Why are males and females 50%-50%?

[cthulu2016]

13. That's attractive but cannot be right

Evolution doesn't plan. We could say "All the species are 50-50 so it appears that only 50-50 species have survived so perhaps only 50-50 species were likely to make it through bottlenecks" but that would only be right if the sex ratio were almost impossible to change. (Which we know it isn't.)

The moment an advantage appeared it would manifest itself. So we would be looking at a world where cats have 90% female kittens or people have 70% males or whatever was most favorable outside the eventuality of a bottleneck.

And we would say, "Cats will be in trouble if their population is reduced to five cats because theres a decent chance they'll all be female."

But if it works in the current environment that's what will happen in the current environment. Always.