The stats I found range from a 0.5 to 3% difference between the male and female population. What is the science behind this? How come there’s not significantly more of one or the other?
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XX and XY chromosomes are part of the story, but not the whole story – the fact is, you get this kind of sex ratio across the animal kingdom, even though there are all kinds of ways sex could be determined, not just via specific chromosomes.
Basically, it comes down to the fact that each child gets half their DNA from the mother, and half from the father.
So imagine you’re a gene who has some influence on whether the kids will be male or female. You’re competing with other genes that also influence that. And suppose that currently, the species produces kids who are 80% boys, and 20% girls. The genes in the 20% are almost certainly going to survive to the next generation, but the genes in the 80% have to be pretty lucky. The 20%, though, includes genes that make kids likely to be girls, so the next generation will have more girls than before. The sex ration is pushed towards 50/50 by natural selection.
The same logic applies if the sex ratio was 20% boys and 80% girls – the next generation will have taken a step closer to 50/50.
So natural selection pushes the sex ratio to close to 50/50 in every species where mum and dad contribute equally to the DNA of the kids.
In species like bees and ants, mum and dad *don’t* contribute equally to the kids’ DNA, and you can do the same kind of maths to calculate what the ratio should be between boy bees and girl bees, and when you work it out, you get the answer observed in nature.
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