The peppered moth, known scientifically as Biston betularia, is a common moth species found across various regions, including Europe, Asia, and North America. These moths typically inhabit woodlands, forests, and even urban areas. The peppered moth holds a notable position in evolutionary biology; its observed changes in coloration over time have made it a classic example of natural selection.
Identifying the Two Variations
The peppered moth exhibits two distinct color variations, each providing a different camouflage advantage. One variation is the “typical” form, known as Biston betularia f. typica. This form displays a light, mottled appearance, characterized by a whitish background speckled with black markings across its wings and body. This coloration allows the typica form to blend against light-colored, lichen-covered tree bark, making it difficult for predators to spot.
The second variation is the “melanic” form, known as Biston betularia f. carbonaria. In contrast to the typica form, the carbonaria form is almost entirely black, with minimal lighter markings. This dark pigmentation results from a genetic mutation that causes an overproduction of melanin, the pigment responsible for dark coloration. The visual difference between these two forms relates to their ability to camouflage against different backgrounds.
The Industrial Revolution and Natural Selection
During the 19th century, the Industrial Revolution brought about significant environmental changes, particularly in industrialized regions of Britain. Burning coal in factories and homes released large amounts of soot and smoke. This pollution settled on surfaces, including tree trunks, turning them dark and killing off the light-colored lichens on their bark.
These environmental shifts significantly altered the landscape and, consequently, the camouflage effectiveness of the peppered moth variations. The light-colored typica form, once well-camouflaged against lichen-covered trees, became highly conspicuous against the soot-darkened bark. Conversely, the dark carbonaria form, previously noticeable on pale trees, now blended well with the blackened environment. This change in background color created a strong selective pressure on the moth population.
Birds more easily spotted and consumed the less camouflaged moths. In polluted areas, the light typica moths were more vulnerable to predation, while the dark carbonaria moths had a survival advantage due to their superior camouflage. Over successive generations, the carbonaria form became significantly more prevalent in industrial areas, a phenomenon known as industrial melanism. Observations and experiments by Bernard Kettlewell provided strong evidence supporting this rapid evolutionary shift driven by natural selection.
Peppered Moths Today and Their Legacy
Since the mid-20th century, many industrialized nations have implemented clean air legislation and adopted cleaner energy sources. This led to a significant reduction in atmospheric pollution. As air quality improved, soot cleared from tree trunks, and light-colored lichens gradually recolonized the bark. This environmental recovery again shifted the selective pressures on the peppered moth population.
With the return of lighter tree bark, the camouflage advantage swung back to the typica form. The light-colored moths once more blended, while the dark carbonaria form became more visible to predators. Consequently, in many regions, the typica form has seen a resurgence, while the carbonaria form has declined. This reversal of industrial melanism provides a clear, observable demonstration of ongoing natural selection.
The peppered moth continues to serve as a frequently studied example of evolution. Its changes in coloration, driven by environmental shifts and predator pressures, make it an important tool for teaching fundamental principles of natural selection. The moth’s story underscores the dynamic interplay between organisms and their environment, illustrating how populations adapt to changing conditions.