Who Suggested Peppered Moths Are an Example of Natural Selection?

The study of how species change over time has long fascinated scientists, with certain cases offering particularly clear examples of natural selection. One such case is the peppered moth (Biston betularia), which became a widely discussed example in evolutionary biology due to shifts in its coloration during the Industrial Revolution.

This phenomenon drew interest from researchers seeking to explain why darker-colored moths became more common in polluted areas. Their work provided key insights into how environmental pressures influence survival and reproduction.

Observations of Color Variation in Polluted Regions

During the 19th century, naturalists in England documented an unusual shift in the coloration of the peppered moth. Traditionally, the species exhibited a light, speckled appearance, providing camouflage against the lichen-covered bark of trees. However, as industrial pollution intensified, reports emerged of a striking increase in a darker, almost black variant known as the melanic form. This shift was particularly pronounced in urban and industrialized areas, where coal soot darkened tree trunks and killed off the pale lichens that once concealed the lighter moths.

Field studies in the late 19th and early 20th centuries confirmed that the melanic form had become dominant in heavily polluted regions. In places like Manchester, where industrial emissions were severe, nearly all observed peppered moths were darker. This pattern suggested a strong environmental influence on survival, prompting further investigation into the causes of this transformation.

By the mid-20th century, controlled experiments provided additional evidence that predation played a key role. Birds, the primary predators of peppered moths, were more likely to spot and consume the lighter-colored moths on soot-darkened surfaces. Conversely, the melanic moths, once conspicuous against lichen-covered bark, now had a survival advantage. This correlation between environmental changes and shifts in moth coloration reinforced the idea that selective pressures were driving the observed variation.

Suggested Mechanisms of Natural Selection in Moths

As the shift in peppered moth populations became apparent, researchers sought to identify the mechanisms driving this change. Natural selection, first described by Charles Darwin, provided a compelling explanation. The increasing prevalence of the melanic form in polluted areas suggested that certain traits conferred a survival advantage, leading to differential reproductive success over generations.

Predation pressure emerged as a primary factor. Birds, particularly insectivorous species such as great tits (Parus major), rely on visual cues to locate prey. Before industrial pollution altered tree surfaces, the lighter, speckled moths were well-camouflaged against lichen-covered bark. As soot darkened tree trunks and killed lichens, the once-advantageous light coloration became a liability. Melanic moths, which had previously stood out, now blended into their surroundings, reducing their visibility to predators. Over time, selective predation disproportionately removed lighter-colored moths, allowing the darker variant to proliferate.

Beyond predation, researchers examined genetic inheritance in maintaining and spreading the melanic trait. Studies showed that dark coloration followed a dominant Mendelian pattern, meaning moths inheriting even one copy of the melanic allele exhibited the darker phenotype. This allowed the melanic variant to spread rapidly in populations exposed to strong selective pressures.

Environmental shifts further reinforced this selection process. As pollution levels declined in the mid-20th century due to regulatory measures, tree surfaces lightened, and lichens returned. The selective advantage reversed—lighter-colored moths were now better camouflaged, while melanic individuals became more conspicuous. Long-term monitoring confirmed a decline in the melanic form as industrial soot diminished, illustrating the dynamic nature of natural selection.

Genetic Factors Under Investigation

Unraveling the genetic basis of color variation in peppered moths required pinpointing the molecular mechanisms responsible for the melanic form. Early studies suggested a single dominant allele controlled the difference, but its precise location remained elusive for decades. Advances in genomic sequencing eventually allowed scientists to identify a specific region of the moth’s genome associated with pigmentation.

Further investigation revealed that the dark coloration resulted from a mutation in the cortex gene, which plays a role in wing patterning across Lepidoptera species. This gene regulates scale development during metamorphosis, and mutations within it have been linked to altered pigmentation in other moth and butterfly species. Researchers identified a transposable element—a mobile genetic sequence—that had inserted itself into the cortex gene. This insertion disrupted normal gene function, leading to the dark-winged phenotype.

The discovery of this mutation provided insight into how rapid evolutionary changes can occur through structural genetic alterations. Transposable elements can drive significant shifts in phenotype by influencing gene expression, and their role in the peppered moth case demonstrated how a single mutation could dramatically alter survival prospects. This finding also underscored the broader significance of transposable elements in evolutionary biology, as similar genetic mechanisms have been implicated in adaptive traits across multiple species.

Broader Relevance to Environmental Adaptation

The case of the peppered moth highlights how species undergo rapid evolutionary changes in response to environmental shifts. Organisms facing strong selective pressures must develop advantageous traits or risk population decline. This principle applies widely, from microbial communities developing antibiotic resistance to plant species adjusting to climate variations. Understanding these processes helps scientists predict how species will respond to ongoing environmental challenges, such as habitat destruction and global temperature changes.

Genetic mutations that enhance survival can spread quickly under intense selection pressures, as seen in the peppered moth. Similar phenomena occur in other species, such as rock pocket mice in desert environments, where coat color variations help individuals avoid predation. These adaptations illustrate how even small genetic changes can shape population dynamics, influencing which traits become widespread over time. Studying these genetic mechanisms across species helps researchers uncover the broader principles governing evolutionary change.