Why is the ratio of men to women roughly equal? As a man can impregnate a woman more quickly than a woman can make a baby, the human race could easily survive on a ratio of, say, 50 women to one man, so is there another reason for the equality?
Julian Harrow , Southampton, Hampshire, UK Answers
Ronald Fisher, who pioneered the mathematical theory of natural selection, wrote in Natural Selection, Heredity and Eugenics: Including selected correspondence of R. A. Fisher with Leonard Darwin and others (edited by J. H. Bennett, OUP, 1983): "There are a number of instances of tendencies which have developed apparently clean contrary to the general interest of the species...I think a good example of this is in the sex ratio of polygamous animals living in flocks and herds, where the economy of the herd as a whole would seem to suggest (and the stock breeder would prefer) a sex ratio of about 5 per cent males, but where nature, through the action of selection insists on producing nearly equal numbers of the two sexes".
Elsewhere he argued that in a population of 5 per cent males any male has 20 times the influence of any female on the genetic make-up of the next generation. This gives an enormous selective advantage to a gene that predisposes an animal to have male offspring. The spread of this gene will increase the proportion of males, but its advantage will decline as the proportion increases towards 50 per cent. This broad argument is still accepted by biologists.The main development in thinking since has been to observe that selection should favour equal parental investment in offspring of the two sexes, not necessarily numerical equality. This means that if the cost of raising a son were twice that of a daughter we would expect a sex ratio of two (females) to one (male).
But other complications arise. For example, R. L. Trivers and D. E. Willard pointed out in 1973 that in many species the variation of male reproductive success is larger than that of females most females are likely to reproduce but only a few males do so, namely the larger dominant ones (Science, vol 179, p 90). This means that a mother, in a healthy condition, is likely to have more grandchildren if she puts her abundant resources into raising large strong males. If she is less healthy then producing more daughters is a better bet. Even so, the overall investment in the sexes at population level is still expected to be about equal.
Murray Lark , Bedford, UK Fisher gave the answer to this question early in the 20th century. If there is a minority of one sex (say males), then there is a selective advantage in being male.
Later analysis has shown that this equality actually applies to the effort put into creating and raising each sex. Therefore, if it takes more resources to raise a female than a male, then the numerical sex ratio will be biased to males. The age at which the sex ratio applies is when the offspring become independent of the parent.
An interesting paradox is seen in red deer. Here Tim Clutton-Brock, a professor of animal ecology, has shown that male calves are more of a drain on the mother. However, the sex ratio is biased towards males. This is explained by the fact that the males leave the maternal herd when weaned. However, the female calves remain and are therefore in competition with the mother, reducing her resources.Jon Richfield , Somerset West, South Africa
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