Sexual Selection and Human Phenotypic Variation

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.

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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.

Tim Gillin

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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.

Reference

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.

ASPM and human evolution

A year ago, Science published an article showing that the ASPM and microcephalin genes continued to evolve even after the emergence of Homo sapiens, being more prevalent in some populations than in others. This finding raised many eyebrows because the evolution of these genes is closely tied to increase in brain size between lower primates and humans.

Eyebrows have been lowering over the past year. Two independent research efforts have failed to find any relationship between ASPM variants and performance on IQ tests. Today, the general feeling is that it is time to move on to other things. This finding, like so many others, seems to have been just a flash in the pan.

Yet some, like me, think otherwise. Two British researchers, Dan Dediu and D. Robert Ladd have recently argued that the latest ASPM and microcephalin variants are associated with populations that speak non-tonal languages (PNAS "Linguistic tone is related to the population frequency of the adaptive haplogroups of two brain size genes, ASPM and Microcephalin"). Their hypothesis might explain the latest microcephalin variant, which is estimated to have arisen 37,000 years ago. The latest ASPM variant, however, is much younger, being only 5,800 years old (95% confidence interval between 500 and 14,100 years ago). It is probably too young to account for the emergence of non-tonal languages. Neither Indo-European languages nor Uralic languages are tonal, so it is likely that a non-tonal language was ancestral to Proto-Indo-European (8,000 - 6,000 years ago) and Proto-Uralic (10,000 - 7,000 years ago).

What cultural change, then, might have favored the latest ASPM variant? This is the subject of an article I have recently written for Medical Hypotheses:

The spread of alphabetical writing may have favored the latest variant of the ASPM gene

Abstract:

ASPM, a gene that regulates brain growth, has evolved considerably in the primate lineage that leads to humans. It continued to evolve even after the emergence of modern humans, with the latest ASPM variant arising about 6000 years ago somewhere in the Middle East. The new variant then proliferated within and outside this region, reaching higher incidences in the Middle East (37–52%) and in Europe (38–50%) than in East Asia (0–25%). Despite its apparent selective advantage, this variant does not seem to improve cognitive performance, at least not on standard IQ tests. At present, we can only say that it probably assists performance on a task that exhibited the same geographic expansion from a Middle Eastern origin roughly 6000 years ago. The closest match seems to be the invention of alphabeticalwriting, specifically the task of transcribing speech and copying texts into alphabetical script. Though more easily learned than ideographs, alphabetical characters place higher demands on mental processing, especially under premodern conditions (continuous text with little or no punctuation, real-time stenography, absence of automated assistance for publishing or copying, etc.). This task was largely delegated to scribes of various sorts who enjoyed privileged status and probably superior reproductive success. Such individuals may have served as vectors for spreading the new ASPM variant.

http://dx.doi.org/10.1016/j.mehy.2007.04.039

Peter in the news ...

In its August issue, Allure magazine ran an article by Maghan Daum on the origins of blond hair, including a summary of my work on the subject. I had talked with Maghan earlier over the phone, in addition to sending her the following capsule summary:

"Until 10,000 years ago, tundra covered Europe and the only food was migrating herds. Only a very able hunter could provide for a second wife, so polygamy was rare and women had to compete for mates. Men were also in short supply because they ran a high risk of death while hunting herds over long distances. The resulting sexual selection favored women with bright, novel, or striking hair and eye colors, such as blond hair."

The article was nicely written. I didn't expect to see this kind of stuff in a fashion magazine!

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On August 10, I was a guest on The Leonard Lopate Show (WNYC/New York Public Radio (93.9 FM/820 AM) for a weekly series titled "Please Explain." The topic was hair – on the head and on the body, e.g., How did different ethnic groups end up with different colored hair, and different amounts of hair on the body? How is hair linked to skin and eye color? What evolutionary purposes does hair serve? Are some ethnic groups more likely to go bald than others?

How did I do? I suspect that my nervousness was audible, at least during the first half of the show. To be honest, I’m not used to giving quick answers to questions, even when they’re nicely put to me. God knows how I would do in a more confrontational situation!