Question

Prior to 1800 in England, the typical moth of the species Biston betularia(peppered moth) had a light pattern. Dark colored moths were rare. By the late 19th century, the light-colored moths were rare, and the moths with dark patterns were abundant.

The cause of this change was hypothesized to be selective predation by birds (JW Tutt, 1896). During the industrial revolution, soot and other wastes from industrial processes killed tree lichens and darkened tree trunks. Thus, prior to the pollution of the industrial revolution, dark moths stood out on light-colored trees and were vulnerable to predators. With the rise of pollution, however, the coloring of moths vulnerable to predators changed to light.

In the late 1900s, England cleaned up its air, and pollution decreased. The bark of trees went from dark to light.

Which of the following outcomes to the populations of peppered moth would you expect given this environmental change?

Answer 1

Normally, evolutionary pressures change only slowly, meaning that genetic change and natural selection moves very slowly. This began to change with the increasing industrialization of human societies.

The Industrial Revolution, in Britain, burned vast amounts of coal, producing sulfur dioxide that killed off all the lichens. Factories also threw out huge amounts of black soot, covering every building and every tree with black grime.

All of a sudden, evolutionary pressure on the peppered moth began to change. Light colored moths resting on a tree now stood out against the black background and were more likely to be eaten. The darker variant, on the other hand, was now camouflaged, and more likely to survive and breed. In a textbook case of industrial melanism , in just a few generations, the dark variant became by far the most common.

In just over 50 years, the dark variety went from making up just 2% of the population to making up over 95%, a change that could not be explained by any theory other than natural selection and industrial melanism.

Genetic drift, where random influences can change the genetic make up of a population over time, is far too slow a process to account for this.

The incidence of industrial melanism is a process called micro-evolution, where selection pressures within a species lead to changes.

In time, when mixed with genetic drift, other mutations and other possible selection pressures, this process of micro-evolution could have led to specialization within the peppered moth population.