The Biston betularia, commonly known as the peppered moth, is a temperate species of night-flying moth found across the Northern Hemisphere, including Europe and North America. These moths are notable for their distinct color forms, providing valuable insights into how species can adapt to their surroundings.
The Typica Moth’s Light Form
The typica form of the peppered moth displays a pale, mottled coloration. Its wings are primarily whitish or ash-gray, adorned with scattered black speckles, giving it a “salt-and-pepper” appearance. This patterning offers effective camouflage against light-colored, lichen-covered tree bark, which was common in many natural environments before the Industrial Revolution. Historically, this light-colored variant was the predominant form of the peppered moth.
The Carbonaria Moth’s Dark Form
In contrast, the carbonaria form of the peppered moth is distinctly dark, appearing nearly black or melanic. This coloration results from a genetic mutation. While rare prior to the mid-19th century, this dark variant became more prevalent during the Industrial Revolution. The carbonaria form’s dark color provides camouflage against surfaces that are similarly dark, such as tree trunks blackened by soot.
Natural Selection and the Moth’s Changing Colors
The shift in peppered moth populations from predominantly light to dark forms, and then back again, serves as a clear example of natural selection. During the Industrial Revolution, factories burning coal released massive amounts of soot and smoke, causing tree bark in industrial areas to darken. This pollution also killed off light-colored lichens that typically grew on trees, further exposing the dark bark beneath.
In this altered environment, the light-colored typica moths, once well-camouflaged, became conspicuous against the soot-darkened trees, making them easy targets for predatory birds. Conversely, the dark carbonaria moths, previously rare, now blended effectively with the blackened tree trunks. As a result, more dark moths survived to reproduce, passing on their genes for melanism to their offspring. The frequency of dark moths in some industrial areas, such as Manchester, rose dramatically from being rare to nearly 98% by 1895.
Following the implementation of clean air legislation starting in the mid-20th century, pollution levels decreased significantly. As the air became cleaner, tree bark lightened, and lichens began to regrow. This environmental reversal shifted the selective pressure once more. The typica form regained its camouflage advantage, while the dark carbonaria moths became more visible to predators. Consequently, the population of light-colored moths increased again, demonstrating the ongoing influence of environmental changes on species adaptation.
A Living Example of Evolution
The story of the peppered moth, with its typica and carbonaria forms, is a clear example of observable evolution. The documented changes in moth coloration over a relatively short period provide direct evidence of natural selection in action. This case illustrates how environmental pressures, such as pollution and predation, can drive rapid changes in the genetic makeup of a population. The peppered moth continues to be a subject of study, reinforcing evolutionary understanding.