WHy does a peacock have big tail feathers? It uses them to attract a mate. Therefor peacocks with bigger brighter plumage are more likely to breed. Because of this bright tail plumage haws evolved over time.
And not only does that happen in nature, but farmers can do some cool stuff, too. Breeding livestock is the only long-term lab we can look at. But it's still a peacock, and makes a terrible racket.