At one point the Milk Snake (Lampropeltis triangulum) comprised two dozen subspecies ranging from Quebec to Ecuador, from drab, saddled snakes to brilliant tricolours, and from bootlace-sized minatures to six-foot behemoths. The difference between a local Eastern Milk Snake and a tangerine-morph Honduran Milk Snake from the pet store is pretty extreme. It had been suggested, I can’t remember where, that the Milk Snake was an example of a ring species, where neighbouring populations interbreed but the end points (i.e., Quebec and Ecuador) are too distantly related.
But the simpler answer is that these snakes are not all one species, and a recent study—an early draft of which I tweeted about in December 2013—suggests that they are, in fact, seven species. The authors divide them as follows: