Observations of moths have captivated scientists, revealing how these delicate creatures adapt to their surroundings. A classic example of environmental influence on living organisms is the dramatic shift in moth coloration. This phenomenon, which saw an increase in dark-colored moths in certain regions, offers insight into the intricate relationship between species and their habitats.
The Appearance of Dark Moths
Before the mid-19th century, most peppered moths (Biston betularia) displayed a light, speckled pattern. This coloration allowed them to blend effectively with light-colored, lichen-covered tree trunks and branches where they rested. However, a significant shift began to occur, particularly in industrialized areas of England. The first recorded dark-colored peppered moth, termed “carbonaria,” was found in Manchester in 1848.
Following this initial observation, the frequency of dark moths increased dramatically. By the late 19th century, in some industrial regions, the dark variant became overwhelmingly common, sometimes accounting for 98% of the peppered moth population by 1895. This rapid increase in dark moths, while the light form became rare, presented a significant biological puzzle.
The Role of Environmental Change
The shift in moth coloration coincided with industrialization in England. Factories, fueled by coal, released vast amounts of soot and smoke into the atmosphere. This pollution drastically altered the environment, particularly impacting tree appearance. Airborne particulate matter settled on tree trunks, blackening them and killing off the light-colored lichens that once covered the bark.
This environmental transformation significantly affected the moths’ ability to camouflage. Light-colored peppered moths, previously hidden against lichen-covered trees, now stood out against the darkened, soot-covered bark. Conversely, dark-colored moths, once conspicuous, blended seamlessly with their changed surroundings. This reversal in camouflage meant dark moths became less visible to predators, providing a survival benefit in industrial areas.
Natural Selection and Survival
Environmental changes brought by industrialization directly influenced the survival rates of different moth forms through natural selection. Birds preyed on moths resting on tree trunks during the day. In polluted areas, light-colored moths were easily spotted against the darkened bark, making them more vulnerable to avian predators. This increased visibility reduced their likelihood of survival and reproduction.
Conversely, dark-colored moths, camouflaged against soot-blackened trees, were harder for birds to detect. This improved concealment led to a higher survival rate for dark moths in polluted environments. Surviving dark moths reproduced, passing on the genetic trait for dark coloration to their offspring. Over generations, as dark moths thrived and light moths diminished, the population in industrial areas shifted rapidly, leading to the widespread prevalence of the dark form. Environmental pressures can thus drive changes in a species over a relatively short period.