> 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.