Moths display a remarkable variety of colors and patterns, which serve numerous functions in their natural environments. These intricate colorations can provide camouflage, allowing moths to blend seamlessly with their surroundings and avoid predators. Some vibrant patterns, conversely, act as warning signals to deter potential threats. The development and changes in these colors over time offer compelling insights into how living organisms adapt to their changing world.
The Peppered Moth Phenomenon
The peppered moth, Biston betularia, is a widely studied example of color change in nature. This temperate species, found across the Northern Hemisphere, exists in two primary forms: the light-colored typica and the dark, nearly black carbonaria. Before the 19th century, the light typica form was common in Britain, camouflaged against light, lichen-covered trees. In 1848, a dark carbonaria moth was sighted near Manchester, England. This rare dark form rapidly increased, reaching 98% of the population in some industrial areas by 1895, illustrating a significant biological change.
Environmental Shifts and Survival
The dark moth population increased rapidly during the Industrial Revolution in 19th-century Britain. Factories burning coal released vast soot and smoke, causing significant air pollution. This pollution blackened tree trunks and killed lichens, making trees in industrial regions dark and bare. This altered landscape impacted the moths’ visibility to their predators, insectivorous birds. Light typica moths stood out against the soot-darkened bark, while dark carbonaria moths blended in, gaining a camouflage advantage.
The Engine of Change
The shift in camouflage effectiveness led to differential survival among moth forms. Birds preferentially preyed upon more visible moths, meaning light moths were caught more often in polluted areas, while dark moths were targeted in unpolluted regions. Moths better camouflaged against their local background were more likely to survive and reproduce, passing on their advantageous coloration. This process, where organisms better adapted to their environment tend to survive and produce more offspring, is known as natural selection. The changing environmental conditions during the Industrial Revolution exerted selective pressure, driving the increase in the dark moth population where pollution was present.
Genetic Blueprint for Color
Peppered moth color variations are rooted in their genetic makeup, with a single gene primarily controlling the difference between light and dark forms. One allele of this gene causes dark coloration, while another leads to the lighter, speckled appearance. The allele for dark body color is dominant, meaning a moth needs only one copy to exhibit the dark phenotype. A mutation involving a “jumping gene,” or transposon, within the cortex gene causes the dark color. This genetic mechanism allowed the dark coloration trait to be inherited and spread.
A Reversible Tale
The peppered moth’s story continued beyond the Industrial Revolution, as countries like Britain implemented Clean Air Acts in the latter half of the 20th century to combat air pollution. These policies reduced industrial soot and smoke, improving air quality. Tree trunks lightened, and lichens recolonized the bark. This environmental reversal also reversed selective pressure on the moths. Light typica moths again became better camouflaged against cleaner trees, while dark carbonaria forms became more conspicuous, leading to an increase in light-colored moths and a decline in the dark form, illustrating adaptation’s reversible nature.