The peppered moth, Biston betularia, offers a clear illustration of observable evolution. Its story highlights the dynamic interplay between environmental changes and the survival of different traits within a population, demonstrating how living organisms adapt to their surroundings.
The Moth’s Distinct Forms
The peppered moth exists primarily in two distinct variations, known as morphs, which differ noticeably in their coloration.
One form is the light-colored, mottled type, scientifically referred to as typica. Its wings are predominantly whitish or ash-gray, adorned with scattered black speckles, giving it a “salt-and-pepper” appearance. This patterning provides effective camouflage against light-colored, lichen-covered tree bark.
In contrast, the second variation is the dark-colored, or melanic, form called carbonaria. This morph appears almost entirely black due to a genetic mutation. Both typica and carbonaria moths belong to the same species, capable of interbreeding and producing fertile offspring.
Natural Selection in Action
The changing prevalence of these two moth forms serves as a well-documented example of natural selection.
Before the Industrial Revolution, the light-colored typica moths blended seamlessly with lichen-covered trees, making them less visible to predatory birds. As industrialization progressed, factories released substantial amounts of soot and smoke, darkening tree trunks and killing off light-colored lichens. This environmental alteration shifted the camouflage advantage.
The light-colored moths, once camouflaged, became conspicuous against the soot-darkened trees, making them easy targets for birds. Conversely, the dark carbonaria moths, previously rare, now blended effectively with the blackened surfaces. As a result, dark moths had a higher survival rate, reproducing more often and passing on their genes for melanism. This phenomenon, termed industrial melanism, led to a significant increase in the frequency of dark moths in polluted areas, with populations in some industrial centers reaching nearly 98% carbonaria by the late 19th century.
Current Population Trends
Following the implementation of clean air legislation beginning in the mid-20th century, pollution levels significantly decreased in many regions. As air quality improved, tree bark lightened, and lichens began to regrow on tree surfaces. This environmental reversal shifted the selective pressures on the moth population.
The light-colored typica form regained its camouflage advantage against the cleaner, lichen-covered trees.
Consequently, the dark carbonaria moths became more visible to predators in these recovering environments. The population of light-colored moths subsequently increased, while the numbers of the dark form declined. This illustrates the dynamic nature of natural selection and how species adapt to changing surroundings.