While we grapple with numerous discoveries of variation in genomic diversity in humans, interest has subsequently risen in understanding their causes/results. Two recent papers describe experiments to determine (a) the effects of marital rules (who gets to marry whom) on genomic diversity (Guillot et al. 2015), and (b) the correlations between effectively random-mating, and inbreeding human populations and various health-related quantitative traits (Joshi et al. 2015).
Relaxed Observance of Traditional Marriage Rules Allows Social Connectivity without Loss of Genetic Diversity, Guillot et al. Molecular Biology and Evolution, 2015.
Marital rules – societal constructs on who marries whom are predominant in several human populations. Biologically, one would hypothesize that these rules also influence genetic diversity of the population, and thus the fitness of offspring. Guillot et al. (2015) attempt via simulations, and analyses of SNP diversity in an Indonesian population to quantify relaxed, or strict adherence to these rules, particularly the MBD rule (or Mother’s Brother’s Daughter) wherein men are required to marry their mother’s brother’s daughter. Key findings of the study include (a) strict MBD marital rules lead to a reduction in genomic diversity under simulations, (b) non-adherence of strict MBD rules in the Rindi community in Eastern Indonesia, an island population in which marital rules have been extensively studied.
Certainly, reduced genetic diversity under a strict interpretation of the APA marriage rules suggests that there was little biological incentive for communities to enforce marriage rules strongly, at least for long periods of time.
Directional dominance on stature and cognition in diverse human populations, Joshi PK et al. Nature, 2015.
While the detrimental effects of inbreeding (and marital rules like in Guillot et al. above) have been extensively studied in Mendelian traits in humans, most fitness traits are complex, and polygenic. Joshi et al. (2015) as part of the ROH (Runs of Homozygosity) consortium investigate 16 quantitative traits that have fitness consequences in humans and their correlations with homozygosity. Analyses of SNP arrays for ROH in more than 300,000 individuals revealed (a) differences in ROH lengths, and demography (with African populations containing the least homozygosity, and isolated populations, including Amish, and Hutterites containing the most homozygosity), (b) an average reduction of 1.2 cm in height, and 137 ml in forced expiratory volumes in offspring of first cousins, (c) 0.3 standard deviations reduction in general cognitive ability, and 10 months’ reduction in educational attainment in offspring of first cousins, and (d) no significant effect in 12 other fitness related traits (particularly to do with cardio-metabolism).
We have demonstrated the existence of directional dominance on four complex traits (stature, lung function, cognitive ability and educational attainment), while showing any effect on another 12 health-related traits is at least almost an order of magnitude smaller, non-linear or non-existent.
Joshi, Peter K., et al. “Directional dominance on stature and cognition in diverse human populations.” Nature (2015) DOI:10.1038/nature14618
Guillot, Elsa G., et al. “Relaxed observance of traditional marriage rules allows social connectivity without loss of genetic diversity.” Molecular biology and evolution (2015). DOI: 10.1093/molbev/msv102