Dark moths had an advantage because they were harder for birds to spot on soot-darkened trees. During the Industrial Revolution, coal pollution killed the pale lichens that normally covered tree bark and coated surfaces in dark soot. Light-colored moths that once blended in perfectly became visible against this darkened backdrop, while dark moths virtually disappeared against it. Birds picked off the easier-to-see light moths, and the dark ones survived to reproduce. Within a few decades, dark moths went from nearly nonexistent to dominant in industrial areas.
How Pollution Changed the Playing Field
The peppered moth is the textbook example. In unpolluted British woodlands, tree trunks and branches are covered in pale, speckled lichens. The light-colored form of the peppered moth matches this surface almost perfectly in both color and brightness, making it nearly invisible to birds. The dark form, by contrast, stands out against lichen-covered bark like a bullseye.
Coal burning during the Industrial Revolution (roughly 1760 to 1840 and beyond) released massive amounts of sulfur dioxide into the air. Lichens are extraordinarily sensitive to sulfur dioxide. Entire lichen populations died off across industrial regions, leaving bare, soot-stained bark behind. Suddenly, the camouflage equation flipped. Light moths were conspicuous on dark bark, and dark moths blended in. The first recorded solid-black peppered moth appeared near Manchester in 1848. By 1895, about 98% of peppered moths near Manchester were dark.
That shift from rare oddity to near-total dominance in roughly 50 years is remarkably fast for an evolutionary change, and it shows just how strong the survival pressure was.
Birds Are the Selection Pressure
The advantage comes down to one thing: not getting eaten. Birds are the primary predators of resting moths, and they hunt by sight. A moth that blends into its resting surface lives longer and produces more offspring. A moth that doesn’t match gets picked off.
The most rigorous test of this came from a six-year experiment by Cambridge biologist Michael Majerus, who released 4,864 moths in an unpolluted woodland where lichens had recovered. He tracked which moths were eaten and which survived. The results were clear: birds ate significantly more dark moths than light ones. Dark moths had a daily survival rate only about 91% that of light moths. That may sound like a small gap, but compounded over a moth’s lifespan and across generations, it’s more than enough to shift an entire population. Majerus documented nine different bird species preying on the moths.
In polluted areas during the industrial era, the same predation worked in reverse. Light moths were the ones getting picked off disproportionately, giving dark moths the survival edge.
The Genetic Mutation Behind Dark Coloring
The dark coloring isn’t a response to the environment during a moth’s lifetime. It’s genetic. In 2016, researchers identified the exact mutation responsible: a chunk of repetitive DNA that inserted itself into a gene called cortex, which helps regulate cell growth during wing development. This insertion causes the moth’s wings to produce far more dark pigment.
Statistical analysis of the genetic data suggests this mutation first appeared around 1819, which lines up neatly with the historical record. The mutation already existed in the population at very low levels before pollution gave it a survival advantage. Once soot darkened the trees, moths carrying this mutation survived and reproduced at higher rates, and the trait spread rapidly through the population.
The Reversal After Clean Air Laws
If the advantage of dark coloring truly depended on pollution, then reducing pollution should reverse the trend. That’s exactly what happened. After Britain passed the Clean Air Acts in the 1950s, coal burning declined, sulfur dioxide levels dropped, and lichens slowly recolonized tree bark. As the trees lightened, dark moths lost their camouflage advantage and light moths regained theirs.
Majerus’s experiment captured this reversal in real time. At his unpolluted study site, the number of dark moths eaten by birds dropped each year simply because fewer dark moths remained in the population. By 2007, birds ate only 4 dark moths compared to 158 light ones, reflecting how rare the dark form had become in clean-air woodlands. The selection pressure that had favored dark moths for over a century was now working against them.
Not Just Peppered Moths
The peppered moth gets the most attention, but it wasn’t the only species affected. Researchers have documented similar dark-form increases in multiple moth species across polluted regions of Britain. Two other species in the same family, the scalloped hazel and the pale brindled beauty, showed strikingly similar patterns: dark forms rose during heavy pollution and declined after clean air legislation. The fact that the same camouflage-based selection pressure acted across an entire community of moth species strengthens the case that bird predation on mismatched moths was the driving force, not some quirk of one species’ biology.
Why This Example Matters
The peppered moth story is one of the clearest documented cases of natural selection acting in real time. It shows how a change in the environment (soot-darkened trees) creates a new survival pressure (visibility to predators), which favors individuals with a specific trait (dark coloring), which then becomes more common in the population because those individuals reproduce more successfully. The entire cycle played out in both directions within about 150 years, fast enough for scientists to track with direct observation and experiments. It’s evolution you can see with your own eyes, measured in decades rather than millennia.