Before the 19th century, the peppered moth of Britain was almost always pale, speckled grey — camouflage that made it nearly invisible resting on lichen-covered tree bark. A dark, near-black version of the same species existed too, but it was rare, because a dark moth resting on pale, lichen-covered bark stood out to every hungry bird nearby. Then Britain industrialised. Coal smoke blackened tree trunks with soot across whole industrial regions and killed off the lichen that used to grow on them, and within a few decades the once-rare dark moth had become the common one in those same regions, while the pale form stayed dominant in unpolluted rural areas untouched by soot. Nothing about the moths changed on purpose. The bark did, and predation did the rest.
Camouflage is a moving target, not a fixed one
Neither moth colour is "better" in any absolute sense — each is a liability wherever it stands out and an asset wherever it blends in. Before industrialisation, a pale moth on lichen-covered bark was nearly invisible to birds and a dark moth was conspicuous; birds preferentially picked off the dark ones, keeping them rare. Once soot blackened the bark and killed the lichen, the exact same dark colouring that used to be a liability became the camouflage, and the pale moths — previously the safe ones — became the easy targets. Natural selection didn't invent a new trait to solve a new problem. The dark form had existed at low frequency the whole time; the environment simply reversed which existing form survived long enough to reproduce, and the population's colour mix shifted accordingly, generation over generation.
This is natural selection in its cleanest form: a trait already varying within a population, an environmental pressure that makes one variant survive and reproduce more than the other, and a shift in the population's makeup over successive generations as a direct result — with nothing about any individual moth needing to change at all.
A shift that ran in both directions
What makes peppered moths such a durable textbook example is that the story didn't stop with industrialisation. Clean air legislation in the mid-20th century sharply cut coal soot pollution across Britain, lichen recolonised tree bark in many of the same regions, and the dark form's frequency declined again over subsequent decades as the pale form's old camouflage advantage returned. The same mechanism ran in reverse when the environmental pressure that had favoured the dark form was removed — a documented, two-way shift within a single well-studied population, not a one-off change that happened to look like evolution.
What we're still unsure about
The classic field experiments behind this story — most famously Bernard Kettlewell's mark-recapture studies in the 1950s, releasing marked moths and tracking how many survived to be recaptured on polluted versus unpolluted bark — were later criticised for real methodological weaknesses: release densities that may not have reflected natural conditions, and assumptions about where the moths actually rest in the wild that turned out to be shakier than the original studies assumed. Those criticisms became, for a time, more widely cited than the more careful, updated field studies that followed — including later work by Michael Majerus — which largely reconfirmed bird predation as a major driver of the shift while correcting the earlier methodology's flaws. The core phenomenon has held up. The textbook version of how it was proven oversimplified a messier and more contested evidentiary history than most summaries let on.
This sits inside Natural Selection & Fitness, one of seven topics in Evolution, one of six domains in Biology, one of seventeen subjects the app can quiz you on.