The wings of the peppered moth are usually white with black speckles—a pattern that renders them invisible against the bark of a typical tree. But in the early 19th century, the trunks of English trees became jacketed in soot, which was belched forth from coal-fired factories. Against these trunks, the once-camouflaged moths shone out like beacons, making them easy prey for birds. But some lucky moths had mutations that gave them all-black wings. They successfully hid against the sooty trunks while their peppery peers were devoured, and as the decades ticked by, they became more and more common. By the turn of the 20th century, almost all the peppered moths in some parts of England were black.
The peppered moth’s turn to the dark side made it an icon of evolution. It’s a wonderfully evocative example of natural selection changing the body of an animal to meet an environmental challenge, by favoring certain genes over others—and we even know exactly which gene was responsible. It’s also the best-known case of industrial melanism—the phenomenon where animals become blacker in areas that are affected by industrial pollution.
Most cases of industrial melanism involve moths, butterflies, and other insects, but Claire Goiran, from the University of New Caledonia, has now discovered a surprising exception. In the waters around Australia, one species of sea snake, which usually has black and white stripes, has also become completely black. And just like the peppered moths, urban pollution is probably to blame.