Most people are familiar with the concept of natural selection in terms of a change in the physical environment surrounding a population of organisms. So, for example, if the weather gets colder, those organisms with bodies that can retain heat to a higher extent have a survival advantage over those with bodies that don’t. Thus the organisms with better heat retention will tend to have more offspring and in a few generations they will be overrepresented in the population. Another type of natural selection is that which occurs in response to the interaction between organisms. Thus, if a predator with a tendency to attack prey with black fur enters an environment, the amounts of animals with black fur will decline overtime while the numbers of those with fur of different color will increase. This is because those with fur of different color have a better chance of surviving, mating, and having descendants.
The above examples of natural selection involve selective pressures that are external (the environment or predators) to the population of organisms. However, there are forms of natural selection that are driven by internal factors. One such modality of selection is sexual selection. Sexual selection occurs when, for example, female birds have a proclivity for mating with males that have specific characteristics such as brighter or more abundant plumage. Sexual selection is paradoxical in that the brighter or more abundant plumage of males, which makes them more appealing to females, may also make them also more prone to being killed by predators. But the enhanced reproduction success of the males possessing these characteristics compensates for their greater death rate and thus the brighter or more abundant plumage in males endures in the population. However, sexual selection does not always result in large or flamboyant body characteristics, and in many cases its effects are quite subtle. All species including humans experience sexual selection to a certain degree.
The poster boy for extreme sexual selection is the male peacock (Pavo cristatus) which has developed an extravagant train of feathers that it uses to court females.
Below is a video of a male peacock (Pavo cristatus) which was shot at the Waldstein garden in Prague, Czech Republic.
Peacock courtship photo by Dick Daniels used here under an Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0) license.