Consider how male peacocks got their bright feathers. These bright feathers have no benefit for survival. In fact they are harmful for survival, being heavy and easy for predators to see. So let's start by imagining peacocks before they had big bright feathers. Since females can only have a limited number of children, females tend to be selective about which males they mate with. They should prefer healthy "good-looking" males since their children will inherit the male's genes. Those males with dull feathers may well have been poorly nourished and unhealthy. Bright feathers were probably a good indication of a healthy male. So females evolved to prefer males with bright feathers. But now it becomes in the male's interest to have bright feathers because if he doesn't, females will reject him and he won't be able to reproduce. So males evolved to have bigger and brighter feathers, not for survival, but for the sexual advantage that allowed males to have more children. At some point these bigger brighter feathers were no longer an indication of good health. So why didn't the females stop preferring such males? This is explained by the sexy son hypothesis. If the female mates with a male who is popular with other females, then her sons will likely inherit the male's traits that made the father popular and so her sons will also be popular and will have many children with many females. This will spread not only the father's genes but also the mother's genes. So those females who are attracted to popular males have an advantage. At this point in the story, male peacocks with big bright feathers are popular with females, so it is to each female's advantage to mate with males with big bright feathers. This is Fisherian runaway selection, a story of evolution gone wild, of evolution not doing what is in the best interest of the specie