More on bison and introgression

3 minute read

Jim Robbins of the NYT has written a long article about genetic introgression of cattle genes into bison populations. The article is mainly concerned about management, and wildlife managers are trying to minimize the proportion of cattle genes in their conserved herds:

Over time, cattle genes have spread into many of the remaining herds of American bison. Since the late 1990s, Dr. Derr and his graduate students have traveled to public and private bison herds around the country, taking blood samples. They have concluded that the vast majority of the 300,000 or so bison in the United States are hybrids, though they look like pure bison. Fewer than 10,000 bison are genetically uncontaminated.

The whole idea of "genetic contamination" implies that there is something bad about this genetic introgression. But we can guess that the cattle genes don't intrinsically reduce fitness, since bison with cattle genes have been greatly increasing in numbers. And these introgressed herds are unlikely to be fixed for any cattle genes, so the original bison alleles still have every chance to compete with the cattle alleles. In other words, the cattle introgression has introduced variation into bison, some of which might be adaptive.

As you can tell, I'm not very sympathetic to the idea that we should prevent "genomic extinction" by insisting on some kind of genetic purity. It seems to me that we want to retain as much variation in our conserved populations as possible, so that they can adapt to changing climatic conditions in the future. We can't predict which alleles will be adaptive.

The geneticists in the article worry that cattle genes will make the bison susceptible to cattle-borne diseases like Texas fever. But making a large herd of genetically uniform bison is hardly the way to prevent disease!

Now, a history of selection for docility on ranches is of more concern:

"Ranchers might get rid of a cantankerous bull, for example," said Curt Freese, a biologist who directs Great Plains bison restoration for the World Wildlife Fund. "Breeding bison to be docile and meaty are the kinds of things that affect the wildness of the bison."

But it's unpredictable what behavioral traits will adapt bison to a conserved herd, which after all must be smaller and occupy a lot less space than many of the ancestral bison herds. They may end up more docile anyway, or just the opposite. I tend to think that selection will sort all this out.

Managers of these herds must also keep a wary eye on hybridized invaders. In Yellowstone, officials found a domestic bison that had wandered into the wild population from a neighboring ranch. And Wind Cave National Park is adjacent to Custer State Park, where the animals are hybridized.
The new approach may change other aspects of management, as agencies move from managing the species to managing the genetics. Dr. Derr is involved in a study, for instance, of whether the hunting of the bison that leave Yellowstone might be selecting certain behaviors from the population because animals that migrate are targeted.

This kind of selection is unavoidable in conserved populations, and might even be desirable -- they do, after all, want to stop the bison roaming out of the park. Roaming out of the park is one of the more noticeable bison phenotypes. I'm more worried about all the selection that is happening but doesn't have obvious effects.

This seems like a good doctoral project for somebody: how do the introgressed bison compare behaviorally with "genetically pure" bison? And the all-important question: how does mean fertility compare between these herds? They've both historically grown very rapidly, but does one maintain higher mean fitness than the other? Are there more animals in the Custer herd that fail to reproduce?

Anyway, there was no way to quantify the introgression until recent molecular techniques made it possible, and Ted Turner and others were happy to breed large bison herds that contained introgressed cattle genes. The only difference now is that wildlife managers know that some herds are "more pure" than others. But making conservation decisions on "purity" seems less relevant than fitness, which they still don't know much about because it's harder to measure. There is a presumption that the originally bison alleles will be more fit, but today's conserved situations are very different from those faced by ancient bison. And the historic bison -- the ones shot up by Buffalo Bill -- were facing a very novel environment compared with their ancestors.

The best we can hope for is a capacity for adaptation, which will maximize the chance of survival. In that context "genetic purity" is less important than genetic variability.