Why has evolution produced a 50:50 sex ratio?

[Gentlygently]

I presume there is some evolutionary reason to why humans (all mammals?) have a 50:50 sex ratio. But what is it? As a species we could probably survive with much fewer men, in a co-operative society. But maybe a race with more men (hence physically stronger) would fight the female dominated race and win?

Male humans and female humans are different sexes, part of the same species.

I always thought that there were more women. No idea where I got that - O level biology maybe - and I am happy to be told that I am wrong.

It's generally true for most sexually-reproducing species due to something called Fisher's principle.

At conception there is 50% channel of male or female chromosomes making up the resulting zygote / embryo.

That's what sexual reproduction is.

Normally 105:100 ratio female to male without femicide I believe.

Bike I think slightly more males are born, and then slightly more die early, and then women live longer, so at any given point it is slightly in favour of women. But I meant something more drastic than 51:49.

Autumn where in my post did I imply otherwise?

50% chance

Evolution, innit?

What benefits the species doesn't 'win' the evolutionary race. It's what benefits the individual. Selfish gene.

Surely it just shows there's somewhere near a 50/50 chance of a fetus being male or female and so the resulting numbers bear this out? As you need one of each sex there's no way for evolution to work on producing more males or females - ie, if you came from a family of all female children you couldn't breed with someone from a family of all female children as they'd need to be male so no way for natural selection to get going. (I know some regions have bigger or smaller percentages but it all comes down to somewhere around 50/50ish)

Bees aren't 50:50 though, that must be evolutionary. IIRC only about 30%, perhaps even less, are male.

I think it's an interesting question. It's so efficient. It's nearly too efficient.

Because at conception it’s down to chance: XX or XY.
It may be that more boys are born due to higher female foetal mortality.

Bees aren't 50:50 though

But bees in a colony are all related to each other. So it's a completely different evolutionary pressure.

The answer to this has been known to evolutionary biology for over 100 years. It's called Fisher's principle.

Sex ratios generally stay at 50-50 even in species where a small number of males impregnate all the females (leaving the other males not to reproduce at all). This is because if your species produced - say - 75% females, but you were an individual who had a gene that made you produce more males you'd have an evolutionary advantage (because males would on average have better odds on reproducing if there are fewer of them). So your male-producing genes would survive and go on doing so until the ratio reached roughly 50-50.

Why it isn't exactly 50-50 I'm not so sure.

Sex in bees is down to haplodiploidy, they effectively choose what sex to make an egg, though not a conscious choice obvs!

Bees (and other insects) have a completely different sex determination system to mammals. en.m.wikipedia.org/wiki/Haplodiploidy

(Some reptiles are also different, sex depending on egg incubation temperature.)

Evolution is about characteristics becoming more prevalent because they make the species more likely to reach sexual maturity and therefore reproduce. I’m not sure if the proportion of males/females is related to evolution or not, but if it is, then clearly the ideal scenario is 50/50 as that’s how it’s turned out! Particularly as we tend to have monogamous relationships it makes sense to have half and half, unlike lions which have one male and lots of females - anyone know if their birth rate is also 50/50?

It is an interesting question why it is 50% males. It could be 60%. Or 30%.

It probably isn't 60% because men tend to kill women and children if allowed to have too much impact on society. It doesn't work with too many males.

Why not fewer males? Well, with slightly more than 50% males, I still have problems finding a good man to have children with.

More scientifically:
Surplus males are where selective pressure happens, especially under favourable conditions.
Some percentage of males are going to be subpar, they won't be selected by females.

If mammals had to rely just on harsh winters and predators to weed out disadvantageous genes, populations that had favourable conditions for a couple of years would be more vulnerable to being wiped out once the weather changes and the wolves can cross the frozen river, etc.

Particularly as we tend to have monogamous relationships it makes sense to have half and half

I wrote a long, boring post about this with lots of long words but what it boiled down to is that we haven't been monogamous long enough to produce an evolutionary effect. Chances are we aren't 'naturally' monogamous, certainly not for longer than about 2 years.

Its 52:48 male : female

And im just hazarding a guess here. But males are more likely to die before and during prime reproducing age through accidents. Probably why the extra 2/100 are needed

Why it isn't exactly 50-50 I'm not so sure

Might be because humanity has, up until rather recently, regularly waged wars in which young men who haven't fathered any children yet are killed.

The more sons a man has, the more likely his Y-chromosome (and thus probably the tendency to father more sons) is to be inherited by future generations.

Once there's too many males, not every male can father offspring, and things even out again.

People are a bit confused about how evolution works. The mechanism is important. How would some men dying younger cause a gene in other males to be passed on? I can't see how.

As a species we could probably survive with much fewer men, in a co-operative society.

A ‘co-operative society’ in which people get to live based on whether we need them or not isn’t one I’d like to live in.

Nor one in which I’d get much say in the matter.