The peppered moth, Biston betularia, is a well-known insect. This species is often studied due to its varied appearances. Its differing forms offer a clear illustration of how organisms can adapt to their surrounding environments.
The Primary Color Forms
The peppered moth occurs in two distinct color forms, or morphs. One is the light, speckled form, scientifically known as typica. This morph displays a pale, whitish or ash-gray coloration with scattered black speckles, creating a “salt-and-pepper” appearance. The other main form is the dark, melanic carbonaria. This morph is almost entirely black. While these two are most prominent, intermediate forms, known as insularia, also exist, showing variability between the light and dark extremes.
The Role of Camouflage
The distinct color forms of the peppered moth serve a significant biological purpose: camouflage. The light-colored typica morph is well-suited for blending in with pale, lichen-covered tree trunks and branches. This coloration provides effective concealment from predatory birds during the day, when the moths rest on these surfaces. Studies have shown that these light moths reflect ultraviolet light in a speckled manner, which further enhances their camouflage against crustose lichens in both visible and ultraviolet wavelengths. Conversely, the dark carbonaria morph finds concealment on tree surfaces that are blackened, such as those covered in soot. This adaptation helps them avoid detection by visual predators like birds, which are a primary threat.
Industrial Melanism and Color Change
The historical shift in peppered moth populations is a prominent example of environmental influence on species, known as industrial melanism. Before the Industrial Revolution, the light typica form was the predominant type in populations. However, with the rise of industrialization, particularly in England, coal-burning factories released significant amounts of soot and smoke. This pollution darkened tree bark and killed off light-colored lichens, fundamentally changing the moths’ resting environments.
In this altered landscape, the light moths became more conspicuous to predators, as they no longer blended with their darkened backgrounds. The dark carbonaria morph, previously rare due to a genetic mutation, gained a survival advantage as it was now better camouflaged against the soot-covered trees. This led to a dramatic increase in the frequency of dark moths; for instance, in Manchester, the percentage of dark moths rose to 98% by 1895. The phenomenon was extensively investigated by Bernard Kettlewell, who demonstrated that differential bird predation was a primary factor in this adaptation.
Following clean air legislation and reduced pollution in the mid-20th century, tree bark lightened, and lichens began to regrow. This environmental reversal shifted the selective pressure again, allowing the light typica form to regain its advantage, and its numbers subsequently increased.