Sex ratio

Sex ratio is the ratio of males to females in a population. It can be expressed as a permille value.

Ronald Fisher, in his 1930 book The Genetical Theory of Natural Selection outlined a model that explains the (approximately) 1:1 sex ratio commonly seen. Although attributed to Fisher, the theory is believed to have originated elsewhere.

Fisher posited that the sex ratio is genetically determined, there being many possible ratios. When one sex is rare, it is advantageous to produce more of that sex. Therefore, alleles that code for that ratio are favored and spread. The result is a stable equilibrium at 1:1. There are mechanistic constraints that might make a ratio 1:1 through meiosis difficult to alter.

The bacterium wolbachia causes skewed sex ratios as it kills males through kin selection.