Why sex ratios are even

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We're so used to our 1-1 sex ratio, that it's actually easy to forget that it's sort of counterintuitive. After all, any given male can impregnate many different females, so from a sort of top-down efficiency perspective, it would be a lot more efficient to have a small number of males with big harems. But, of course, evolution isn't a matter of top down planning. Here's Dawkins's explanation from The Selfish Gene:
In mammals, sex is determined genetically as follows. All eggs are capable of developing into either a male or a female. It is the sperms that carry the sex-determining chromosomes. Half the sperms produced by a man are female-producing, or X-sperms, and half are male-producing, or Y-sperms. The two sorts of sperms look alike. They differ with respect to one chromosome only. A gene for making a father have nothing but daughters could achieve its object by making him manufacture nothing but X-sperms. A gene for making a mother have nothing but daughters could work by makin her secrete a selective spermicide, or by making her abort male embryos. What we seek is something equivalent to an evolutionary stable strategy (ESS) although here, even more than in the chapter on aggression, strategy is just a figure of speech. An individual cannot literally choose the sex of his children. But genes for tending to have children of one sex or the other are possible. If we suppose that such genes, favouring unequal sex ratios exists, are any of them likely to become more numerous in the gene pool than their rival alleles, which favor an equal sex ratio?

Suppose that in the elephant seals mentioned above, a mutant gene arose that tended to make parents have mostly daughters. Since there is no shortage of males in the population, the daughters would have no trouble finding mates, and the daughter-manufacturing gene could spread. The sex ratio in the population might then start to shift towards a surplus of females. From the point of view of the good of the species, this would be all right, because just a few males are quite capable of providing all the sperms needed for even a huge surplus of females, as we have seen. Superficially, therefore, we might expect the daughter-producing gene to go on spreading until the sex ratio was so unbalanced that the few remaining males, working flat out, could just manage. But now, think what an enormous genetic advantage is enjoyed by those few parents who have sons. Anyone who invests in a son has a very good chance of being the grandparent of hundreds of selas. Those who are producing nothing but daughters are assured of a safe few grandchildren but this is nothing compared to the glorious genetic possibilities that open up before anyone specializing in sons. Therefore, genes for producing sons will tend to become more numerous and hte pendulum will swing back.

For simplicity, I have talked in terms of a pendulum swing. In practice, the pendulum would never have been allowed to swing that far in the direction of female domination, because the pressure to have sons would have started to push it back as soon as the sex ratio became unequal. The strategy of producing equal numbers of sons and daughters is an evolutionarily stable strategy, in the sense that any gene departing from it makes a net loss.

The result, then, is that we get an even sex ratio. This applies even in massively polygamous situations where the result (as in seals) is that a few males have a big harem whereas the rest get nothing at all.

4 Comments

The 1 to 1 birth ratio also makes sense when you take into account the conditions when man was developing. Men had a much larger mortality rate then women due to their roles in society. In the early years being exposed to the elements and animal attacks while hunting caused men caused a higher death rate. Then as man developed he turned to warfare which kept the death rate up. So even with a 1 to 1 ratio you would have had more women then men for procreation.

Actually, I suspect that this is wrong. Until very recently the maternal mortality rate was incredibly high. Dobbie , for instance, estimates over 2.5% per birth in the 16th to 18th centuries.

The actual ratio at birth is 1.05 boys for 1 girl. Except in parts of China and India where female infanticide, selective abortions or simply less food and medical care for baby girls has led to a skewed ratio of 1.14 to 1.

Sorry, I hadn't seen the previous post.


Interestingly, it seems like the law of supply and demand is starting to kick in in India and China. Men are getting so desperate to find mates in India that they are resorting to formerly unthinkable means like marrying across castes. Also, sometimes men can only marry if they have a sister that can be reciprocally married to a male member of their new wife's family. This is leading to a slight change in attitudes where people are starting to think of having baby girls "so her brother can marry"...

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