XIV.5 The ratio of male and female progeny born is close to one with surprising frequency
Although a single male can frequently ensure the production of sufficient microgametes to cover the needs of the entire population, the ratio of the number of progeny of the male and female sex, i.e. the secondary sex ratio of the population, is close to one with surprising frequency.The mortality of males and females frequently differs during maturing and finally also in adulthood.Consequently, in adulthood, the tertiary sex ratio can be very different, usually biased in favor of females.In contrast, the primary sex ratio, i.e. the ratio of male and female zygotes immediately following fertilization of the eggs, is, to the contrary, usually biased in favor of the sex whose embryos die more frequently prior to birth.For example, in human beings, there are 160 male zygotes for every 100 female zygotes, while only 106 boys are born for every 100 girl babies (Dorak et al. 2002).It is obvious at first glance that a secondary sex ratio equal to one is not optimal from the standpoint of the population for a great many species.If more females were to be born at the expense of males and every male were to fertilize a greater number of females, the population as a whole could grow faster than when the numbers of males and females are approximately equal.
This paradox has long drawn the attention of a number of biologists.Consequently, a number of hypotheses have emerged in to explain its existence.Hypotheses considering the same numbers of males and females to be a consequence of a genetic mechanism of determining the sex of the embryos are currently falling into disfavor.A genetic mechanism of determining sex should primarily affect the ratio of the two types of heterogametes and thus the ratio of male and female zygotes immediately after fertilization.This ratio, the primary sex ratio, however, frequently deviates substantially from a value of 1 and, as already mentioned above, approaches a value of 1.6 in favor of male zygotes in humans (Dorak et al. 2002).Similarly, comparative and experimental studies have demonstrated beyond the shadow of a doubt that a genetic mechanism of determining the sex of a zygote is extremely plastic at both the interspecies and intraspecies level.It is known that completely different mechanisms that, together, could theoretically ensure a quite arbitrary ratio of males and females in the progeny, exist in the individual taxa.Simultaneously, it is apparent that a population exposed to a selection pressure for a change in the sex ratio usually reacts quite easily to the given pressure and changes the sex ratio in the appropriate manner (Orzack & Gladstone 1994).It is thus apparent that a secondary sex ratio of 1 is not a consequence of the mechanism employed to determine the sex of the embryo but rather a result of quite specific selection pressures and that it is adaptive.