Sexual selection is a special case of natural selection, which consists of an individual choosing another, of the opposite sex, because of some differential attribute that it contains. Thus, the “chosen” individual is more likely to fertilize his gametes, ensuring the perpetuation of his genes and even transmitting such an evolutionary advantage to the offspring. As a result, such traits tend to become more and more frequent over time and generations to come.
Plumage, colors, horns, aggression, physical strength, or even singing; are aspects that can be different in this case, making the potential partner more attractive than another from the same population.
Generally, such attributes refer to secondary characters of males, and it is up to the female to define her future partner, as these indicate that the chosen animal is healthier and more resistant than other individuals in the population.
In amphibians, for example, females tend to choose larger males with more elaborate singing. As these animals are nocturnal, with less developed eyesight, a lower pitched song indicates to these animals that their potential mate is larger than other individuals in the population. Thus, such a factor may indicate that he survived all the weather, such as predators and environmental pressures; being, therefore, more capable than many who have not reached that level.
In the case of peacocks, males with longer and showy tails tend to be more attractive and sought after by females. Such a choice may be related to the fact that being able to balance with a large and heavy tail indicates health and resistance.