Sex Ratio

The proportion of females indicates the reproductive potential of a population. Sex ratio also reflects a number of life history traits, such as the importance of sexual reproduction, mating system, and ability to exploit harsh or ephemeral habitats (Pianka 1974).

A 50 : 50 sex ratio generally indicates equally important roles of males and females, given that selection would minimize the less-productive sex. Sex ratio approaches 50 : 50 in species where males select resources, protect or feed females, or contribute necessary genetic variability. This sex ratio maximizes availability of males to females and, hence, maximizes genetic heterogeneity. High genetic heterogeneity is particularly important for population survival in heterogeneous environments. However, when the sexes are equally abundant, only half of the population is capable of producing offspring. By contrast, a parthenogenetic population (with no males) has little or no genetic heterogeneity, but the entire population is capable of producing offspring. Parthenogenetic individuals can disperse and colonize new resources without the additional challenge of finding mates, and successful colonists can generate large population sizes rapidly, ensuring exploitation of suitable resources and large numbers of dispersants in the next generation.

Sex ratio can be affected by environmental factors. For example, haploid males of many insect species are more sensitive to environmental variation than are diploid females, and greater mortality to haploid males may speed adaptation to changing conditions by quickly eliminating deleterious genes (Edmunds and Alstad 1985, J. Peterson and Merrell 1983).

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