Sexual selection is the evolutionary pressure that selects for individuals that are more attractive to members of the opposite sex. Sexual selection was theorized by Charles Darwin, along with natural selection and artificial selection.
The handicap principle suggests that honest signals of fitness must also be costly signals. In this way, fitness can be shown off by handicapping behavior, or morphology that effectively lowers actual fitness. The typical example is a peacock's tail; the main purpose of the tail is to exist as a handicap to show off. Traits such as this signal the ability to squander a resource.
To flaunt the squandering of resources signals that the individual must be fit in other ways. Individuals who were not fit would not be able to afford to squander resources; they would instead need to devote resources to more crucial areas of their morphology
Sexy Son Hypothesis
One explanation for the preference for more attractive individuals is that it is a type collective action problem. Individuals select mates with traits considered more attractive because they know that those traits will be passed on to their children, and they want their children to be more attractive. Individuals want their children to be more attractive because it increases their potential evolutionary success, because of the aforementioned preference for more attractive individuals. Thus, sexual preferences are self-reinforcing.
Sexual selection plays a role in sexual dimorphism, or the difference between the two sexes of the same species.