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Researchers have found sexual selection important in the evolutionary history of humans, and a lot of researchers are focusing on the roles of mate choice and life history in major transitions in human evolution. I find the transition from hunter-gatherer to agricultural civilization the most interesting. This week I’ve read three interesting papers on three interesting facets of human sexual selection. These studies also did things in three different ways: there is a study of psychological study of modern (i.e. living) human mating preferences, a study of pre-industrial humans using historical data and a theoretical study using mathematical and computer models.

The first study is a twin study of human mating preferences in men and women. Brendan Zietsch and Karin Verweij at the university of Queensland, and Andrea Burri from King’s College, London gave surveys to men and women in both monozygotic (MZ, “identical”) and dizygotic (DZ, “fraternal”) twin pairs, asking them to rank the qualities of mates that they found desirable. The traits were qualities like “kind and understanding,” “healthy,” “intelligent,” “good earning capacity,” “good housekeeper,” “wants children,” and of course “physically attractive.” The most amusing characteristic of their raw data was, of course ,the differences between the sexes. Members of both sexes valued “kind and understanding” the most, and in line with other studies, men valued physical attractiveness more than women did. The least important qualities to men and women were “religious” and “university graduate.”

The evolutionary part of this study was using the twins to find the ability of natural selection to affect these preferences. MZ twins have the same genetic material, therefore differences between them are due to the environment. We can get more information on the role of environment by looking at DZ twins, who share some genes. From these data the researchers quantified the amount of additive genetic variance, dominance genetic variance and environmental variance. When individuals differ in their survival or reproductive success and it’s because of additive genetic variation — the number of certain copies of particular genes — then all those differences in fitness are passed on to their offspring. Additive variation is therefore the raw material for selection. These researchers found that the mating preferences with the highest heritability (biggest differences in preferences due to genes) were those that ranked highest on people’s priority lists. The researchers also questioned why the heritabilities should be so low (around 20% in most cases): though they entertained many possibilities, I think the most promising is that selection is acting very strongly on women’s and men’s mating preferences right now.

That brings us to the second study, from a multinational group of researchers that was published in Proceedings of the National Academy of Sciences (PNAS). The data on marriage (i.e. mating), births and deaths was mostly compiled from church records, family bibles and tax records in pre-industrial Finland. Another really interesting church-record data set producing results on human evolution, was derived from records of the pioneering Mormons of Utah in the late 1800s. The researchers studying the Finnish population asked if they could determine whether natural and sexual selection was acting on this population over the time period of their data, and what the crucial periods in the lifetime were: what fitness components contributed to overall fitness? Was it survival to adulthood? Was it how many times you married? Was it how many children you had? They found that in this society, where serial monogamy (i.e. remarriage) was common, and extramarital affairs and divorce were severely punished, that survival to adulthood made the biggest difference. However, they also found that the raw material for sexual selection in this population: interestingly from my perspective, most men remarried younger women who could still produce more children.

They also found that sexual selection might be able to act on the ability to remarry in women as well as men in this population. Also interesting: the wealth of individuals (whether or not they owned land) was totally unimportant to any fitness component, either under natural or sexual selection. The authors of the paper emphasized that natural and sexual selection can still act in our species, despite the demographic changes that came with the agricultural revolution. This is an important finding, and their data is totally awesome.

The third study I read this week used mathematical models to study the possible transition from a promiscuous mating system in human ancestors to our more-familiar system with long-term pair-bonds. Sergey Gavrilets of UT Knoxville authored this study, also published in PNAS. This is a contentious area, with many sources of interest, including anthropologists, social theorists and evolutionary biologists. I’m glad to see that a theory paper is getting some popular attention, and particularly that anthropologists are paying attention to it.

Gavrilets studied four different models of how males can obtain mates, and how females derive their fecundity, at least partially from male behavior. He used these models to ask if there wa sa relationship between the fighting ability of males and how much they provisioned their mates: we often assume, as is standard in economics as well, that each organism has a finite amount of resources to devote to various activities, so he divided male activities into fighting versus something else. All these models led to a state where males did nothing but fight, and females had lower fitness than if they got some direct, material benefits (food) from their mates. This is a low-fitness state: good for males who can fight to gain more mates and thereby more offspring, but not so good for females, who could have more offspring and survive better if the dudes would just cut it out.

Then Gavrilets added two wrinkles: a negative correlation between male fighting ability and mate provisioning, and female variation in faithfulness to their mates. Using a computer model, he then simulated evolution to show that populations would move more toward monogamy and long-term pair bonds. Gavrilets’ conclusion is that low-ranking males (not as good at fighting) would could increase their reproductive success by provisioning their mates; females can reward and reinforce this by not fooling around, which would force these males to provision offspring who got genes from an aggressive, promiscuous male.

All these studies show some really interesting ways that sexual selection works in humans, and may have played a role in our past. There is male and female mate choice going on. There are factors other than simple mate choice, for example in Gavrilets’ models. There are also life-history factors: as the Finnish study showed, survival to adulthood is a necessary prerequisite for mating success. Also, knowing that sexual selection can still act in humans, who are mostly monogamous, is really exciting.

A few caveats are in order, however: all of these studies emphasize selection. So do I, since I study selection. However, it’s easy to get caught up in the idea that selection is the evolutionary mechanism, when in fact there are other forces that are potentially much more important, particularly genetic drift, which randomly causses alleles to disappear from populations. We often think of genetic drift as something that produces variation between populations: the differences in appearance between Europeans and Asians, for instance, are conceivably due to drift. But who wants to think that critical parts of our identity — for example our mating behavior — could be due to a process even more stupid than natural selection? Drift is not just stupid, it’s stochastic — even worse! In other words, selection at least has the appeal that “it’s not totally random!” as biologists are often heard to say to religious zealots. Drift, on the other hand, is completely random. Not very pretty. The trap of thinking that everything interesting is due to selection is called adaptationism.

The other caveat is that most studies of humans assume that our current monogamous mating system is derived, in other words a recent adaptation. Most studies I hear of, be they from anthropologists, psychologists, or evolutionary biologists, assume that our ancestors were promiscuous or polygynous. This is intuitively appealing for scientific reasons — men are, on average, larger than women — and for social reasons — we like to think of ourselves as new, developed, derived and interesting. Whatever we are doing right now is often seen as a good thing, and we know that in the past what those people did was not a good thing. However, I have yet to see any data that supports this idea. The specific significance of Gavrilets’ paper hinges on the idea that our ancestors were not monogamous. However, this could be a good case of The Platypus Fallacy: just because gorillas and chimpanzees have different mating systems from modern humans does not mean that our ancestors did.

Thanks for reading.

  • Zietsch BP, Verweij KJ, & Burri AV (2012). Heritability of preferences for multiple cues of mate quality in humans. Evolution 66 (6), 1762-72 PMID: 22671545
  • Alexandre Courtiol,, Jenni E. Pettayd,, Markus Jokelae,, Anna Rotkirchf, and, & Virpi Lummaaa,b (2012). Natural and sexual selection in a monogamous historical human population PNAS DOI: 10.1073/pnas.1118174109
  • Sergey Gavrilets (2012). Human origins and the transition from promiscuity to pair-bonding PNAS DOI: 10.1073/pnas.1200717109
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