Natural Selection with reference to Industrial Melanism:
Before the industrial revolution in England, in the early part of the 19th century, the peppered moth with a dull grey color (Biston betularia) was observed in excess and the dark variety (carbonaria) of the same moth was very rare. Within a couple of hundred years, it has almost completely replaced the dull grey-colored moths.
(1) Before the Industrial Revolution, the tree trunks were covered with grey-colored lichen and the grey-colored moths resting on the tree trunks were escaped unnoticed from the predatory birds. But the dark-colored moths were easily noticed by the predatory birds. So, these black moths were predated by birds and grey color moths escaped from death. Thus, the population of grey-colored moths (B. betularia) was abundant before industrialization.
(2) With the advancement of heavy industrialization a sufficient quantity of smoke was produced which caused the appearance of dark-colored lichen on the tree trunks. As a result, the grey moths became more conspicuous than the black variety and hence predator birds selectively picked the grey moths for predation. So, the frequency of black-colored moths inhabiting the industrial areas increased.
The dark variety moth could not survive in the non-sooty forest or in rural areas because birds could locate them on the lichens. But with industrialization, their population increased and they completely replaced the light-colored moths. Here is the environmental factor that has selected the dark variety against the grey forms in the bird predation. Natural selection has operated in the direction of eliminating the gene for light color and gradual increase of gene for dark color.
Presently in advanced industrialization, oil, and electricity have replaced coal in industries and there is minimal production of smoke. It has caused less deposition of soot (carbon) on the tree trunks and ultimately, the tree trunks are again grey in color due to the accumulation of lichens. Consequently, light-colored moths have again increased in number. This reveals the example of industrial melanism.