From a Mastodon post by Rajini Rao (@madamscientist@mas.to):

When humans migrated across the Bering Strait to N America during the last Ice Age, they brought along domesticated dogs. Soon after, black coats appeared in the native Gray Wolf population, caused by increased melanin due to changes in a single gene.

Known as the K gene (Japanese kurokami for ‘black hair’), the dark coat resulted from a variant introduced by interbreeding with dogs. Wolves carrying one or two copies of the KB version are black.

The frequency of black wolves increases from the Arctic to Mexico. Black coats are more common in regions with canine distemper outbreaks. It turns out that the K gene is a natural antibiotic (“defensin”) that confers immunity against distemper.

But the black coat variant also carries a fitness defect: few pups with 2 copies survive to adulthood. Gray-coated wolves produce larger litters and are more aggressive than black-coated wolves during territorial conflicts.