The peppered moth (Biston betularia) illustrates how species adapt to changing environments. Between 1848 and 1900, this moth underwent a significant transformation. The period marked a significant shift in its populations, driven by external forces that reshaped its habitat. This period reveals the dynamic relationship between organisms and their environment.
The Moth Before the Transformation
Before the mid-19th century, the typica peppered moth had a light, speckled coloration, with wings primarily white or ash-gray with black markings. This patterning provided effective camouflage against light, lichen-covered tree trunks where moths rested, allowing them to blend seamlessly and making them difficult for bird predators to detect. The dark carbonaria form was rare in these environments, comprising a small percentage of the population. Dark moths would have stood out against the pale bark, making them more susceptible to predation.
The Environmental Catalyst
Air Pollution
The middle to late 19th century brought about profound environmental alterations, particularly in industrialized regions of Britain. The Industrial Revolution, fueled by the burning of vast quantities of coal, led to unprecedented levels of air pollution. Factories and homes released dense smoke and soot into the atmosphere, which then settled on buildings, vegetation, and especially tree trunks. This deposition of soot drastically darkened the surfaces of trees, transforming their appearance from a light, mottled background to a uniform, dark one.
Impact on Lichens
The increased air pollution also had a detrimental effect on lichens, the small, symbiotic organisms that typically covered tree bark. Sulfur dioxide emissions, a common byproduct of coal combustion, were particularly harmful to these sensitive organisms, causing them to die off in polluted areas. The loss of these light-colored lichens further contributed to the darkening of tree surfaces. This environmental shift created a stark contrast to the pre-industrial landscape, directly impacting the conditions under which the peppered moth had previously thrived.
The Population’s Dramatic Shift
The environmental changes between 1848 and 1900 had a profound effect on the peppered moth population. The first recorded sighting of a dark peppered moth in Manchester, an industrial city, occurred in 1848. This dark carbonaria form, once rare, began to increase rapidly in frequency in industrial areas. By the mid-19th century, the number of dark moths had noticeably risen, and by 1895, the carbonaria form constituted as much as 98% of the peppered moth population in Manchester.
This dramatic shift was a direct consequence of altered camouflage and differential predation. On soot-darkened tree trunks, the light-colored typica moths, previously camouflaged, now became highly conspicuous to bird predators. Conversely, the dark carbonaria moths, which had been easily spotted on light bark, were now well-camouflaged against the sooty background. Birds selectively preyed upon the more visible moths, leading to a higher survival rate for the dark forms in polluted regions. This differential survival meant that dark moths were more likely to reproduce and pass on their genes, causing their numbers to surge while the light forms declined.
A Classic Case of Natural Selection
The observed changes in the peppered moth population during the Industrial Revolution provided compelling evidence for the theory of natural selection. This phenomenon, often referred to as industrial melanism, demonstrated how environmental pressures could lead to rapid evolutionary change within a species.
The peppered moth quickly became a widely recognized example of Darwin’s theory in action, illustrating how advantageous traits become more prevalent in a population over time. Scientists like J.W. Tutt, in 1896, recognized the peppered moth’s transformation as a clear instance of natural selection, where the environment dictated which forms were more likely to survive and reproduce.
The story of the peppered moth offered a tangible, real-world case study, making the concept of evolution by natural selection more accessible and understandable. Its significance endures as a foundational example in biology, showcasing the dynamic interplay between genetic variation, environmental change, and the forces of selection.