The notable discovery is that, where the evidence a few decades ago was that the bison were breeding in their historical herds (so multiple, smaller genetic pools), they now appear to be breeding between herds (so a single larger, more diverse genetic pool).

AIUI with small populations, more variation in breeding between groups is a good thing, because it spreads genetic diversity across the whole population.

You probably mean inbreeding. Interbreeding is good. It is good that the bison herds mingle and interbreed.

Anyway, for mammals an initial population of a couple dozen individuals (assuming they're reasonably genetically diverse in the first place) is plenty enough to produce a population of any size without problems.

There's a general guideline called the 50-500 rule. You need at least 50 animals to avoid immediate inbreeding (and also stochastic extinction from a fire or flood or disease etc), and about 500 to have a genetically healthy population. That varies some after a bottleneck event since your genetic population will be functionally less than your actual physical one, but it's a decent way to approach the problem.

Having a single breeding population across the park creates more genetic diversity than would be present in isolated herds.

> Bison like those in Yellowstone once suffered a population crisis that conservationists call the "population bottleneck" of the 19th century. By the early 1900s, American bison numbers had been reduced by 99.9% across North America and only 23 wild bison were known to have survived poaching in Yellowstone.

So at their worst this particular population only had 23 individuals left. Interbreeding is bad insofar as it increases the chances of passing harmful recessive genes to younger generations.

Compared to the alternative of the species not surviving at all, it seems like the better option :)

Besides, it seems like they think it's genetically healthy, so doesn't seem like a problem. I'm assuming they've verified this somehow.

> Today, the Texas A&M researchers report that the Yellowstone bison population appears to be functioning as a single and genetically healthy population that fluctuates between 4,000 and 6,000 individuals.

> Compared to the alternative of the species not surviving at all

How about compared to two distinct herds?

Getting that "living on top of a volcano" risk feeling :-)

Biology is not my area of expertise, but: Interbreeding is bad when it’s a small population interbreeding for a long long time. From the article it sounds like they aren’t worried about the genetic diversity of this 6k bison herd. I’m sure it would be better to have more diversity, but that’s hard to achieve with animals brought back from near-extinction.

There are a lot of private herds. But a lot of them have been bred with domesticated cattle and do not have the pure bison DNA in them. They can be used as a last resort. The solution here would be to slowly start separating herds to more locations away from Yellowstone. Over generations, the genetic makeup will separate enough to be considered separate populations.

At least two groups are now breeding as a single population. The genetic diversity might be more spread out over the population. As I understand the article there were two functionally separate groups as late as 20 years ago (already 100 years after the introduction of the Texas bison to the original Montana heard) and now they are recorded as being a single population.