I’ve just read The Mating Mind: How Sexual Choice Shaped Human Nature by Geoffrey Miller. The book’s argument is that many of the typical characteristics of human behaviour are best understood as products of sexual selection. Sexual selection is the process where you start with some ancestral bird species where the females have a bit of a thing for longer tails, and a few thousand generations later, they’ve evolved into peacocks. I suppose there are two key insights I’d take from the book, neither of them completely new to me but worth being reminded of.
The first is this – it’s easy to think of natural selection as being driven by adaptations for survival, with sexual selection being almost cosmetic in its effects. It doesn’t matter how good-looking you are if you starve, die of disease or get eaten by a lion before you get a chance to breed. But from an evolutionary perspective, there’s no point in living to a ripe old age if you can’t attract any sexual partners. Both scenarios are evolutionary dead-ends. Also, selection is not necessarily an either-or process (either you survive or you don’t; either you breed or you don’t). Rather, it’s driven by differential rates of reproductive success. And within a well-established species, it’s easy to see how the biggest single factor in determining reproductive succcess will often be the ability to attract a mate. The results of sexual selection will often appear cosmetic – coloured feathers, or an attractive song – but that’s just because those are the things a potential mate is able to perceive. It doesn’t mean that sexual selection is a less powerful force than ‘normal’ natural selection. In a sense, this is an obvious insight; anyone who has ever heard a nightingale or a blackbird singing must be aware of how much effort it is costing them, and there are few more spectacular adaptations than the plumage of a bird of paradise. And just because sexual selection mainly operates on external features, it doesn’t mean that it is limited to those features. Applied to humans, it doesn’t have to be limited to skin colour, breast size and hip-waist ratios. There’s no reason why it can’t also operate on people’s ability to hold a conversation, or dance, or sing. The only requirement is that there must be some genetic component.
The other insight is that anywhere in nature where we see an oganism with a physical feature or behaviour that doesn’t seem to have any survival benefit, it’s worth considering sexual selection as the explanation. Natural selection is inherently thrifty – we should never expect to see energy being expended without there being some reason for it. If that reason isn’t survival, pretty much the only other possibility is an attempt to attract mates, either directly or via increased status. And sexual selection can take almost any form. There are reasons why some adaptations are more likely than others, but the process is essentially arbitrary; once some trait becomes associated with sexual attractiveness, it’s a self-sustaining trend. The explanation is almost too powerful – you can see how it would become a lazy assumption faced with anything slightly unexpected, but as far as I can see, it’s very difficult to disprove. Geoffrey Miller certainly sees sexual selection everywhere – he uses it to explain sport, art, poetry, music, language, dancing and indeed just about everything that makes us human.
I find this argument moderately persuasive, I must admit. As ever, there are questions about which human behaviours can really be seen as written into our genes; can music making really be seen as an evolved trait? Or sport? They seem to be human universals, so it’s not a ridiculous idea, but I’m still slightly wary about making the assumption. But for more obviously evolved traits, like language, it seems very plausible that sexual selection would be the principle driving force.
On the whole, though, I found the parts of the book about human behaviour less interesting than those about sexual selection generally. I’ve read about sexual selection before but to have it treated in depth as a subject in its own right was helpful. For example, the classic examples used to illustrate sexual selection involve dramatic sexual dimorphism, as with the drably-plumaged peahen, or the difference in size between male and female elephant seals. But Miller points out that those are a special case where a few successful males account for the vast majority of offspring. Even in species which form largely faithful pairs, there is an advantage in being able to attract the best (healthiest, most fertile) mates. In that situation, the effects of sexual selection will be less dramatic, but will still be present. For example, in bird species where colourful plumage is found in both sexes, they have traditionally been referred to as ‘species markers’; Miller suggests that these could still be the results of sexual selection.
So I would have liked more of the book spent on sexual selection in general, with more illustrative examples from other species, and slightly less of the human stuff at the end. But it’s a good book, and I recommend it.