Genetic variation in intelligence amongst humans is very unlikely to be due to architectural improvements, and is probably due to some combination of design tradeoffs and the relative prevalence of deleterious mutations.
Moreover, behaviorally relevant genetic variation in human intelligence seems to be a nearly a priori prediction of the contemporary understanding of evolution, and so to provide relatively little evidence about the nature of intelligence. SupportMutations that improve intelligence--rather than moving along some tradeoff between intelligence and other useful features--seem likely to improve fitness and spread throughout the population. Moreover, substantial architectural differences amongst humans do not appear to be plausible in light of sexual recombination.
Consequently, mutations that improve intelligence without negative consequences are likely to be very prevalent throughout the human population. At face value, suboptimal alleles are likely to represent deleterious mutations that degrade performance rather than novel performance-enhancing mutations. Even given strong positive selection, such mutations would persist in the population simply because the rate of selection will not be high enough to remove all mutations as they are introduced.
It seems likely that enough mutations in the human genome would result in seriously degraded cognitive performance. In light of this fact, we should expect mutations to continue to accumulate until the regime where they are behaviorally relevant, since prior to that point they cannot meaningfully be selected against by evolution. The extent of this variation would be expected to be larger if intelligence had a smaller effect on fitness, if the mutation rate was higher, or if individual mutations had a higher impact on intelligence. |