Sex allocation bias

A second sex allocation problem is understanding bias in allocation towards one ofthe sexes overall, such that in dieocious populations there is a biased sex ratio, or in cosexuals greater investment in one sex function. Our understanding of this problem originates largely with Fisher (1930) (though see Edwards (1998); Seger and Stubblefield (2002)). Fisher pondered the reasons why organisms tend to display 50 : 50 sex ratios. His argument is one of frequency-dependent mating success. Consider a gene that biases offspring production towards one sex. In the next generation that sex will find itself more common, and will on average have lower mating success than the opposite sex, hence, production of grand-offspring is reduced. Selection will favour genes that bias the sex ratio in the opposite direction, until the sex ratio reaches 50 : 50 again. An equal sex ratio is therefore an Evolutionarily Stable Strategy (ESS) because selection will immediately tend to counter any bias. This verbal argument of evolutionary stability is slightly different to the fitness optimization problems mentioned in the last chapter in that the fitness of any strategy depends on the strategies adopted by the rest of the population, so the optimum shifts as evolution proceeds. This presents novel challenges when attempting to predict evolutionary outcomes, and these have led to a range of theoretical innovations.

Observing Fisher's equalizing selective process in action has not been widely possible because of a general absence of genetic variation in the sex ratio in organisms displaying a 50 : 50 sex ratio. However, a small marine fish, the Atlantic Silverside (Figure 5.2), showed its practical validity for the first time (Conover and Vanvorhees 1990). In this fish, the temperature at which offspring develop determines sex. Since the fish has a large geographic distribution, the temperature that results in an equal sex ratio is

Fig. 5.2 The Atlantic silverside, Menidia menidia, which displays environmental sex determination under frequency dependent selection, thus maintaining a equal sex ratio. Photo by R. George Rowland and provided courtesy of David Conover.

lower in higher latitudes where the water is colder. In an elegant experiment, populations of fish from different regions were reared at abnormal water temperatures, and the population sex ratio was naturally biased, but over time the sex ratio equalized because of selection on the temperature threshold at which sex switches. This is exactly the process Fisher envisaged, showing it works in nature as well as on paper.

In the general absence of studies like this, the validity of Fisher's argument has been largely tested indirectly from the association between an observed sex ratio bias and the breaking of one of Fisher's implicit theoretical assumptions. Charnov (1993) has extended his ideas of'invariance' relationships (Chapter 4) by reference to Fisher's assumptions: the 50:50 sex ratio can be understood as a life history invariant resulting from two underlying 'symmetries'. The first is that of inheritance: an offspring receives half of its genes from its father and half from its mother. This tends to make both male and female offspring equally valuable because half of any offspring's genes will come from males and the other half from females. If one sex ever became a more profitable way of transmitting genes than the other, this symmetry would be broken and there would be the potential for sex ratio bias. The second symmetry is that of proportional gains through sons and daughters: when mothers switch investment from one sex to another, their reproductive gains and losses are constant for all mothers and in all circumstances. The alternative is that certain mothers may gain more fitness than others from investing in a particular sex.

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