Why does almost every species settle at a roughly 50:50 ratio of males to females? Set a lopsided starting population, press play, and watch natural selection drag it back to parity — generation by generation. Blue = male, pink = female.
Every child has exactly one mother and one father. So across a whole generation, the total number of children credited to all males equals the total credited to all females — the two sexes always split parenthood evenly, as a group.
That means whichever sex is rarer divides the same reproductive pie among fewer individuals — so each rare-sex individual has more children on average. A gene that nudges parents to produce the rarer sex therefore leaves more grandchildren, and spreads.
As that gene spreads, the rare sex becomes less rare — until neither sex has an advantage. That balance point is exactly 50:50. It is self-correcting from either direction.
Model: a haploid sex-ratio allele under Fisherian selection. Small populations can drift or even collapse — that's real biology, not a bug.
The top row is your starting population. Each row below is the generation its parents produced. Every row is the same total size — blue males packed left, pink females right — so the moving boundary is the sex ratio. Watch it march toward the centre line: parity.