Genetic Variation In this environment there are two types of butterflies. There are white butterflies and brown butterflies. The brown butterflies have an advantage the white butterflies don't have, and that is camouflage. The brown butterflies are able to blend into the trees and allow them to hide from birds. This is something the white butterflies cannot do.
Overproduction & Struggle for Survival These types of birds like to eat any butterflies they see. This is the problem for the white butterflies, they don't have any type of protection. They can't camouflage into their surroundings, so they are easilly being eaten by the brids. On the other hand the other butterflies, the brown ones are able to camouflage, and hide from the birds.
Differential Survival and Reproducion Since the brown butterflies are able to camouflage and survive the attack form the birds, they are finding mates and reproducing offspring, and the more they do this the more brown butterflies come out. Of course there can possibly be an outcome of white butterflies but at the moment they are not surviving.
Change in the Gene Pool In the begging the population had more white butterflies. Then birds started to attack because they were hungry. The brown butterflies had an advantage and were able to camouflage, so the white butterflies were being eaten. Since the brown ones were surviving they were also reproducing offspring. So now the population of brown butterflies increased while the population of white butterflies has decreased.