The Evolutionary Default Hypothesis and Negative HBD

Jayman (2016) argues:

There is no reason to suspect that human groups that have been separated for tens of thousands of years in vastly different environments would be the same in all their cognitive and behavioral qualities. In fact, a priori we should expect them not to be, since such equivalence after so many generations of separate evolution is nigh impossible.

We can quantify the expectation.

When it comes to quantitative genetic trait differences between populations, the evolutionary default expectation is that differences will be commensurate with the degree of drift (not to be equated with neutral mutations). For diploids, the formula is:


VA G,B is the genetic variance between groups
VA, C is the additive genetic variance in a common ancestral population
2FST is 2 times the fixation index with respect to low mutation rate biallelic polymorphs of the type that underlie the traits in question (see: Edelaar and Björklund, 2011)

Discussing the default, Leinonen, McCairns, O’Hara, and Merilä (2013) note:

Yet, because most quantitative traits of evolutionary, ecological, economic and even of medical interest — such as body size and intelligence quotient — are known or thought to have a polygenic basis distinguishing neutral and selective patterns of population differentiation at the phenotypic level is not easily accomplished with standard FST estimates… The value of QST for a neutral quantitative trait that has an additive genetic basis is expected to be equal to the FST for a neutral genetic locus. This finding — which is based on the work of Sewall Wright — provides a basis for evolutionary inference… If QST > FST, trait divergence exceeds neutral expectation, and is likely to have been caused by directional selection. If QST < FST, trait divergence among populations is less than expected by genetic drift alone; this pattern is sug¬gestive of uniform selection or stabilizing selection across the populations. [Emphasis added.]

The SNP Fst between major continental races is about 0.12 and the additive genetic variance for e.g., IQ is 135. Plugging the numbers into the equation we get an expected VA B, G of 32.4/135 = 0.24. The expected phenotypic variance (VA, P) would be a function of this and the narrow heritability of the trait. Simply multiplying the genetic variance value by a h^2 of 0.6 gives us = 0.14, which could be treated as eta-squared and converted into a Cohen’s d — of about 0.75.

This is quantitative refutation of Lewontin’s (1978) (second) fallacy, where, in reply to Mitton (1977), he acknowledged that “when enough loci are looked at” races do not overlap, but then went on to argue that this “misses the point” which concerns expected quantitative difference.

Now, renowned behavioral geneticist Robert Plomin considers continental race difference to be uninteresting because they are small. Yet if they are much smaller than is predicted given the evolutionary default, uniform selection must be at play — which is interesting. We might call the absence of expected difference “Negative HBD” and note that this is something to be explained.

There is a question about whether IQ is a trait that has been under selection. Verweij (2012) lists the predictions of various evolutionary models. Except for the high additive genetic variance, IQ fits the profile of a mutation-selection trait. So drift — and time of separation — is probably not a good index magnitudes of differences. Yet, it just so happens that genetic distance tracks cultural and social differences along with differences in evolutionary environments, which act as crude indexes of selective pressures.

Returning to Jayman’s point, the question is whether we should expect uniform selection. I can not see how this could be considered as an evolutionary (divergence and modification) default. (Though, I’ve seen this argued, incredibly.) Nonetheless, uniform selection can’t be ruled out a priori. Perhaps all around the world, life demands were such to select for roughly the same ability profile across different groups, but, of course, not within them. If the reigning uniform selection model turns out to be correct, there will be something truly interesting to be explained — something which may have relevance for future Eugenics programs. Maybe there are sociological barriers to the evolution of highly cognitively competent societies.


  1. Winthorp

    Do you know of any traits, or categories of traits, that indicate uniform selection?

    • Chuck

      Clearly uniform (negative) selection with respect to most diseases. See: Wu and Zhang’s (2011) “Different level of population differentiation among human genes.”

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