Why nearly every species makes sons and daughters in equal numbers: Fisher's grandchildren argument

Boys and girls are born in nearly equal numbers, about 105 boys for every 100 girls worldwide, even though one male could father far more children than one female can bear.

The reason is bookkeeping. Every child has one mother and one father, so sons as a group and daughters as a group must share the same total of children. When males are rare, a son is worth more than a daughter, and when they are common he is worth less.

Evolve a population of 300 mothers, push it off balance with a disaster, and watch the ratio return to an even split, then see where Hamilton's wasps break the rule.