The Peppered Moth, Biston betularia, is a temperate species of night-flying moth known for its striking color variation. Its original form, known as typica, is predominantly white with black speckles, giving it a salt-and-pepper appearance. A naturally occurring mutation also produces the dark or melanic form, called carbonaria, which is nearly black. This difference in wing coloration makes the Peppered Moth a recognized subject for illustrating the process of evolution by natural selection.
Primary Predators of the Adult Moth
The primary predatory pressure on adult Peppered Moths comes from insectivorous birds, which hunt during the day when the moths are resting. Moths emerge at night to fly and mate, but they spend daylight hours immobile on surfaces like tree trunks and branches, making them vulnerable to visual hunters. The species of birds that most commonly feed on resting moths include common garden and woodland species such as robins, nuthatches, sparrows, great tits, and blue tits.
These avian predators possess excellent eyesight and search tree bark for any movement or break in the surface pattern. Experiments have demonstrated that the survival rate of a moth is directly related to how well its color matches the background tree bark. This predation pressure drives the shift in moth populations, as birds preferentially remove the individuals that are easiest to spot.
While diurnal birds are the primary selective agents, the adult moths must also contend with predators during their active, nighttime hours. Bats are significant nocturnal hunters that use echolocation to detect flying insects. Although bats consume both light and dark forms, this predation does not exert the same differential selective pressure based on coloration as the visual hunting of birds.
Predation Beyond the Adult Stage
The threat of predation extends across the entire life cycle of the Peppered Moth, impacting its larval and pupal stages. The larvae, or caterpillars, feed on the leaves of various trees, including birch, willow, and oak. To avoid being eaten by birds, the caterpillars have evolved camouflage known as masquerade, where their bodies closely resemble small, broken twigs.
This twig-mimicry is effective because the caterpillars can adjust their color to match the specific shades of brown or green of the branches they are resting on. This color change is induced by the visual input from their surroundings, providing them with protection from birds that hunt visually. The pupal stage, where the moth overwinters, takes place in the soil or leaf litter at the base of the host tree, which introduces a new set of predators.
Small, ground-dwelling mammals, such as shrews and mice, actively forage through the leaf litter and soil for hidden insect pupae. Shrews prey on both moth larvae and pupae, making them a major source of mortality for the overwintering generation. Furthermore, the larvae are susceptible to various invertebrate threats, most notably parasitic wasps that lay their eggs inside the caterpillar’s body.
The Role of Camouflage and Background
The mechanism by which predators shape the Peppered Moth population is linked to camouflage, a concept known as crypsis. The survival of the adult moth hinges on its ability to blend into the background of the tree bark where it rests during the day. In unpolluted environments, tree trunks are often covered in pale-colored lichens, making the light-colored typica moth virtually invisible to predators.
However, in areas affected by industrial pollution, soot darkened the tree bark and killed off the light-colored lichens. This change in background color meant the previously camouflaged typica form now stood out sharply against the dark surface, making them easy targets for birds. Conversely, the dark carbonaria form, which was rare before industrialization, became highly cryptic against the soot-darkened bark.
The fitness of each color form is determined by this background matching; a moth that does not match its background is much more likely to be eaten. This selective pressure resulted in the rapid increase of the carbonaria form in polluted areas, a phenomenon known as industrial melanism. Differential predation by birds was the driving force that caused this shift, favoring the survival and reproduction of the better-camouflaged form.
As air quality improved in post-industrial areas, the lichens returned to the trees, and the bark lightened again. This environmental reversal immediately shifted the selective advantage back to the pale typica moths, whose numbers have since increased. The entire process demonstrates how predator behavior, specifically visual detection, acts as a powerful agent of natural selection based on environmental context.