Genetically determined differences in sex ratio that are often observed within and among populations and species suggest that this trait is subject to evolution by natural selection. As described above, the optimum sex ratio for a given individual in a given population depends on both the existing sex ratio ofthe population and on the relative costs of producing offspring of each gender. Empirical studies have found that both the optimum and the observed sex ratios of wild species reflect other parameters as well, such as environmental quality and life history.
Within some species, individuals adjust their sex ratios in response to the quality of the environment in which they reproduce, producing more offspring of the gender likely to contribute most to their individual fitness. In this case, environmental conditions (local or potentially short-term) affect the optimum sex ratio and, consequently, the pattern of natural selection on it. Under these conditions, natural selection can operate on the sensitivity and responses of animals or plants to environmental cues until individuals produce either male- or female-biased offspring sex ratios (if either yields higher fitness than a 1:1 ratio). In this case, sex ratio adjustment appears to be the adaptive outcome of natural selection operating to optimize individual responses to changes in local environmental conditions. One example can be seen among the Seychelles endemic warblers (Acrocephalus sechellensis). These birds are territorial, but individual territories may differ greatly with respect to the productivity of the insects on which the warblers depend for food. Male offspring disperse when they fledge, while daughters remain with their parents to assist in the raising of subsequent cohorts. Territories with high food availability can support both parents and adult daughters and in such conditions mothers produce 90% daughters. In territories where insects are scarce, however, the disadvantages of producing helpers (daughters) outweigh the benefits, and mothers produce ^80% sons. Molecular markers show that the biased sex ratios are the result of gender-biased egg production and not a result of differential mortality.
While environmental variation 'within' populations may explain the maintenance of variation in sex ratio among individuals, sustained environmental differences 'among' populations can cause evolutionary divergence at the population level. This is likely to occur where differences in the optimal sex ratio are sustained over many generations due to ecological differences among populations. An example of this appears to occur among populations of the gynodioecious plant species Silene vul-garis in the Allegheny Mountains of Virginia, USA, where populations differ greatly in the proportions of female versus hermaphroditic individuals (populations range from 0 to 75% female). In this species, gender is determined by the interaction between cytoplasmic genes that cause male sterility, and nuclear 'restorer' genes that reestablish it. Females, which produce only female offspring, also produce more seeds than hermaphrodites when fully pollinated, but they have the disadvantage of needing to receive pollen from hermaphrodites in order to be fertilized. Indeed, as the proportion or isolation of females increases, their seed production declines relative to that of hermaphrodites. This suggests that pollinator abundances may determine the equilibrium frequencies of females, which should be higher where pollinator service is more reliable.
The previous two examples illustrate the role of environmental variation in maintaining variation in sex ratio within and among populations. The life history of a species can also affect the evolutionary outcome of natural selection on sex ratio. When one compares species with similar life cycles, subtle differences in the degree to which offspring disperse prior to mating can have a profound effect on the evolution of their sex ratios. In particular, local mate competition influences the optimal ratio of sons to daughters. Where competition among males is minimized, natural selection favors offspring sex ratios that are highly female-biased. Fig wasps provide perhaps the best-known example of sex ratio evolution in response to variation in local mate competition. Fig wasps, which include many genera (e.g., Pegoscapus, Courtella, Alfonsiella, Allotriozoon, and Liporrhopalum), lay their eggs in figs (Ficus spp.) from which the female offspring do not emerge until after they mate with newly emerging males. When a fig contains the offspring of only one female, males do not compete with any unrelated males for access to females (their sisters). By contrast, when a fig contains the offspring of multiple females, males have to compete with unrelated males, but also have access to larger numbers of newly emerged females. Accordingly, in species of fig wasps that lay their eggs in figs that contain the brood of only one female, the sex ratio approaches 1:20 (sons:daughters). This low value maximizes the number of grand-offspring because a very small number of sons can fertilize all of the daughters. By contrast, in fig wasp species in which multiple reproductive females occupy and oviposit in a single fig, sex ratios are less female-biased. As a mother's sons have the opportunity to mate with females other than their siblings, selection favors increased investment in male offspring.
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