These two peacocks are male. As you can see there is genetic variation. What I mean is that some will have a trait for a larger fan of wings while the others will have the trait for a fan of shorter wings.
The female peacocks are attracted more to other peacocks with a larger fan of feathers.
The female peacock will mate with the peacock that has the larger fan of feathers.
The trait of having a large fan of feathers will pass to their offspring. Since some peacocks might be eaten by predators, meaning not all will survive into adult hood,the female peacock will over produce.
Unfortunately for the other male peacocks who have a fan of short feathers they won't able to reproduce. This is because of differential survival and reproduction. In this case the peacock has a trait that prevents him from being able to attract a mate.