The peppered moth, Biston betularia, presents a remarkable biological puzzle, showcasing how populations can undergo striking transformations. This dramatic shift in coloration, observed during a specific historical era, illustrates a fundamental principle of biological change: the dynamic interplay between an organism and its environment.
The Mystery of the Changing Moths
The peppered moth exists in two distinct forms: a light, mottled typica and a dark, nearly black carbonaria. Before the mid-19th century in Great Britain, the light typica form was overwhelmingly common, blending seamlessly with pale, lichen-covered tree trunks. By 1848, the first dark carbonaria specimen was noted in Manchester, England. This dark form, once rare, rapidly increased in industrial regions. By 1895, the carbonaria form constituted as much as 98% of the peppered moth population in heavily industrialized areas like Manchester, a dramatic reversal of their previous rarity.
Pollution’s Role in Natural Selection
The dramatic change in moth coloration was closely linked to the Industrial Revolution. Coal-fueled factories released soot and sulfur dioxide into the atmosphere. This pollution blackened tree bark and killed lichens that once covered tree surfaces. In these darkened, polluted environments, the light-colored moths, previously well-camouflaged, became conspicuous against the sooty trees.
This altered environment created a strong selective pressure from predatory birds. Birds more easily spotted and consumed the light-colored moths resting on the darkened bark, reducing their survival and reproduction. Conversely, the rare dark carbonaria moths, which had once stood out, gained a significant advantage, as their dark coloration provided effective camouflage against the soot-stained trees, allowing them to evade predators. As a result, dark moths were more likely to survive and reproduce, leading to a rapid increase in their numbers in polluted areas. This process, where environmental conditions favor certain traits, is a classic example of natural selection.
The Genetic Blueprint for Color
The color variation in peppered moths has a genetic basis, controlled by a single gene. Specifically, dark coloration, or melanism, is linked to a mutation involving a transposon inserted into the cortex gene. This alteration, occurring around 1819, affects pigment production, resulting in the moth’s black appearance.
The cortex gene is involved in regulating wing patterns and coloration. A dominant allele for dark body color means a moth with one copy will display the dark phenotype. This genetic mechanism allowed the dark form to spread rapidly through populations once environmental conditions favored its survival.
A Reversible Transformation
The story of the peppered moth did not end with the dominance of the dark form. As environmental awareness grew, particularly in the mid-20th century, legislation like the Clean Air Acts were implemented in the UK and US. These regulations significantly reduced industrial air pollution, leading to cleaner air and a visible lightening of the environment.
With the reduction of soot, tree bark lightened, and lichens recolonized. This environmental recovery reversed the selective pressure that had favored the dark moths. Light moths became camouflaged against cleaner backgrounds, while dark moths became more conspicuous to predators. Consequently, the population of light-colored peppered moths began to rebound, demonstrating the dynamic and reversible nature of natural selection in response to changing environmental conditions.