Proportional gains asymmetry

The second Fisherian symmetry is that of proportional gains from sons and daughters. There are two famous ways in which asymmetric gains can result in biased sex allocation. The first was realized by Hamilton (1967). Many organisms display a 'subdivided' mating structure, meaning that females obtain their mates from a relatively local pool of males. In such groups there is competition among males for mates, and mothers contributing offspring to the mating pool will achieve greater production of grand-offspring if they bias their production of offspring towards females. The reasons are two-fold: first, they do not waste effort on sons with low mating success; second, they instead produce females that will be mated (Figures 5.4,5.5). In populations with less spatially structured mating (i.e. more females contribute offspring to each mating pool), sex allocation should approach equality. This is observed in numerous wasp species that lay their eggs on resource patches where the offspring also mate (hosts insects in the case of parasitoids, figs in the case of fig wasps) (see Werren 1980; Herre 1985). In plants that habitually self-pollinate, there should be similar selection pressure for a reduction in male allocation. There is comparative evidence from numerous plants that allocation to male function decreases with the selfing rate (Charnov 1982).

Fig. 5.4 The evolution of female biased sex ratios under local mating. Two situations are shown, one with a 1 : 1 male to female ratio (left), and one with a 1:3 ratio of males to females. Large circles represent offspring groups which mate among themselves before dispersal to other patches. Closed small circles are females and open small circles are males. Small arrows represent mating. Under a 1 : 1 sex ratio, two mated females disperse to form two patches of offspring in the next generation. Under a 1 : 3 sex ratio, however, three females disperse, leaving more descendents. Thus, selection favours a female bias.

Fig. 5.4 The evolution of female biased sex ratios under local mating. Two situations are shown, one with a 1 : 1 male to female ratio (left), and one with a 1:3 ratio of males to females. Large circles represent offspring groups which mate among themselves before dispersal to other patches. Closed small circles are females and open small circles are males. Small arrows represent mating. Under a 1 : 1 sex ratio, two mated females disperse to form two patches of offspring in the next generation. Under a 1 : 3 sex ratio, however, three females disperse, leaving more descendents. Thus, selection favours a female bias.

The second significant way in which asymmetric gains can emerge is when one sex benefits more from investment than the other sex in some environments (Trivers and Willard 1973; Charnov et al. 1981) (Figure 5.6). Many polygynous cosexual fish, for example, are female early in life, and switch to become males later in life when they are also larger. Presumably, taking over and defending a harem of females is only possible for males if they are large, but it is then highly profitable. Females, however, can produce offspring even when small. Many cosexual plants also bias their allocation towards pollen and away from ovules under conditions of stress, presumably because pollen is small and much less resource-dependent

Fig. 5.5 Subdivided mating structures in nature. Pollinating fig wasps, Pleistodontes froggatti, inside a Moreton Bay fig, Ficusmacrophylla, that has been cut open. The wasp in the right foreground is laying an egg into a fig flower. The female wasps have no wings as these are lost while pushing their way into the fig through the narrow entrance. The offspring of these females will mate with each other inside the fig; hence there is local mate competition and selection for a female biased sex ratio. The full width of the photo is approximately 2 cm. Photo courtesy of James Cook.

Fig. 5.5 Subdivided mating structures in nature. Pollinating fig wasps, Pleistodontes froggatti, inside a Moreton Bay fig, Ficusmacrophylla, that has been cut open. The wasp in the right foreground is laying an egg into a fig flower. The female wasps have no wings as these are lost while pushing their way into the fig through the narrow entrance. The offspring of these females will mate with each other inside the fig; hence there is local mate competition and selection for a female biased sex ratio. The full width of the photo is approximately 2 cm. Photo courtesy of James Cook.

than fruit. Of course, underlying all these sex allocation biases is a sex determining mechanism. Next we will investigate how these mechanisms might evolve.

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