Ok, first the boring part - how do humans end up with a 50-50 sex ratio?
Each of us has two sex chromosomes. A woman is XX, and a man is XY. An egg or sperm cell has half of our genes in it (so that when the two halves meet in fertilisation, a full set is created). Thus, an egg cell, made by the woman, must have one of her Xs in it. A sperm cell, made by a man, can have either his X or his Y chromosome. As it happens, one Y chromosome is enough to direct the body to make a male. The options, therefore, are (female contribution first, of course):
- The one of the female's Xs + the male's X = female
- The one of the female's Xs + the males' Y = male
- The other of the female's Xs + the male's X = female
- The other of the female's Xs + the male's Y = male
These are the only four options, so it is easy to see that you have equal odds of producing males or females.But, the above answer misses the point slightly. Yes, the XX, XY situation guarantees a 50-50 split, but why did THAT system evolve? After all, it would theoretically be just as easy to evolve a different system that, say, made the split 72-28, for example. What is it that is so evolutionarily favourable about the 50-50 sex ratio?
The puzzle was solved by the British biologist R.A. Ficher, and I'll try to paraphrase it here. Imagine that there was a genetically inherited ratio of three times as many females as males (75-25). Fisher realised that this would be unsustainable. Why? Well, consider what would happen when they mated.
A female HAS to mate with a male in order to produce offspring. However, in the above example there are much more females than there are males. Thus, any particular male is likely (on average) to leave MORE descendants than a female is.
Imagine a hypothetical case of 100 birds where there are 75 matings (to make the maths easy). Therefore (since the ratio is 75-25), each female must have mated on average once. But, since there are only 25 males, and 75 matings, each male must have mated an average of... THREE TIMES.
Therefore, each individual member of the minority sex (males in this case) will tend to leave behind a disproportionate number of descendants (genes), compared to the majority sex.
So consider the 'options' to a parent. Clearly, if you want to spread your genes, it would be better to make male offspring, since you would leave an average of THREE TIMES more genes behind. And so it becomes evolutionarily favourable to produce male offspring. Since this is a successful strategy, it will tend to spread, meaning that more and more parents will produces males... which equalises the sex ratio!
The above is true no matter what the initial sex-ratio tendency. The only evolutionarily stable situation is 50-50, since any minority sex automatically becomes a more profitable way to propagate your genes. And this in turn favours a bias towards making more of the minority sex, which eventually equalises out the ratios.
Well, almost. This answers your question about humans, but for other animals that differ in their sex ratios, you need to use Fisher's broader idea that it's actually the INVESTMENT in males and females, rather than their actual numbers, that will tend to be 50-50. For example, if it is twice as 'expensive' to produce a male than a female, the stable sex ratio will be 2:1 in favour of the females. But this post is long enough already - if you want more detail, drop me a line.