In this section, we shall discuss a classic example of natulal selection in action. In the preceding unit, it was stated that natural selection always aims at eliminating alleles, which are less adapted to the environment. Conversely selection indirectly aims to promote those alleles or individuals that are well adapted to a given environment.
The promotion and establishment of an allele and the resultant phenotype may take several hundreds of years in natural populations. But there are indeed instances where the selective forces have brought changes in the character of individuals within historical times.
The peppered moths Biston betillaria occur in two forms, the melanic and non-melanic. As shown in Fig. the melanic forms are black in colour and non-mel?nic ones are mottled grey. Until the early part of the 19th century, the melanic forms were regarded as rare and prize collections by insect collectors, and the light non-melanic forms were abundantly present in natural populations. The rarit.? of black forrns was essentially due to the higher rate of predation by birds. The black forms resting on light coloured and lichen encrusted trees were the easy targets for the birds. The light coloured non-melanics blend with their background, and therefore, are not easily visible to the predators. With the onset of the industrial revolution the distribution of melanic forms particularly in the industrial centres underwent a great change. An examination of museum collection of insects over the past 100 years showed that the melanic forms increased in numbers and the non-melanics were becoming rarer and rarer. What could be the possible reason for such an occurrence? In industrial regions, the sooty smoke emanating from factories blackened the bark of the trees and prevented the growth of lichens. This resulted in the black coloured forms resting on such trees becoming invisible to the p~edators and the light coloured ones being increasingly susceptible to the predator attack because of their contrast with the background. In industrial areas the birds predated upon non-melanics in extremely large numbers whereas in other areas melanics were preferentially caten. Since the proportion of melanics increased due to industrial activity, the examplc is often referred to as industrial melanism.