The peppered moth, a species known as Biston betularia, presents a compelling biological puzzle. The core mystery revolves around why populations of this moth underwent a dramatic change from predominantly light-colored to dark-colored, and then back again. This phenomenon offers insights into how species can adapt to their changing surroundings.
The Peppered Moth Before the Change
Before widespread industrialization, the typical peppered moth, often referred to as the typica form, displayed a light, mottled appearance. Its wings were primarily white with scattered black speckles, resembling salt and pepper. This coloration provided effective camouflage against the pale, lichen-covered tree trunks and branches where the moths rested during the day. This blending offered protection from visually hunting predators, primarily birds, which would otherwise easily spot and consume them. The dark-colored variant, known as the melanic or carbonaria form, was naturally present but very rare in these unpolluted environments.
Industrial Pollution and Environmental Impact
The Industrial Revolution in Britain brought about significant environmental changes, as factories burning coal released vast amounts of soot and other pollutants. This led to the widespread darkening of urban and surrounding landscapes. Tree trunks, once light and covered with pale lichens, became coated in black soot. Air pollution proved detrimental to lichens, causing them to die off. This transformation drastically altered the resting surfaces for peppered moths, turning their once camouflaged environment into a stark, dark background.
Survival of the Fittest: A Camouflage Story
The altered environment created selective pressure on the peppered moth population. As tree trunks darkened with soot, the light-colored typica moths, previously well-camouflaged, now stood out conspicuously, making them highly visible to predatory birds and leading to increased predation. Conversely, the previously rare dark carbonaria moths, with their nearly black coloration, gained an advantage; their dark pigmentation provided superior camouflage against the soot-darkened trees, allowing them to evade detection more effectively. More dark moths survived to reproduce, passing on their genes for melanism, leading to a rapid increase in the dark moth population and a significant decline in the light form in industrial areas, a phenomenon known as industrial melanism. For example, in Manchester, the dark form rose from being rare in 1848 to 98% by 1895.
Reversal and Enduring Scientific Legacy
After industrial pollution peaked, environmental conditions began to improve; as air quality improved, soot levels decreased, and tree trunks gradually lightened, allowing lichens, sensitive indicators of air quality, to return to tree surfaces. This environmental reversal shifted the selective pressure back towards the light-colored moths. The typica form again gained a camouflage advantage against the lighter, lichen-covered bark, while the dark carbonaria moths became more visible to predators. Consequently, the light form increased, and the dark form declined in these areas, demonstrating a reversal of the earlier evolutionary trend. Pioneering experiments by Bernard Kettlewell in the 1950s, and later studies by Michael Majerus, provided strong evidence for the role of bird predation and camouflage in this color change, solidifying the peppered moth as a classic example of evolution by natural selection.