Sex Ratio

Y. Sapir, S. J. Mazer, C. Holzapfel

Research output: Chapter in Book/Report/Conference proceedingChapter

19 Scopus citations


Sex ratio affects both the growth rates and the evolutionary trajectories of wild populations. The sex ratio of the population affects, and is affected by, birth, death, immigration, and emigration rates. It is measured as the ratio of the number of individuals of one sex to that of the other sex, or the ratio of allocation in each. The production of males and females in a ratio of 1:1 is generally the most common evolutionary stable strategy (ESS), led by frequency-dependent natural selection due to competition for mates among individuals of the same sex. Natural selection often appears to determine the differences within and among populations and species in sex ratio. The optimum sex ratio for a given individual in a given population depends on both the existing sex ratio of the population and on the relative costs and benefits of producing offspring of each gender. Patterns of natural selection on sex ratio may be affected by the quality and stability of the immediate habitat, as well as by life-history traits, competition and dispersal, which affect local competition on mate or resources. Environmental effects, both temporal and spatial, can create a biased sex ratio by excess production of the sex that is cheaper to produce under poor environmental conditions. Moreover, empirical studies have found that the sex ratio in plants can be modified at the individual level, following seasonal changes in the resources available.

Original languageEnglish (US)
Title of host publicationEncyclopedia of Ecology, Five-Volume Set
Number of pages6
ISBN (Electronic)9780080454054
StatePublished - Jan 1 2008

All Science Journal Classification (ASJC) codes

  • General Agricultural and Biological Sciences
  • General Environmental Science


  • Adaptive sex ratio
  • Dioecy
  • Environmental control
  • Evolutionary ecology
  • Evolutionary stable strategy (ESS)
  • Gender modification
  • Hermaphrodite
  • Local mate competition
  • Mating system
  • Monoecy
  • Resources limitation
  • Sex allocation


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