After the industrial revolution, dark-colored moths outnumbered light-colored moths in certain regions of england. within the past 40 years, factories in these regions have added scrubbers and air purifiers to their smokestacks, and the relative number of light-colored moths has increased. the probable reason for this increase is that

The reason for the increase is that the light-colored moths are no longer "selected against" by predators. These birds could only see the light ones against a sooted background. After removing the dark soot, these predators could no longer distinguish the light ones from their natural light background. Thus more light-colored moths survived to reproduce, passing on more of their recessive genes for light color to their offspring. After several hundreds of generations, most of the later generations were light, due to selective advantage of camouflage to survive predation.