The dark color in peppered moths was caused by a genetic mutation, specifically a “jumping gene” that inserted itself into a gene called cortex, which controls wing pattern. This mutation produced dark-winged moths that happened to survive better in pollution-blackened industrial environments, allowing the trait to spread rapidly through the population. The story of how it happened is one of the most famous examples of natural selection ever documented.
The Genetic Change Behind the Dark Color
At the molecular level, the dark coloring traces back to a single type of mutation: a large, repetitive piece of DNA called a transposable element inserted itself into the first section of the cortex gene. Transposable elements are sometimes called “jumping genes” because they can copy themselves and land in new locations within an organism’s DNA. When this particular piece landed inside the cortex gene, it changed how the gene behaved, switching the moth’s wing color from a speckled salt-and-pepper pattern to solid black.
A 2016 study published in Nature confirmed this mechanism. The finding was notable because it showed that a major, visible change in an animal’s appearance can result from a single genetic event rather than a slow accumulation of many small mutations. The dark form of the moth, known as carbonaria, carries this transposable element insertion. Moths without it have the classic pale, speckled wings.
How Pollution Made Dark Moths Survive
The mutation alone didn’t make dark moths common. What changed their fortune was the Industrial Revolution. Before widespread coal burning, peppered moths rested on tree bark covered in pale lichens. Their speckled wings blended in perfectly, making them hard for birds to spot. Dark moths existed in small numbers but were easy targets against that light background.
Starting in the mid-1800s, factories across England pumped massive amounts of soot, ash, and sulfur dioxide into the air. The sulfur dioxide killed the lichens on trees, and the soot coated the bare bark in a layer of black grime. Suddenly, the camouflage advantage flipped. Pale moths now stood out against the darkened trees, while dark moths blended in. Birds picked off the conspicuous pale moths at higher rates, and the dark moths survived to reproduce. Over just a few decades, the dark form went from a rarity to the dominant type in industrial cities.
The first recorded sighting of a dark peppered moth was in Manchester in 1848. By 1895, 98% of peppered moths in Manchester were dark. That shift, from near zero to near total dominance in roughly 50 years, represents one of the fastest documented examples of natural selection in a wild population.
Birds as the Selective Force
The link between moth color and survival comes down to bird predation. A landmark experiment conducted by Cambridge researcher Michael Majerus between 2002 and 2007 tracked predation on both color forms at an unpolluted site where trees had recovered their natural pale appearance. Over six years, birds consistently ate more dark moths than pale ones. In 2007, for instance, birds took 158 pale moths but only 4 dark ones from the study population, reflecting how few dark moths were left and how visible they were on clean bark.
Nine different bird species were observed eating the moths in this experiment. The daily survival rate for dark moths at the unpolluted site was about 91% that of the pale form, meaning dark moths faced roughly a 9% higher daily risk of being eaten. That may sound like a small difference, but compounded over a moth’s lifespan and across generations, it’s more than enough to shift the entire population’s color balance within decades.
The Recovery After Clean Air Laws
The story didn’t end with pollution. After the United Kingdom passed Clean Air Acts in the 1950s and 1960s, sulfur dioxide levels dropped, lichens regrew on trees, and soot-blackened bark gradually lightened. The selection pressure reversed again. Pale moths regained their camouflage advantage, and the frequency of dark moths began a steady decline.
By the early 2000s, researchers studying peppered moths in the Manchester area noted that industrial melanism in this species was “almost past.” Dark moths still exist, because the transposable element insertion hasn’t disappeared from the gene pool, but they’ve returned to being uncommon in most of England. The population essentially tracked the pollution levels of its environment with a lag of a few decades in each direction.
Why This Case Matters Beyond Moths
The peppered moth isn’t alone. During the nineteenth century, dark color variants increased in frequency across numerous moth species in industrialized regions, a phenomenon broadly called industrial melanism. The peppered moth became the iconic example because the color change was so dramatic, the timeline so well documented, and the mechanism so clean: one gene, one environmental pressure, one predator group, one outcome.
It also demonstrated something important about how evolution works in practice. The dark color mutation existed in the population long before it was useful. It only became advantageous when the environment changed. Natural selection didn’t create the mutation. It simply favored moths that already carried it once conditions shifted. When conditions shifted back, selection favored the original form again. The whole cycle, from rare dark moth to dominant dark moth and back, played out in about 150 years.