How did this affect the variation in the population of peppered moths?

During the start of the industrial age, darker moths had some sort of survival advantage in the newly-darkened landscape because they used coal to fuel their factories. All the pollution allowed the dark moth to camouflage itself from predation. However, in recent years, the burning of cleaner fuels and the advent of Clean Air laws has changed the countryside of England even in industrial areas, and the sootiness that prevailed during the 19th century is all but gone from urban England. Coincidentally, the prevalance of the carbonaria form has declined dramatically. In fact, some biologists suggest that the dark forms will be all but extinct within a few decades.

At this time there was much pollution around the country, therefore in able to stay alive, over time the moth evolved and changed its appearance in able to blend in with this pollution