Sex ratio is a parameter that affects both the growth rates and the evolutionary trajectories of wild populations; consequently, it is a focus for both ecological and evolutionary research (Table 1). The sex ratio of a population is defined as the ratio of the number of individuals of one sex to that of the other sex, but this definition can be evaluated at three points in time:
(1) 'primary sex ratio': at the time of fertilization;
(2) 'secondary': at birth/hatching or germination; and
(3) 'tertiary': postjuvenile stages at all ages. For the last stage we have most information since it is easiest to assess. At the individual level, the sex ratio of an individual is measured as the ratio of its sons to its daughters. These operational definitions are straightforward when individuals are unisexual (gonochorism in animals; dioecy in plants) but in cosexual individuals (e.g., hermaphroditic plants), the allocation of resources to different sex functions (e.g., sperm vs. eggs) defines the sex ratios. In plants, a variety of morphological analyses can be used to estimate the sex ratio, ranging from simple counts of unisexual male versus female individuals or flowers (where flowers are unisexual) to estimates of the ratio of the number of pollen grains to the number of ovules (the pollen:ovule ratio). Obtaining estimates of sex ratio for organisms or life stages (such as seedlings) that do not exhibit clear morphological differences between the genders at the time of investigation requires physiological or molecular techniques (at least where sex determination is genetically based).
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