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?

I thought slightly more boys died in infancy and youth at least in part because of X linked recessive disorders such as haemophilia. So a slight excess at birth has evolved for the reasons explained in the Fisher links upthread, to yield around 50:50 after puberty.

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

They aren't needed, as such. They just aren't there at reproductive age to provide a disadvantage to the genes of parents who have too many sons.

What is interesting is how China managed to keep its cultural tendency to prefer sons for such a long time.
Culture isn't genetic, but it is just as subject to evolution as genes are.
You would have expected people to observe that it was harder and harder to marry off young men (and thus get grandchildren) and stop aborting female fetuses before things got as extreme as they are now.

Men might be more likely to die in war/accident. But look at the rate of women dying in childbirth through history.

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.

How would that explain the same 50:50 balance in kangaroos? Or humpbacked whales? Or polar bears?

I think any explanation that involves some kind of exceptional human behaviour resulting in exactly the same outcome as other mammals should be treated with suspicion.

The mother already has 50% likelihood of having passed on any genetic predisposition contributing towards dying during childbirth though, eg a narrow pelvis, as will her daughter, so that won’t have an impact on evolution except that she might have only 1 child instead of 10! But the characteristic would never die out, hence female humans still struggle with childbirth. But modern medicine has ruled that out of evolution now, along with many other genetic things.

If 90% of us were female, the males would disproportionately get to reproduce their DNA. Therefore, natural selection would favour any mutation that led to having more sons, and there would be a trend towards increased number of sons until there was no advantage to having sons. If the ratio were reversed, natural selection would favour daughters.
Unless there are social reasons which favour an uneven sex ratio (viz, bees) most species tend to finish up with a roughly 50:50 ratio.

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.

You can't see that?
But it is quite easy.

Let's say we have a village in Africa where 20 boys and 30 girls live to adulthood. Let's assume, to keep things simple, that they only marry within their own generation.

All 20 boys will find a wife and have children, some will be able to father children with more than one woman. If only 2 of those 20 boys have the same father, he will make double the contribution to the gene pool compared to all other men in the village. Parents of boys will have more grandchildren because one man can father children with two women, which is double the amount of children a girl could have.

If you have 30 boys and 20 girls, then either 10 boys will not have children at all, or, with polyandry, some males will not get to father as many children as others. In any case, the number of possible babies born is limited by the number of women.

So, if girls are rare, the parents who have daughters have an advantage in passing their genes to the next generation.

If you have 33 boys and 30 girls, but 3 boys die before reaching adulthood, then the other 30 can father a maximum number of children, the previous existence of three more boys doesn't change anything at all.

You would have expected people to observe that it was harder and harder to marry off young men (and thus get grandchildren) and stop aborting female fetuses before things got as extreme as they are now.

That assumes that human societies are able to view and judge their own cultures in a detached, objective way and then effect mass change to that culture based on those judgements. Even in our modern more enlightened times, that doesn't really seem to happen much.

... I just re-read my post and noticed I said Africa. I think I wanted to include sickle-cell anemia as example for selective pressure.

But that'd get too long.

Fascinating question. I love this sort of stuff but must say I am not qualified or anything.

Some percentage of males are going to be subpar, they won't be selected by females.

This is far too simplistic, suggesting as it does, that the only reason a females genes would not be selected is due to her pickiness. Or that the above par males all chose other females.

Chances are we aren't 'naturally' monogamous, certainly not for longer than about 2 years.

I don't agree with this and think that most (all?) Of our attitudes towards sex are evolutionary. I don't see how a male who has dozens of children to different partners would be a good bet (safe sex being a recent invention). We still (especially men it seems to me) have strong mate guarding instincts resulting in jealousy and "heartbreak" undignified pleading etc. Why does love even exist if we are not supposed to be monogamous ?

Interesting thread.

That assumes that human societies are able to view and judge their own cultures in a detached, objective way and then effect mass change to that culture based on those judgements. Even in our modern more enlightened times, that doesn't really seem to happen much.

It only assumes that they are able to see their own advantage, really, but that's probably expecting too much, still.

Apparently, by now, a young man in China must have his own car and own flat to even dream of getting married, whereas women can have their pick.
That could have been predicted, obviously.

But @HestiaParthenos the example the PP had was of men in their middle age dying, not men who hadn't reproduced yet.

gently it is a really good question: not how we have a 50:50 ratio (e.g half the sperm are X half are Y etc....) but why ? why did evolution lead to this ratio?

Farmers don't keep 50: 50 bulls and cows in their breeding stock because you don't need all those bulls to sire the next generation, so why waste all that food on them. So why does evolution end up with 50:50? (or not quite 51:49 or whatever...)

As Mrs Terry Prachet the answer is what benefits the species doesn't 'win' the evolutionary race. But its also not "what's what benefits the individual" either ....its what benefits the individual gene (i.e. leads to the gene being replicated more often in the next generations) . This is what "the selfish gene" means. It is a totally mind blowing idea in biology.

So you have to think of it from a genes-eye view. Being a gene that is carried in a female organism represents quite a different risk/return prospect than being a gene that is carried in a male . Female humans can have something like between 0-10 live offspring in a lifetime with probably the most likely number being a couple surviving (pre antibiotics etc.). Males can have between 0 and thousands of offspring, with the most likely number probably being 0 (many young men die in battle before having a child etc...but some become rich men with many wives, concubines, mistresses etc...). So for the genes being carried in a female organism is like betting on black at roulette, you may well win, but not big, whereas being carried in a male is like putting everything on a single number. The gene will probably loose (end up not being passed on), but occasionally it will win big by being passed into a lot of grandchildren.

So you can think about different gene options competing with each other over rounds and rounds of reproduction. A gene mutation that leads to a man producing more viable Y sperm is a higher risk prospect (for the gene) and can easily get wiped out (there are no grandchildren to carry the gene in one generation), whereas a gene mutation that leads to a man producing mainly viable X sperm (mainly daughters) is at less risk of being wiped out but will tend to get overtaken in the evolutionary race by the gene which results in more equal X/Y sperm -- so the evolutionary equilibrium tends toward.

....I'm not sure if i've explained that well!

China together with other paternalistic societies favour sons as inheritance of land and money are usually through the male line and this continues despite the cultural revolution. The lack of social welfare means parents would need a son to look after them in their old age. Daughters would be married off and help to look after her in laws.

clearly the ideal scenario is 50/50 as that’s how it’s turned out

A 50/50 sex ratio is highly inefficient. From the point of view of maximizing the reproduction of the species, it would be must more effective to have (say) 65% female, 35% male or even more extreme. But 50/50 persists because an unbalanced sex ratio (either way) is unstable. Even though 65-35 is better overall, it can't be sustained because 'cheats' (who have more male offspring) would prosper at the expense of everyone else.

We still (especially men it seems to me) have strong mate guarding instincts

It all boils down to egoism. For a man, it is evolutionary advantageous to have as many women as possible who bear his children year after year. If they get pregnant by someone else, he misses a chance to spread his genes.

For a woman, on the other hand, it is advantageous to not put all her eggs in one basket, so to speak, and have a different father for every child, so a disease is unlikely to kill all her children. But it also promotes the children's survival if there's another person who feeds and protects them, not just the mother.

Monogamy is a compromise.

In humans, it is hard to say whether monogamy evolved genetically or culturally, but seeing as there are many patriarchal cultures that don't expect men to be monogamous, it may well be cultural.

...scienceroar's explanation is much clearer and simpler than mine!

But @HestiaParthenos the example the PP had was of men in their middle age dying, not men who hadn't reproduced yet.

Which post do you mean? I couldn't find a reference to middle aged men.
But the principle that other men's genes profit if one man is taken out of the race early still applies right up until that man would have been out of the dating pool of women of reproductive age, anyway.

Surely natural selection would have little effect on division of the sexes as how would you select a mate based on their past children? How would you know? Other characteristics like strength etc can be demonstrated, shown via physical characteristics or communicated through pheromones etc but how would you select a mate who's more likely to sire males/females?

It only assumes that they are able to see their own advantage, really, but that's probably expecting too much, still.

To whose advantage, though? Most men do marry, so there are many people benefiting from the current system through having both the prestige of a son and grandchildren. And these people are going to be over-represented amongst the kind of social and economic elites that influence culture. What's in it for them to try to change things?

It has been mentioned twice and linked to- why are you all arguing about something that by all accounts appears to be a fairly well settled principle?

A 50/50 sex ratio is highly inefficient. From the point of view of maximizing the reproduction of the species, it would be must more effective to have (say) 65% female, 35% male or even more extreme. But 50/50 persists because an unbalanced sex ratio (either way) is unstable. Even though 65-35 is better overall, it can't be sustained because 'cheats' (who have more male offspring) would prosper at the expense of everyone else.

If 50:50 wasn’t efficient we wouldn’t have it. Nor would other mammals.

I’m not sure what you mean by ‘cheats’ either.

To whose advantage, though? Most men do marry, so there are many people benefiting from the current system through having both the prestige of a son and grandchildren. And these people are going to be over-represented amongst the kind of social and economic elites that influence culture. What's in it for them to try to change things?

This looks like a cultural explanation.

I don’t see the need for a cultural explanation to explain the 50:50 balance in humans when most other vertibrates seem to reach the same balance without ‘culture’ as such.

Why look for a cultural explanation for the balance in humans when snakes reach the same balance?

If you come up with a cultural explanation you are implying that without it humans wouldn’t have a 50:50 balance, making is virtually unique.

Humans really aren’t that exceptional.

There are cultural factors that can make it unbalanced - as in China - but we don’t need a cultural explanation for when it is balanced.

ScienceRoar thanks, I think your answer makes the most sense.

Seriously, do people really believe that we have a 50:50 balance because that’s what the cultural elites want?