This week's Nature has a news article by Emma Marris about bison conservation and genomics. I've been very interested in cattle and bison as an example of introgression in large mammals; in this case between two genera separated by over a million years of divergence. Possibly all, and certainly most of the bison left today have cattle genes in them. The article profiles geneticist James Derr, who sees these cattle genes as a conservation problem:
Wildlife managers have considered the genetic diversity of animals for some time, and animals in captivity have often been bred to preserve genetic diversity. But those were blunt approaches. Now, armed with genomic tools, researchers are starting to look at specific sequences in the genome, and are raising questions about what the fundamental unit of conservation should be. Most people see preserving wildlife as a matter of saving individuals; if all the individuals die out, the species becomes extinct. But that reasoning looks simplistic when considered at the genomic level. If the genes of a species change enough — through interbreeding, for example — that species will cease to exist even if individuals that look something like the original continue to thrive.
This issue is quite threatening to the entire idea of endangered species preservation. One argument for extending protection to multiple populations of species like chimpanzees is that you are preserving gene pools that have unique evolutionary histories. You can't just preserve one tiny corner of a species and expect to retain the genetic diversity that was present in the whole species' range.
But if your species' genetic diversity is already compromised by introgression, either from within the same species or from other more distant lineages, this argument is weakened. And if it's OK to preserve a fragment of diversity in an interbred population, then why not simply introgress the endangered species' genes into a more common, cosmopolitan relative. That is, why save the wolf, if you've already got lots of dogs with wolf genes in them? Why save the polar bear, when its genes will continue to exist in brown bears?
As the article notes, this question is really academic for most threatened species, whose population histories may not lend themselves to such promiscuous gene mixing. But bison are an interesting case nonetheless --
In 1905, then US President Theodore Roosevelt and William Hornaday, head of the New York Zoological Society (now known as the Wildlife Conservation Society), founded the American Bison Society, which collected bison and established herds in a few reserves in Montana, Oklahoma and South Dakota. A small herd, perhaps 30 in number, was still roaming Yellowstone National Park. According to Derr, all the bison in the United States today — there are now up to a million of them, mostly on private ranches — can probably be traced back to fewer than 200 bison.
Other scientists argue that the most important thing may not be unique genes, but instead unique cultural inheritance and status within ecological communities:
"There are more important things than genes," says Rurik List, an ecologist at the National Autonomous University of Mexico, who works with a herd that spans the US–Mexico border. These bison have some cattle genes, but they also have institutional memory. If List were to remove them and replace them with pure animals, would the bison still be able to find the water holes that the current herd knows so well? "They have been behaving like bison for 80 years," says List. "They have been fulfilling an ecological role."
So far all the genetic estimates of introgression are based on only 14 markers -- probably good enough as a test for the fraction of introgression dating back within the last 150 years, but it's not going to give any information about the dynamics or even the identity of genes that have moved into bison. Many more markers are going to be necessary in this and other cases of reintroductions and hybridization with domesticated varieties.
Marris E. 2009. Conservation: The genome of the American West. Nature 457:950-952. doi:10.1038/457950a