Throughout the evolution of primate and early hominid species, more complex social environments have been associated with increases in the size of the neocortex. However, despite the continuing increase in the social complexity of the human environment, evidence indicates that in recent millennia the human brain has not only stopped growing, but may have started to decrease in size. Several hypotheses have been suggested to explain this apparent decrease, including the collective intelligence hypothesis and the self-domestication hypothesis. Methodological advances now allow for the detection of signals of selection in human genetic data, meaning selection pressure affecting human social complexity and structural characteristics of the human brain can be evaluated. Pleiotropy, a mechanism underlying several hypotheses explaining the evolution of brain size, can similarly be examined using genetic data. Such analyses may aid in our understanding of how the modern environment may affect the human brain in the long term.
14Chapter 1 social complexity or brain size, it is possible to assess the effects of recent selection on these traits.A notable example of a study which used a large genetic database to study human evolution using (among others) MR and SDS analyses is the study by Song et al (2021), in which the authors examined selection pressure for 870 human traits across four different time scales (since human speciation, pan-Neolithic period, 2,000-3,000 years and modern), using summary statistics from genome-wide association study (GWAS) studies carried out in previous studies. An issue with this study is that the GWAS results used for the analyses were found in previous studies, which did not appear to anticipate that these results may be used for studies about selection pressure in the future. As a result, most likely all (or most) of these studies removed SNPs which deviated significantly from Hardy-Weinberg equilibrium (HWE), as this is considered a necessary test of genotyping quality. However, deviation from the Hardy-Weinberg equilibrium may also occur in case of selection. Hence, the removal of SNPs that deviate from HWE may preclude the detection of signatures of selection. We have included a supplementary analysis at the end of this chapter to demonstrate the effect of recent selection on HWE.Another overview of methods of measuring selection using human genome-wide data is provided in the review by Guo et al (2018). They mention that selection could affect allele frequencies at focal loci as well as at linked loci and it may affect linkage disequilibrium at loci under selection. In loci under selection, minor allele frequencies and effect sizes could become correlated, as alleles of larger effects are expected to be more affected by selection. The authors also mention several methods to leverage between-population genotypic differences, for example by examining the relation between trait-related loci and the principal components resulting from the principal components analysis (PCA) which are usuall...