I have two choices for my mate. One has bland feathers and the other has bright beautiful feathers.
The one with bright feathers is very impressive. I choose him.
In this situation there is variation between the color of the male peacocks feathers. There is also an overproduction of peacocks. There are more peacocks than can survive to reproduce.
In this case there is not an real "struggle for existence" but there is a struggle for reproducing. The male peacocks with brighter feathers attract more mates.
Over time the population of bright feathered peacock will go up and the population of the bland feathers peacocks will go down. This will happen because the bright feathered peacocks will get to pass down their traits more often because they are more often chosen a mate.