Agent-Based Modelling shows that Operation of Fisher's Principle does not explain Zero Heritability in Human Offspring Sex Ratio

The evolutionary basis of the ratio of male to female births has been a focus of research for over a century. Fishers principle suggests that offspring sex ratios remain roughly equal due to negative frequency-dependent balancing selection: individuals with alleles that produce the rarer sex have greater fitness, increasing these alleles frequency until the sex ratio is balanced. However, recent analysis of Swedish births since 1932 reveals that offspring sex ratio is not heritable, challenging Fishers principles application to humans. Here, we test the hypothesis that Fishers principle uniquely depletes offspring sex ratio heritability at equilibrium. We used agent-based modelling to simulate Fishers principle over 5000 generations and found that Fishers principle at equilibrium does not eliminate genetic variation. These findings, along with evidence of non-heritable offspring sex ratios in humans, suggest Fishers principle is not a suitable explanatory framework for human offspring sex ratio.