Answer/Explanation:
Differential reproductive success.
When the Birch(white) trees were covered in soot(making them black). The black pepper months could blend in to the tree while the white moths stood out.
When the Birch(white) trees were clean and white. The white pepper moths could blend in to the tree while the black moths stood out.
This is an example of differential reproductive success. Who ever could blend in with the tree had the better genes because it made them difficult to be spotted. The other type of moth would be not so lucky, easily spotted an eaten. The better gene lead is "successful," because that moth could live longer because of its characteristics..