Peppered moth

The peppered moth (Biston betularia) is a temperate species of geometrid moth that became the iconic demonstration of natural selection in action after the industrial melanism episode of 19th-century England. The species exists in two principal morphs — a light, speckled form (typica) and a dark, melanic form (carbonaria) — whose relative frequencies track the soot load of local forests with almost embarrassing fidelity. The case is not merely a textbook example; it is a boundary object between evolutionary biology, the history of science, and the sociology of scientific controversy, having survived staged-photo scandals and creationist attacks to remain the most cited instance of observable evolution in a wild population.

The genetic basis of the carbonaria morph has been identified: a transposable element insertion in the cortex gene produces the melanic phenotype. This is significant because it shows that a large phenotypic shift can be caused by a single regulatory mutation, not by the gradual accumulation of many small changes. The peppered moth is therefore a case study in how standing variation and mutation together constitute the raw material of rapid evolutionary response — and how a single gene can redraw the fitness landscape of an entire population.