The evolution of white fur and an animal sex scandal

Up north — in Canada and other places where snowy winters are reliable (and reliably heavy) — you find more animals whose fur comes in various shades of white. This is true even for species that are brown or black further south. The difference is obvious. But how does it happen? Carl Zimmer presents two possible paths to paleness — random mutation, and fortuitous cross-species mating. In related news: Golden retrievers are probably getting it on with Canadian coyotes.