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Comment by seph-reed | Hacker News Reader

Seems unlikely. Honestly, I'm surprised that the male to female ratio is anywhere near 1:1. Every male baby is one less womb, and one less branch on the evolutionary tree.

Does anybody know why it's so close to 1:1 instead of women outnumbering men? Specifically in non-pair-bonding species?

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I just googled it out of curiosity and I gather from Wikipedia, if a species has more females, the families with male children will reproduce more than the families with female children, and the give birth to male genes will then rise up to 50 percent of the population.

https://en.m.wikipedia.org/wiki/Fisher%27s_principle

Weird. It seems like the female children would almost always propagate their genes, while the male children would either rise to the top and have many children or lose the competition and have very few or none.

It's always weirded me out for harem species in particular... the odds of producing a male that is competitive enough to be a harem leader seems pretty low.

Even in harem species, on average, each male gets exactly the same number of offspring as a female. If say 75% of the specimen would be female, being a male would yield on average thrice as much children compared to a female, so it would be evolutionary beneficial to be male.

The only species where the ratio is different are those where sex is determined with different genetic mechanisms. For example, in hymenoptera (ants and wasps), males are born from unfertilized eggs, which results in a 1:3 ratio between females and males.

Ah. The on average bit is what I was missing. It keeps going against my intuition though... like: on average, buyers of lottery tickets get back ~90% of their investment.

I guess the difference between lottery tickets and genetics is that one is compulsory, with no benefit whatsoever for not playing.

If you are a member of a species with 3 females to every 1 male, that means the average male of your species has 3 times as many offspring as the average female.

Under those circumstances, if you have a mutation that causes you to produce all male offspring, you will have many more grandchildren than average.

This pressure will always favor producing whatever sex is in short supply, pushing sex ratios to 50%.

There is some sort of math behind why a 1:1 ratio is almost always selected for. It intuits like, if all of the other elephants are breeding 1:2, and you breed 2:1, you will be significantly more likely to spread your genes.

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