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A smarter population?

By razib on September 7, 2006.

A few weeks ago I explored the issue of smart individuals generally not being particularly fecund. I thought some readers might find this table from Cavalli-Sforza and Bodmer's Genetics of Human Populations interesting:

Sperm Donors
% Females Participating (p)
IQ
 
Mean of selected males
 Minimum of selected males
 % of males selected
 1
 5
 10
 20
 50
 100
115 105 38 100 100.2 100.4 100.8 100.9 103.8
130 122 6 100.1 100.4 100.8 101.5 103.8 107.5
145 140 .36 100.1 100.6 101.1 102.2 105.6 111.2
160 156 0.008 100.1 100.8 10.5 103.0 107.5 115.0
175 171 0.0001 100.2 101.0 101.9 103.8 109.4 118.8
(page 769, Table 12.2)


In short, what you see above are two parameters which are being varied

  • a constraint (increased selectivity) of males who are the parents of the F1 generation
  • an expansion of the proportion of females who mate with the selected males

The mean IQ is standardized as 100 in the parental generation, so as you increase the threshold & mean IQ of the males a smaller % become part of the pool of parents (operationally, sperm donors) for the next generation. Heritability, the proportion of trait variation within the population that is attributable to genotype is assumed to be 0.5, or half the variation in IQ being due to genetic variation. This explains why the effect is dampened in proportion to what one would expect, some of the superiority of the males selected is due to chance or some environmental factor, and so is not passed down to their offspring.

The expected mean IQ calculated in the offspring (as depicted in the leftmost nine columns) is derived from this equation:

(mean in generation 1) = (proportion of females) X [(mean of selected sperm donors) - (mean in general population)]/2 X heritability + (mean in the general population)

The relationship of this to the breeder's equation is pretty clear, and one can also see now why sexual selection can be so powerful on polygynous species characterized by a great deal of reproductive skew. But humans aren't really strictly a polygynous species, and our reproductive skew in pre-modern circumstances was probably rather mild....

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genetics

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