Broad-scale comparative surveys provide a powerful way to detect evidence for the role of natural selection and for the kinds of mechanisms described above in determining the typical sex ratio of species. For example, in a study of 23 bird species that vary both in sex ratio and in the difference in size between males and females (sexual size dimorphism), the sex ratio was biased toward the smaller sex; this pattern reflects equal parental investment in both sexes. Among 13 species of fig wasp, species in which females rarely lay eggs alone in a single fig have less female-biased broods than species in which single-female broods are common. These examples illustrate that differences among populations or taxa in the direction or strength of natural selection that persist over evolutionary time contribute to interspecific differences in sex ratio.
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