If there are two cells in the same organism that differ only in the number of chloroplasts they contain, and the second cell is the one with fewer, the second cell would not be able to produce as much food because it couldn't capture sunlight.
The pollution from the Industrial Revolution darkening the trees. After the trees darkened from all the soot in the air, the white moths were easier to see and more vulnerable to predators. The dark moths blended into the dark tree bark.