Invertebrates
Peppered Moth
Biston betularia · also called pepper-and-salt moth
No insect has done more teaching than the peppered moth. Its story — a pale, speckled moth that turned soot-black as Britain’s cities filled with coal smoke, then turned pale again once the air was cleaned — is in almost every biology textbook as the clearest case of natural selection anyone has watched unfold.
Overview
Biston betularia is a medium-sized moth with a wingspan around 45 to 62 mm, active at night from May to August across Europe, temperate Asia, and North America. The typical form is white dusted with fine black speckling; the melanic form, called carbonaria, is nearly solid charcoal. The caterpillar is a superb twig mimic that feeds on birch, oak, willow, and many other broadleaved trees; the adult does not feed at all.
Industrial melanism
The dark form was first noted in Manchester in 1848. As coal smoke blackened tree bark and killed the pale lichens that grew on it, birds could easily spot pale moths against the soot but not dark ones. By 1895 the dark form made up around 98 percent of the population in that city — an extraordinarily fast shift. After the Clean Air Acts cut smoke pollution in the mid-twentieth century, lichens returned, bark lightened, and the dark form fell back to a few percent.
One jump of DNA
In 2016, researchers pinned the carbonaria form to a transposable element — a piece of mobile DNA — that inserted into the cortex gene around 1819, decades before pollution made it advantageous. The twig-mimicking caterpillars, meanwhile, can sense bark colour through their skin and adjust their own colour to match, even with their eyes covered.
Status
The peppered moth is Least Concern, common and widespread. What changed over the last two centuries was not its abundance but the balance between its two colour forms, which has tracked air pollution up and then back down.
Peppered Moth: common questions
Why is the peppered moth used to teach evolution?
Because it offers a rare case of natural selection observed within human lifetimes, driven by a change people caused and then reversed. Before industrialisation almost all peppered moths were pale and speckled, well hidden on lichen-covered bark. As coal smoke blackened tree trunks in nineteenth-century Britain, a dark form that had been vanishingly rare became the majority in polluted areas within about fifty years, because birds could no longer see it against the soot while they readily found the pale form. When air quality laws cleaned the atmosphere in the second half of the twentieth century, the pale form recovered as the bark lightened again. The direction of selection flipped with the environment.
Was the peppered moth research ever discredited?
The core finding has held up, though some early experiments were flawed. Bernard Kettlewell's 1950s studies, which involved releasing marked moths and recording predation, used higher moth densities than occur naturally and sometimes placed moths on tree trunks in daylight, and critics in the 1990s and 2000s highlighted these problems. In response, Michael Majerus ran a large, carefully designed predation experiment in the 2000s, published after his death in 2012, that confirmed birds eat the more conspicuous form more often and that this fully accounts for the observed frequency changes. The moth remains a valid and well-supported example.
What is the dark form of the peppered moth?
It is a naturally occurring variant called carbonaria, in which the wings and body are almost entirely black rather than pale and peppered. It results from a dominant version of a pigmentation gene, and in 2016 the specific cause was identified as a transposable element, a mobile stretch of DNA, that jumped into the cortex gene around the year 1819. The mutation existed at very low frequency for decades before pollution gave carriers a survival advantage. Once soot darkened the trees, the dark form spread rapidly, then declined again after clean-air legislation.
Do peppered moths only live in Britain?
No. Biston betularia is found across Europe, through temperate Asia, and in North America, where the local form is sometimes treated as a subspecies. Industrial melanism in the species was documented not just in Britain but also in continental Europe and in North American industrial regions such as around the Great Lakes, and the same rise and later fall of dark forms tracked the rise and fall of heavy coal pollution in those places too. Britain simply has the longest and most detailed records, which is why its populations are the most studied.