An interesting follow-up to my recent posting on timber wolves, research cited in the journal Science indicates that black wolves get their coat from cross-breeding with domesticated dogs - animals originally descended from wolves - that were bred for their dark coat some 50,000 years ago.
As the first humans crossed the Bering land bridge into North America some 15,000 years ago, they brought with them their dogs - the black dogs carrying a mutated gene, beta-defensin, that is part of a family of genes thought to aid in fighting infections. Apparently it was easier for the dogs to mingle with wild animals in North America than in Europe and so the black wolf has been exclusive to the forested Canadian Arctic and icy tundra.
More black wolves occur in the forested areas than in more open icy areas (67% occurance vs. 7% respectively), and that is thought to be because of the black fur's ability to help the wolf catch prey in the forest and the added help the beta-defensin gene provides the wolf in fighting infectious agents that occur primarily in the forest.
It serves as an interesting example of Darwin's theory of natural selection - once introduced into the species from domesticated dogs, the black fur trait slowly diminished where it did not serve to enhance survival of the species (black fur not necessarily being an advantage in the icy tundra and its infection-fighting ability being of less value in that region).
Read L.A. Times article.