In most parts of the world black wolves are absent or very rare. In Yellowstone they are becoming increasingly more common.
Sometime in the last 10,000 years, as humans migrated across the Bering Strait to North America, they brought dogs with a gene that ultimately gave Gray Wolves black coats. Thousands of years later, conquistadors landing in South America brought their dogs and with them a virus that causes distemper.
Over the thousands of years since these two events collided, whenever there is an area where distemper outbreaks occur frequently, as in this section of the Rockies, there are more black wolves.
Why? Hitching a ride on the gene that codes for black coats is an increased immunity to respiratory viruses like distemper.
Wolves get two genes that dictate coat color, one from each parent. That means they either have two black genes, also called alleles, two gray genes, or one of each. For those who have one of each, the black allele is dominant and the wolf, as a result, is black.
So now when you see mated pairs of wolves, you'll often find one grey colored coat and one black colored coat. I wonder if in time more and more wolves in Yellowstone will be black.