Does sexual selection explain many phenotypic differences among human populations? This hypothesis was first put forward by Charles Darwin and has been given a new twist by Henry Harpending. The following is a question raised about its falsifiability on the hbd mailing list and my reply.
I've been thinking about Prof. Henry Harpending's paper "Human Diversity and it's History" (see online copy at http://harpend.dsl.xmission.com/Documents/prize.pdf). You probably are familiar with it's ideas already but I will recap and you can thus check whether I understand it correctly.
Essentially Harpending argues that human phenotype differences have their origin in sexual selection and that this process is inherently conservative. That's 'conservative' in contrast to 'normal' gene flow processes. So external phenotype characteristics are more likely to stay in place versus "neutral" or "unseen" (and thus not sexually selected) characteristics. Thus phenotypes are more likely to reflect the actual human migration history, than analysis of genotypes or other neutral measures, e g blood types etc.
This hypothesis, if true, means that the last generation or so of 20th century physical anthropologists (eg Hooton, Coon, Howells, Birdsell etc) who took metrical analysis of phenotype differences to the max were "barking up the right tree", and a subsequent generation who focused on measuring phenotypical neutral traits (esp blood etc) may have been "barking up the wrong tree".
I'm slowly coming to my point so forgive me for dragging it out.
As best as I can tell the Harpending hypothesis here doesn't outline any prospective falsification tests. I was wondering if you had heard of, or had any ideas that could be used to test this elegant hypothesis?
I'm interested in this as Harpending's hypothesis would seem to me to be in a position to bolster Joseph Birdsell's trihybrid hypothesis of the historical origins of the Australian aboriginals, although of course, if correct, it would have universal applicability.
There are two tests:
1. Look at human populations with intense male-male competition for mates. This kind of mate competition only partly selects for physical traits preferred by the sex in short supply (females). It also selects for physical traits that help males intimidate or fight off other males (e.g., increased body size, higher bone density, larger muscle mass, higher testosterone levels, etc.).
2. Look at human populations with intense female-female competition for mates. This kind of mate competition selects more for those physical traits preferred by the sex in short supply (males). Such traits will stimulate mate-choice algorithms or any mental algorithm that monitors the visual environment. The selected traits thus tend to be vividly colored. At high intensities of mate competition, "color polymorphisms" will develop: novel colors will have a slight edge over less novel ones, so that vividly colored phenotypes will not only proliferate but also diversify. When any one phenotype becomes too common, the selective pressure shifts to others that are rarer and more novel.
In most species, the first scenario is much more common. Sexual selection is usually about too many males competing for too few females. This is because reproduction predisposes females to invest more in their offspring, particularly during pregnancy and early infant care. During these times of life, females are unavailable for reproduction and drop out of the "mate market." Unless males can match female reproductive investment, they can best serve their reproductive interests by inseminating other females. So, at any given moment, too many males will be competing for too few females.
Unlike the situation in most species, human males have the potential to match female reproductive investment, in part because their offspring are dependent for a longer time and in part because humans have colonized temperate and arctic environments where women are less able to provide for themselves through food gathering. This 'paternal investment' is less important among the agricultural peoples of sub-Saharan Africa and New Guinea, where year-round agriculture enables women to be self-sufficient. Women even become net providers of food. The costs of polygyny thus become negative and men best serve their reproductive interests by acquiring as many wives as possible.
So we can test the Harpending hypothesis by comparing low polygyny/high paternal investment populations with high polygyny/low paternal investment populations. This kind of comparison was done when Winkler and Christiansen (1993) studied two Namibian peoples, the !Kung (hunter-gatherers and weakly polygynous) and the Kavango (agriculturalists and highly polygynous). The latter were found to have markedly higher levels of both total testosterone and DHT, as well as a much more robust physique. The authors suggested that this hormonal difference may account for the !Kung’s neotenous appearance, i.e., sparse body hair, small stature, pedomorphic morphology, and light yellowish skin.
Winkler, E-M., & Christiansen, K. (1993). Sex hormone levels and body hair growth in !Kung San and Kavango men from Namibia. American Journal of Physical Anthropology, 92, 155-164.