In the name of purity
By Jon Bridle in the New Scientist, 30 th August 1997
When a Scottish farmer shot a wildcat in 1988, he was prosecuted under the 1981 Wildlife and Countryside Act. However, the farmer was acquitted because the prosecution could not prove that the animal was not a hybrid between a wildcat and a domestic cat - only pure wildcats are protected by the law. As the case proved, defining species for conservation is a complex issue.
In the past, deciding if an animal was a hybrid was largely based on its coat or skeleton. However, by the early 1990s, far more sensitive tools were at hand to help zoologists detect hybridisation. Armed with powerful genetic techniques, they sought "foreign" genes in native populations that outwardly seemed completely pure, so revealing any history of interbreeding with non-native species. Once this was established, the zoologists could assess the threat to the survival of the species.
Hybrid animals are often not protected by environmental law--even if they appear to be identical to pure animals. In the US, the 1973 Endangered Species Act only protects animals that have not been "contaminated" by foreign genes. Some long-established conservation programmes are now under threat. For example, in 1994, when Michael Roy of London's Institute of Zoology confirmed that American red and grey wolves have interbred with coyotes, calls were made to end funding for their conservation.
The problem is that legislation tends to protect rare species rather than rare genes. The laws are based on the traditional view that hybridisation is a rare process that always results in one genome completely swamping another. However, studies of real populations suggest that this is not the case. Although hybridisation transfers many genes across species, the genes that move most easily into natives will be those that have little influence on the way they are adapted to their habitats.
By contrast, natural selection can work strongly against a hybrid offspring that has lost genes that are important to the unique biology of the native species. Such selection is strongest in healthy native populations living in healthy native habitats. However, as habitats become more disturbed, introduced mammals, which are well used to humans and their environments, become more common while native populations become smaller and more isolated.
Elizabeth Balharry and her colleagues at the Institute of Terrestrial Ecology in Banchory, Scotland, suggest that any concept of "genetic purity" is suspect, considering how radically the environment of native organisms has been altered (Joint Nature Conservation Committee Report, no 154, Hybridisation in British Mammals, 1994). They argue that the genes favoured in today's fields and parks bear little relation to those favoured thousands of years ago. So if a modern native wolf is very different from the native wolf of the past, what can the law mean when it talks about "pure" wolves?
Populations can only be as pure as the habitat in which they evolve. Red deer in Britain, for example, are threatened by the spread of Japanese sika deer genes into their populations, which seem to do better in Britain's modern fragmented and modified landscapes. So foreign genes influencing behaviour and morphology may actually confer an advantage to native animals living in the habitats they now find themselves in--making their transfer all the more likely.
Current law concentrates on protecting designated species by captive breeding and reintroduction, rather than tackling the trickier issue of preventing the destruction of habitat. The issue of safeguarding habitats is tangled and politically complex. But unless habitats are protected, where can captive populations be reintroduced? More to the point, what will stop foreign genes being introduced into the population again?
A better long-term approach would be to protect the type of habitat that promotes the survival of native genes and so restricts the spread of introduced ones. This may be the value of such endangered mammals, regardless of their hybrid status. Their great strength is that they are cared about. They encourage the protection of large areas of primary habitat that are home to many more diverse groups of organisms with less fur and charisma. To many people, these wide-ranging natives are symbols of the world to which thousands of us try to escape every weekend. With the current policy in force, even their continued protection may be under threat.
JON BRIDLE is a member of the Ecology and Evolution Group of the Department of Biology at the University of Leeds
New Scientist, 30 th August 1997
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