Sexual selection is a term coined by Darwin to refer to the traits of speciation that are determined by the sexual appreciation of the members of one biological sex. These traits are often exuberant, spectacular and concentrated in the males of the species. In fact, most of the speciation traits developed by this kind of selection do not favour individual survival, and this is the main reason why they concentrate in the species male.
One of the best-known species with evident and exuberant speciation traits is the peafowl. The male peafowl (also known as peacock) has a huge and odd fatherly tail, vivid cobalt blue feathers covering part of its back and its chest and neck to the head, and vivid orange feathers on the tips of the wings. All this exuberance and exhibitionism turn the peacock into an easy target to predators. On the other hand, the peafowl female (the peahen) has a short and discrete tail, its body covered by feathers of an uninteresting greyish. They easily go unnoticed in their natural habitat and seem to deal quite well with that.
It is easy to find many of these examples within the birds. The birds of paradise are a group of birds which, with no exception, males exhibit traits of sexual selection. However, some mammal species also exhibit speciation traits which seem to be of sexual selection nature. The male lion mane is an example as well as the colours of male mandrill monkeys or the antlers of male deer are. There are also many species of fishes who exhibit sexual selection traits. The guppies, a common fish well known by aquarium lovers, are an instance.
Richardia flies live in the Amazon forest and are one of the most astonishing and most eccentric examples of sexual selection. These flies have antler-like projections coming out from their heads on the region where the eyes should be and the eyes (the parts sensitive to light) are located on the tips of these projections.
David Attenborough explains how one male of the Richardia genus flies makes the stalks that sustain their eyes.
But how does sexual selection works, and why the traits are seen almost exclusively in species males?
First, we must acknowledge that most of these traits do not directly increase the chance of individual survival. The fatherly tail of the peacock, the eccentric ornaments of the birds of paradise, the lion’s mane, the long and colourful tail of the guppies, and the fake antlers of Richardia flies only survived to evolution because the females of the species prefer males with these characteristics. In fact, in all these species the male role on perpetuating the species resumes to copulation. All the work is done by the female. Thereby all that it is needed from the male is that he reaches adulthood and become sexually mature to be able to procreate. Actually, all males from the species previously mentioned only developed the eccentricities after achieving sexual maturity. Before that they have a discrete look, often similar to the females, making easier for them to survive and become procreating adults. They are, then, ready to be chosen by one of the females which is carrying both genes: the ones that produce sexually mature exuberant males and the ones that make the females prefer sexually mature exuberant males, assuring the passage of the eccentric traits to the future generations.
Equally fascinating is the role of sexual selection among humans. Truthfully, many are the voices affirming that some of the ethnic characteristics like the Asian eyes, the hair and eyes colour, or height of individuals, that are in the origins of all human variety, are, indeed, traits resulting from a sexual selection. Of course, culture, and not only genetics, played a crucial role in human societies, but, even though, the selection still is sexual, in the sense that, depending on the region of the globe and the ethnicity inhabiting there, genes and culture got together to produce the most diverse expression of human beauty – and that is amazing!