Peppered moths get their name from their appearance. The original form of the species has white wings covered in small black speckles, like someone shook a pepper grinder over them. That salt-and-pepper pattern is the moth’s default look and the reason it’s been called the peppered moth for centuries. But the story behind that name opens up one of the most famous examples of evolution ever documented.
What the “Peppered” Pattern Looks Like
The peppered moth (known scientifically as Biston betularia) comes in several forms, all controlled by a single gene that determines how dark pigment spreads across the wings. The original form, called typica, has a white or pale background with a sprinkling of black marks scattered across the wings and body. This speckled pattern closely resembles cracked bark covered in lichen, the crusty, pale organisms that grow on tree trunks in clean-air environments. When a typica moth rests on a lichen-covered oak or birch during the day, it virtually disappears.
There’s also a nearly solid black form called carbonaria, and several intermediate shades in between. These darker versions carry a different version of the same gene, which floods the wings with dark pigment instead of scattering it in specks. The existence of these contrasting forms is what turned a quietly named moth into a biology textbook staple.
How the Industrial Revolution Changed the Moth
Before the mid-1800s, almost every peppered moth in Britain matched its name: pale, speckled, and well hidden on lichen-covered trees. Then coal-powered factories spread across England. Soot blackened tree trunks, killed off the pale lichens, and transformed the moths’ resting surfaces from light to dark. Suddenly, the classic peppered pattern was a liability. A white-speckled moth sitting on a soot-blackened tree stood out to every hungry bird in the canopy.
Dark moths, previously rare, now had the advantage. They blended into the darkened bark while their lighter relatives got picked off. Naturalists in Manchester began documenting a rapid increase in the black carbonaria form through the late 1800s, and by the early 1900s it dominated populations in and around industrial cities. The mathematician J.B.S. Haldane estimated in 1924 that the dark form carried a survival advantage as high as 30 percent over the light form in polluted areas, a strikingly strong selection pressure by evolutionary standards.
The genetic change behind the dark form has since been traced to a specific event: a chunk of mobile DNA inserted itself into a gene called cortex, which controls wing pigmentation. Researchers estimate this mutation first appeared in Britain around 1819, right as industrialization was accelerating.
Kettlewell’s Landmark Experiments
In the 1950s, the geneticist H.B.D. Kettlewell set out to test whether bird predation was actually driving the shift. He released marked moths of both forms into two different woodlands: one polluted and soot-darkened, the other clean and lichen-covered. Then he recaptured as many as he could.
The results lined up perfectly with the camouflage theory. In polluted woodland, a greater proportion of dark moths survived to be recaptured compared to light moths. In unpolluted woodland, the pattern reversed: light, speckled moths survived at higher rates. Birds were selectively eating whichever form was easier to spot against the local background. This gave direct experimental support to the idea that the peppered moth had evolved through natural selection in real time.
The Caterpillars Have Their Own Trick
Adult peppered moths rely on their wing pattern for camouflage, but the caterpillars take a completely different approach. The larvae look like small twigs, a survival strategy known as masquerade, where an animal resembles an inanimate object rather than simply blending into a background. They feed on the leaves of birch, willow, and oak trees, and can actually shift their body color from brown to green depending on the branches they’re resting on.
This color-shifting ability is continuous, not just a toggle between two options. The caterpillars use visual cues from their surroundings to closely match both the color and brightness of nearby twigs. Research published in PeerJ confirmed that caterpillars resting on mismatched branches were more likely to be eaten by birds. The flexibility likely evolved because wind can blow caterpillars onto unfamiliar plants, so they need to adjust to whatever environment they land in rather than being locked into one color.
Interestingly, the adults and larvae use entirely different evolutionary strategies for the same goal. Adult wing color is genetically fixed, while larval color is a flexible response to the environment. Both protect against the same threat: sharp-eyed birds.
What Happened After Clean Air Laws
Britain’s Clean Air Acts, passed in 1956 and 1968, gradually reduced coal soot in the atmosphere. Lichens recolonized tree trunks. The environment shifted back toward favoring the original peppered form, and the moth population followed. In the Manchester area, where the dark form was first documented, carbonaria moths dropped from about 90 percent of the population in 1983 to below 10 percent by the early 2000s.
This reversal is just as important as the original shift. It confirmed that the population changes weren’t a one-time fluke but an ongoing process of natural selection responding to environmental conditions in both directions. The peppered moth earned its name from a simple visual description, but it earned its place in science by demonstrating evolution on a timescale humans could actually observe. The name stuck because the speckled form is once again the one you’re most likely to see.