchange of genes with other species. This allows more rapid evolution than would be possible if all organisms exchanged genes because it allows genetic adaptation to local conditions. Species change naturally with time and can eventually give rise to new species. Otherwise, evolution would be at an end. Neither does a species exist in glorious isolation. It is reliant on its habitat. If that is modified, the species will change and evolve to suit a new set of conditions. So, as the authors of the JNCC report put it: 'The native, genuine, 'real' wildcat or red deer are only as native, genuine and 'real' as the habitat in which they are presently evolving.'
Professor Steve Jones of University College, London, a biologist and former Reith lecturer, isn't much bothered about hybrids. 'The question is, do we worry about the species or the genes?' he says. 'In hybrids we may lose the species as we know it, but its genes are still there in the hybrid.'
Professor Jones thinks we have become obsessed with cataloguing and pigeon-holing species and that conservationists should bother themselves about far more important issues, such as destruction of traditional habitats.
That is fine if it all happens naturally. Who can quibble with trout and salmon - both native species in many of our rivers - producing a few talmon (or strout)? And should we bother if sika deer, originally from China and Japan, make it with our enigmatic reds, a deer well adulterated with a motley collection of other deer genes anyway?
What is more difficult to accept is the loss of a species through hybridisation with another species that it would not naturally encounter. It is difficult for conservationists to wave goodbye to a species they have struggled to retain. Knowing that its genes are still around is not much consolation.
'Hybridisation in British Mammals', JNCC Report No. 154, by Elizabeth Balharry, Brian Staines, Mick Marquiss and Hans Kruuk.