Normally we understand the (near) 50-50 male/female population sex ratio with this simple model: if there were more males than females then individuals who are genetically disposed to have female children will have more grandchildren because their female children will find more mates. Thus females will increase in proportion, restoring the balance.

But here’s the interesting thing. That model doesn’t work for humans (and many other species) and in fact 50-50 is highly unstable, with potentially catastrophic consequences.

Suppose that a male has a mutation on his Y chromosome which causes him to produce Y-chormosome sperm that swim faster than his X-chromosome sperm. Then he will have only boys. And his boys will have the same gene and the same super Y-chromosome sperm.

Now suppose that his male children have an equal chance of mating as all other males in the population. Then our original mutant will have more male grandchildren than other males of his generation. Thus, the proportion of this super-Y gene increases in the population, and this trend continues generation after generation.

The balance is not being restored anymore. In fact eventually the super-Y’s dominate the male population. And that means that all offspring in all matings are boys. That means very little reproduction can happen because there are so few females. And the species goes exctinct.

I learned this from a paper by W. D. Hamilton called Extraordinary Sex Ratios.