Opposing levels of selection can cause neutrality: mating patterns and maternal-fetal interactions.

A biallelic viability model based on human data for maternal-fetal interactions reported by Hedrick (1997) gives the interesting result of neutral stability at all gene frequencies. I show that there are two levels of selection, within and among families, acting in opposing directions in this model and that the neutral stability occurs when the two levels of selection exactly balance one another, as they do in a randomly mating population. Deviations from random mating disrupt the balance and consequently destroy the neutral stability. However, with inbreeding avoidance, which characterizes the human histocompatibility loci, within-family selection is strengthened and among-family selection is weakened. This favors the invasion of new alleles and contributes to a high equilibrium level of genetic diversity at loci with maternal-fetal interactions affecting offspring viability in the pattern described by Hedrick. This pattern of selection is remarkably similar to that observed for the maternal effect selfish genes, Medea in flour beetles and scat in the mouse, and the Gp-9 gene in the fire ant.