I believe the answer is: The dark colored moths will experience disruptive selection
The bird prey on moth and possibly identify the moth by visual sense. The moth has two type of color: dark and light. The trees in the area is more light coloured so the black moth will be identified easier as their color is more contrast to the tree. As the dark moth easier to identify, they will be predated more and their population will be reduced. When a trait has advantage than the other, disruptive selection happens.
