The peppered moth, Biston betularia, stands as a widely recognized example in biology for illustrating evolutionary principles. Its distinctive coloration has allowed scientists to observe natural selection in action. This insect’s varying hues provide a clear, observable case study for understanding biological adaptation.
The Natural Color Variations
The peppered moth naturally occurs in two main color forms. One is known as the typica form, characterized by a light, often whitish or pale gray body and wings, speckled with small black dots. This pattern provides excellent camouflage against tree trunks covered in light-colored lichens, making the moth difficult for predators to spot.
The other primary form is carbonaria, which is predominantly black or very dark, with much reduced speckling. Before significant human-induced environmental changes, this dark form was rare in natural populations. Both forms existed as part of the species’ inherent genetic diversity.
How Environment Shaped Moth Colors
The Industrial Revolution dramatically altered the environment, leading to a phenomenon known as industrial melanism in the peppered moth. As factories burned coal, soot and pollution darkened tree trunks and killed off light-colored lichens, especially in urban and industrial areas. This environmental shift made the light-colored typica moths highly visible against the darkened bark, increasing their vulnerability to bird predators.
Conversely, the dark carbonaria moths, once conspicuous, now found themselves well-camouflaged against the soot-blackened trees. This change in background gave the dark form a significant survival advantage. As a result, natural selection favored the carbonaria moths, leading to a rapid increase in their population frequency in polluted regions. For instance, in Manchester, the dark form increased from being rare in 1848 to making up 98% of the population by 1895.
Following the implementation of clean air legislation and a reduction in industrial pollution, the environmental conditions began to reverse. Tree trunks gradually became cleaner, and lichens started to regrow. This reversal in environmental conditions again shifted the selective pressure, favoring the light typica form once more. Consequently, the frequency of the dark carbonaria form has steadily declined in many areas.
The Science of Moth Color Inheritance
The distinct color forms observed in the peppered moth are determined by specific genes passed down through generations. Scientists have identified that the dark (melanic) coloration, particularly the carbonaria form, is controlled by a dominant allele. This means that a moth needs only one copy of this specific gene variant to display the dark phenotype.
In contrast, the light (typica) coloration is governed by a recessive allele. For a moth to exhibit the light, speckled appearance, it must inherit two copies of this recessive allele, one from each parent. Recent research has even pinpointed a specific gene, cortex, which plays a role in this color determination. A mutation within this gene, specifically a “jumping gene” or transposon, is responsible for the dark coloration.
Understanding this genetic mechanism helps explain how the moth populations could shift their dominant coloration so rapidly in response to environmental changes. The presence of both dominant and recessive alleles within the population allows for the expression of different phenotypes, which natural selection can then act upon.