The peppered moth (Biston betularia) offers a clear illustration of how species adapt to their surroundings. Its historical changes in coloration serve as a prime example in evolutionary biology, showcasing how populations respond to environmental shifts.
The Moth Before Industrialization
Before the Industrial Revolution transformed landscapes, the majority of peppered moths exhibited a light, speckled appearance, known as the typica form. These moths possessed a natural camouflage, blending seamlessly with the light-colored, lichen-covered tree trunks and branches where they rested during the day. This coloration made them difficult for predatory birds to spot, offering a significant survival advantage. While a dark-colored variant, the carbonaria morph, existed due to a genetic mutation, it was exceedingly rare, comprising perhaps less than 1% of the population because it stood out against the pale bark.
Environmental Changes from Industrialization
The Industrial Revolution, particularly in England during the 18th and 19th centuries, brought about significant environmental alterations. Factories, reliant on coal, released large quantities of soot and pollutants into the atmosphere. This air pollution directly impacted the moth’s habitat: soot blackened tree trunks and walls, and sulfur dioxide emissions killed off lichens. As a result, light-colored surfaces in industrialized regions became uniformly dark, altering the moth’s resting backdrop.
The Population Shift by 1900
By the turn of the 20th century, a significant transformation occurred within peppered moth populations in industrialized areas. The previously rare dark carbonaria form became dominant. For instance, in Manchester, a heavily industrialized city, the frequency of the dark morph reached as high as 98% by 1895. This shift meant the light typica form, once common, had declined in these polluted environments. The phenomenon’s geographical spread directly linked to industrial activity and pollution.
Natural Selection in Action
The observed population shift in the peppered moth demonstrates natural selection. As tree trunks darkened with soot, light-colored moths became conspicuous to visual predators, primarily birds. Conversely, the dark carbonaria morph, once easily seen, now had superior camouflage against the sooty bark. This differential visibility led to selective predation; birds more readily consumed the less camouflaged moths. Dark moths, with their survival advantage, were more likely to reproduce, passing on their genes for dark coloration and increasing their frequency.