The reason for the loss of color after the industrial revolution is that the light-colored moths were "selected against" by predators. These birds could only see the light ones against the newly dark, sooted background. Over time, these predators could no longer distinguish the dark ones from their natural dark, sooted background. Thus more light-colored moths stood out against the dark soot, and were eaten. And more dark-colored moths eluded the birds, survived to reproduce, passing on more of their dominant genes for dark color to their offspring. After several decades of hundreds of thousands of generations, most of the later generations were dark, due to selective advantage of camouflage to survive predation.
