Most people understand how natural selection drives evolution. The variants within a species best adapted to their environment wind up winning the reproductive contest and their offspring inherit a piece of the earth. At least until an even better adapted variant comes along and takes it from them.

Less well known is that sexual selection can sometimes have an even bigger influence on how species evolve. Darwin figured this out first and said it best.

“He who admits the principle of sexual selection will be led to the remarkable conclusion that the nervous system not only regulates most of the existing functions of the body, but has indirectly influenced the progressive development of various bodily structures and of certain mental qualities."

"Courage, pugnacity, perseverance, strength and size of body, weapons of all kinds, musical organs, both vocal and instrumental, bright colors and ornamental appendages, have all been indirectly gained by the one sex or the other, through the exertion of choice, the influence of love and jealousy, and the appreciation of the beautiful in sound, color or form and these powers of mind manifestly depend on the development of the brain.”

For more on Darwin's pioneering contributions to psychology, see:

UCLA psychiatrist Andrew Shaner is an expert on the modern extensions of Darwin's early insight relating sexual selection to human evolution. He writes:

"Darwin argues that brain function, and the psychology that arises from it, are not just a product of evolution but also a major driver of evolution. Sexual choices help determine which members get to reproduce and which of their physical and psychological features are favored over time."

"In survival selection, the harsh physical environment kills off individuals with anything other than perfect form. So, for example, nearly all the birds in a species have exactly the same wing length and it is exactly the right length for flight."

"In sexual selection, the animals themselves, not the physical environment determine which traits are passed on."

"You might think that female birds would go for males with perfect wing length, but in many species they focus, instead, on some extravagant feature- like the size and beautiful intricacy of a peacock's tail. And, if a peacock with an even bigger tail shows up, he gets all the attention and gets to have the kids."

"From a peahens perspective, bigger is better- even though an enormous tail would seem to be quite a handicap to survival because of the significant investment in energy needed to grow, maintain, and display it."

"But peahens prefer big tails precisely because they are such a burden. The peacock able to grow the biggest tail has convincingly passed the fitness test- the tail is proof he has the best genes for feeding, fighting parasites, escaping predators and anything else necessary to help her future offspring survive."

"Quite a paradox. The peahen's pursuit of the peacock with the biggest impediment to survival ensures that her chicks inherit the best genes for their own survival."

"Human mental abilities may have evolved in a similar way, not so much for survival of the individual, but to attract mates and ensure reproductive success. Our human skills in language, dance, music, and art may have evolved as fitness markers facilitating success in the evolutionary mating game, and are very much influenced by it. Extreme talent is sexually attractive because it indicates good genes for survival.”"

Fascinating. Thanks Dr Shaner. Natural selection works ruthlessly and randomly in favor of adaptations that put food on the table. Your genes don't get to survive long enough to reproduce unless you can collect enough energy to provide for your own survival, for reproduction, and for the survival of your offspring. Of course, adaptations that reduce the risk of your becoming someone else's lunch are also crucial.

Natural selection is blind- it has no purpose beyond selecting those creatures best adapted to make it through each day.

In contrast, sexual selection has an eye for beauty- though of course the nature of the beauty is always in the eye of the beholder. She- and it is usually she- gets to have some say in shaping the course of future evolution.

Our psychology has been shaped by a sometimes uneasy balance of natural and sexual selection- the practical need to acquire our daily bread and the romantic need to find love and produce offspring.

Darwin, Charles (1871). The Descent of Man and Selection in Relation to Sex. Prometheus Books, New York, 1998, p. 641, originally published London 1871.

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