Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

A value of the secondary sex ratio equal to one can also be explained by the action of individual selection.The effect of this factor on the secondary sex ratio is expressed in the Shaw-Mohler principle (Shaw & Mohler 1953).Translated from the language of mathematics to normal language, this principle says that, at the instant when, because of the momentary ratio of males and females in the population, it is preferable to produce members of one sex rather than members of the other sex, those individuals, who produce more progeny of momentarily more valuable sex, will be at an advantage.

            Under normal circumstances, a population is in equilibrium in the numbers of males and females.The selection value of males (most readily expressed as the number of progeny that they leave behind) is the same as the selection value of females.Simultaneously, it is not important that all the females in the population have approximately the same number of progeny, while there are frequently enormous differences amongst males in the number of progeny.The variance value has no effect on the selection value of a member of a certain sex, only the value of the average number of progeny per member of that sex is important.If males predominate because of a random fluctuation in the population, then those individuals that, on the basis of their genetic predisposition, produce more progeny of the female sex are at an advantage.Thus, the population gradually returns to equilibrium.The temporary increase in the sex ratio amongst humans in the post-war years has been cited as an example of this phenomenon in the past.However, newer studies have shown that, for example, the men that returned to England from the battlefields of the I^st World War were more than 3 cm taller than those that died.As taller men exhibit a higher sex ratio in their progeny, the increased sex ratio in the post-war years can be fully explained by the higher death rate of shorter men in the military conflicts {13730}.

            It follows from game theory that the optimal strategy for an individual is to invest the same amount of energy into production of sons as into production of daughters.Under conditions where the production of sons is just as costly as the production of daughters, the ratio of young of both sexes in the population settles at a value of one.

            This explanation of maintenance of equal numbers of the two sexes was apparently first proposed by R.A. Fisher in 1933 (Fisher 1958).However, it must be admitted that later mathematical analyses of the relevant model demonstrated that establishment of equilibrium through such individual selection is too slow and that some other mechanisms are apparently also active in a great many species (James 1995).