Nehaveigur

We’ve been on earth all these years and we still don’t know for certain why birds sing […] If the lyric is simply “mine mine mine,” then why the extravagance of the score?

Annie Dillard: Pilgrim at Tinker Creek

Natural selection, defined by genes that benefit survival becoming more frequent, is generally well understood. Sexual selection is more subtle. It acts on genes that benefit mating success but sometimes reduce survival. Runaway selection, proposed by statistician and geneticist Ronald Fisher in the 1930s, is a special case. If females of a species have a slight preference for males with a certain trait, that trait will lead to greater mating success. Because both the trait and the preference are heritable, they become genetically correlated: females preferring the trait mate with males expressing it, producing offspring that inherit both the trait (in sons) and the preference (in daughters). This generates a positive feedback loop. Extravagant ornaments like the tail of peacocks are likely due to Fisherian runaway selection.

Birds, Sex & Beauty by Matt Ridley is about sexual selection. It covers the history of the idea, going back to Charles Darwin’s The Descent of Man, and its ups and downs since then. Because sexual selection is a subtler theory than natural selection, it has met with more resistance. Ridley explores different aspects of the theory, stitching the parts together with his observations about birds and specifically grouse. I don’t mind birds, but I’m not that interested in the specifics of grouse behavior and skipped over some of those parts.

Of course, sexual selection isn’t limited to birds. It happens in monogamous species like humans too. Ridley argues that its influence is underappreciated:

Yet to this day the vast majority of ideas about the evolution of the human mind evoke natural, not sexual selection. Where sexual selection is mentioned it is to explain differences between males and females, not between human beings and Chimpanzees. Popular theories to explain the rapid expansion of the human brain starting some three million years ago include the ‘social brain’ hypothesis, whereby human ancestors evolved big brains to keep track of relationships, plots and schemes within large and complex social groups. But explaining the unique expansion of the human brain using such natural selection explanations runs into the brick wall of a simple question: why did it not happen to other species? If it is so valuable to have extra gray matter, why don’t all sorts of birds, monkeys an apes have it? Why are Bottlenose Dolphins the only creatures that come close to rivalling our brain size relative to body size? Sexual selection specialises in creating unique, runaway expansions found in no other species, so again it is a candidate that deserves consideration […] Sexual attractiveness alone can be a sufficient explanation for almost any human mental trait. It should therefore be the null hypothesis.

I’d particularly recommend Birds, Sex & Beauty to the authors of this recent paper on the function of love.