So, given that we have logical reason to hypothesize about differences in cognitive abilities, why would we expect to measure these by using a single number such as IQ, which suggests there must be a hierarchy of cognitive function? The prediction surely is that each population will adapt to be better at the particular cognitive tasks that are most important for survival in its own environment. If this is the case, then identifying these (potentially adaptive) differences in cognitive ability, and searching for associations with genetic variants, could provide fascinating insights into how our brains work.
He then goes on to add:
This makes good sense; if human populations have indeed undergone some level of genetic adaptation to meet differing cognitive demands (which seems entirely possible given what we know about recent human evolution),
then investigating group differences may provide useful insights into the molecular architecture of cognition.
I want to emphasize the molecular architecture aspect here. Variation in IQ, like height, is probably due to genotypic variation on many genes. In fact, the effect sizes for genes controlling variation in IQ is probably smaller on average than for height (perhaps far less than 1% per gene). Unlike some I believe that it selection can operate on quantitative traits; otherwise agricultural genetics as a field probably wouldn't exist. But I also recall from basic population genetics that the fitness implications of highly heritable continuous traits tend to be weak. The reasoning being that if selection was operative then variation would quickly be eliminated as the population fixes into the "fit" state. That being said, there is the possibility of balancing selection pushing the population toward a stable optimum. What could push individuals off the optimum? Here's a candidate: Intelligence tests with higher g-loadings show higher correlations with body symmetry: Evidence for a general fitness factor mediated by developmental stability:
Just as body symmetry reveals developmental stability at the morphological level, general intelligence may reveal developmental stability at the level of brain development and cognitive functioning. These two forms of developmental stability may overlap by tapping into a bgeneral fitness factor.Q If so, then intellectual tests with higher g-loadings should show higher correlations with a composite measure of body symmetry. We tested this prediction in 78 young males by measuring their left-right symmetry at 10 body points, and by administering five cognitive tests with diverse g-loadings. As predicted, we found a significant...relationship between each test's rank order g-loading and its body symmetry association. We also found a substantial correlation...between body symmetry and our most highly g-loaded test (Ravens Advanced Progressive Matrices). General intelligence is apparently a valid indicator of general developmental stability and heritable fitness, which may partly explain its social and sexual attractiveness.
If general intelligence is a fitness indicator, then the high genetic variance (and substantial heritability) of general intelligence in humans is really a special case of the high genetic variance of general fitness inmost species. Biologists recently have focused on the importance of mutation-selection balance inmaintaining heritable variation in general fitness...For example, Eyre-Walker and Keightley...estimated that over most ofhuman evolution, an average individual has had 1.6 new, harmful, phenotypically expressed mutations that neither parent had; Crow...argues that the number should be closer to three new mutations per individual. With mutations accumulating at this rate, selective pressures appear too modest to reduce the heritability of general ability below its frequently observed levels. ... If pleiotropic mutations do create much of the heritable variation in general fitness, morphodevelopmental stability, neurodevelopmental stability, and general intelligence, then this has several important implications for psychology. First, it suggests that the g factor is best understood at the genetic level as an index of mutation load, rather than at the psychological level as a distinctive "cognitive capacity" that might be localized in some brain area or identified with some distinctive set of information processing operations...The different g-loadings of different psychological adaptations simply reflect the susceptibility of those functions (and their underlying neural mechanisms) to disruption by deleterious mutations, and need not be given any cognitive interpretation. Second, there is no theoretical conflict between the existence of a unitary g factor at the level of individual differences in psychological functioning....and the existence of many psychological adaptations at the level of species-typical cognitive architecture, as advocated in the "massive modularity" view of evolutionary psychology...Third, the importance of possible "intelligence-boosting genes" (i.e., alleles at specific genetic loci that increase intelligence above the population average...) will need to be considered in the context of variation in intelligence maintained by fitness-reducing (and hence intelligence-reducing) mutations.
The normal distribution whereby there is a long tail above and below the median seems to imply that the distribution of mutational load has a high variance and expectation within the population.