Peppered MothEdit
The peppered moth, scientifically known as Biston betularia, is one of the clearest, most accessible illustrations of natural selection in action. In its native range across Europe, including the United Kingdom, the species exhibits at least two common wing-color morphs: a pale, speckled form and a darker, melanistic form. The frequency of these morphs shifted dramatically in the 19th and 20th centuries in response to changing environments, making the peppered moth a staple in biology classrooms and a touchstone for discussions about evolution, adaptation, and the role of environmental change in shaping life on Earth. Biston betularia natural selection industrial melanism
History and biology
The peppered moth belongs to the order Lepidoptera and undergoes complete metamorphosis: egg, larva (caterpillar), pupa, and adult. In many populations, the pale morphs blend in with lichen-covered or light-barked trees, while the melanistic morphs resemble soot-darkened surfaces. The two color forms are controlled by genetic variation, with the darker, carbonaria morph arising from a dominant allele and the typical, pale morph representing the ordinary allele. This setup makes the species an excellent example of a single-gene polymorphism contributing to differential survival in a heterogeneous environment. See the genetics of color in carbonaria versus typica morphs and their role in camouflage and predation. carbonaria typica camouflage Lepidoptera
Historically, the distribution and abundance of the morphs tracked the color of the environment. In regions with heavy industrial pollution, tree trunks became darker from soot, giving melanistic moths a camouflage advantage. In cleaner eras, when surfaces brightened again due to pollution controls, pale moths gained the upper hand. This dynamic has been documented across multiple sites and over several generations, illustrating how quickly natural selection can operate in nature. See discussions of environmental effects on phenotype in industrial melanism and related studies of pollution and habitat change. industrial melanism pollution
Industrial melanism and evolution in action
The mid-20th century brought concrete, observational demonstrations of these ideas. Field researchers conducted experiments to test whether predation by birds favored one morph over the other in different environmental contexts. Notably, field experiments and observational data suggested that darker moths had higher survival on soot-darkened trees, while lighter moths fared better on lighter backgrounds. This pattern aligned with predictions from the theory of natural selection and helped settle debates about how quickly selection could operate in natural populations. See the work of H. B. Kettlewell and subsequent analyses in the broader literature on industrial melanism and the peppered moth. H. B. Kettlewell industrial melanism
Over time, clean-air reforms in many regions reduced soot on tree surfaces, and the balance shifted again toward the pale morph in some populations. The case thus illustrates not only selection in action but also how environmental policy and industrial history can indirectly shape evolutionary trajectories. For a broader view of the phenomenon, see industrial melanism and air pollution history.
Controversies and debates
Like many famous demonstrations in biology, the peppered moth story has attracted scrutiny and debate. Some critics argued that early Kettlewell experiments relied on questionable methods, such as the way moths were marked or released, or that the ecological context was more complex than a simple background-matching story. These criticisms led to a broader call for replicable experiments and more nuanced interpretations of selection in the field. See debates surrounding the methodology of H. B. Kettlewell and later work by critics and supporters alike in the literature on natural selection.
From a conservative or mainstream science perspective, the core insight remains robust: natural selection can produce rapid changes in phenotype frequencies when environmental conditions shift, and the peppered moth provides a relatively clean, observable example of that process. Critics who frame the case as a failure of science or as an indictment of evolutionary theory often emphasize methodological disputes or politicized readings of the data. Proponents argue that while no single study is perfect, the weight of evidence—from field experiments, observational data, and modern genetic analyses—supports the basic conclusion: differential survival based on camouflage drives changes in morph frequencies over generations. See discussions of the modern genetics of coloration in carbonaria and typica and the broader topic of evolution in natural populations. carbonaria typica evolution
In short, the peppered moth remains a durable example of adaptive change in response to environmental pressures, even as scholars continue to refine the details of how best to interpret and measure those changes. Some criticisms have highlighted legitimate questions about experimental design and ecological complexity, but the fundamental mechanism—natural selection acting on color polymorphism in a changing environment—continues to be supported by the corpus of research, including subsequent echoing observations in other species and systems. See entries on Majerus and related work for extended discussions of the evidence and its interpretation. Majerus