Answer: Melanism

The most readily identifiable pigmentation disorder in both humans and animals is albinism. Nothing stands out in nature more than a creature best known for a dark and relatively camouflaged body, like a crocodile or deer, stark white against the greens and browns of their habitat.

While albinism is a genetic variation characterized by a complete or partial lack of pigmentation, melanism is a genetic variation found in animals characterized by hyperpigmentation. In the photo here, as an example, you can see two European adders, Vipera berus, side by side. The two snakes are from the exact same species but one, with melanism, is jet black.

Not all melanism is a one-off genetic quirk, like a random black snake in a brood. It can also be adaptive. The most widely taught example of adaptive evolution—the changes in the peppered moth in the United Kingdom during the industrial revolution—is an example of adaptive melanism. As soot from the increasing number of factories darkened everything in London from building exteriors to tree bark, the peppered moth slowly became “sooty” itself, as its offspring with darker and more peppered wings blended in better to the dirty environment. This type of adaptive evolution in response to pollution is common enough that it even has a name: “industrial melanism”.

Another well known example of melanism is that of black panthers and jaguars. Long thought to be distinct feline species, modern genetic testing has revealed that the black jaguars of the Americas are actually regular jaguars, Panthera onca, with melanism. The same for black panthers in Asia and Africa, which are leopards, Panthera pardus, with melanism.