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Evolutionarily speaking, the nine month gestation means you need quite a few females to propagate the species, but only a few men are necessary. So why are there so many men? In other words, why didn't we evolve so that females were more likely to be born than males (leading to the maximum offspring potential for the species)?
The answer is not obvious, but there is actually a good reason. If it happened that we were evolutionarily inclined to produce more females than males, then either people are monogamous - in which case many females are left with no partners and their genes do not survive - or else people are promiscuous - in which case the situation appears to succeed.
However, consider what happens when a woman comes along with a genetic mutation that makes her more likely to give birth to males than females. In a promiscuous setup, each male has more children than the females, so this unusual woman's sons would each pass on multiple copies of their genes to their own children. Each of the grandkids of the original mutation would carry this genetic predisposition to have relatively more males. On the other hand, if this unusual woman had a normal sister (who tends to give birth to females who have less children), the sister's children (mostly women) would have fewer grandchildren to pass along the 'normal' genes that incline sex towards females.
Thus over time stability is going to be maintained at a roughly one to one ratio regardless of the promiscuity of the species. Males may seem to play a smaller biological role in reproduction, but an unbalanced proportion of sexes will eventually correct itself. This was shown in the 1930's by mathematician R.A. Fisher. Note that the same applies in the other direction, with more males than females. The only evolutionarily stable situation is for the sexes to be about equally proportioned.
So if one looks at the situation in China, e.g., where significantly more boys are born than girls, in time the situation should theoretically correct itself. However, the disproportion there is actually caused largely by abortion of female fetuses before they can be born, thanks to technology now allowing families to identify gender before birth. In Chinese society, male offspring tend to be more highly valued because they can carry on the family line in the patrilineal society.
Because of this technological and social intervention, the self-correcting feedback loop of nature cannot fully kick in and fix the imbalance over time; this leads to a need for social fixes. Thus, at the beginning of 2005, the Chinese government outlined a plan to make sex-selective abortion illegal.
This situation demonstrates the fact that evolutionary changes are made by processes other than just the familiar natural selection. In the case of Chinese abortion, natural selection has effectively been overridden by artificial selection. Sexual selection is yet another factor in human evolution: men are getting taller over time because women tend to choose to mate with men who exhibit tallness, thus accumulating that characteristic in the population and selecting for ever taller genes.
Indeed, while natural selection will always apply where a population is competing for survival (usually because of limited resources), other forms of selection are at work shaping the evolution of humans today, and many of them - like abortion trends - are social rather than purely biological.
In some sense, by attempting to counteract one trend or cause another (like the Chinese government's move to ban such abortions), we have taken control of our own evolution. No doubt future technological developments in genetics and biotechnology will only increase our genetic self-determination, but what direction that change will take, no one can predict with certainty.