Fisher's Principle
Fisher's principle explains why for most species, the sex ratio is approximately 1:1. Bill Hamilton expounded Fisher's argument in his 1967 paper on "Extraordinary sex ratios" as follows, given the assumption of equal parental expenditure on offspring of both sexes.
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- Suppose male births are less common than female.
- A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
- Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
- Therefore the genes for male-producing tendencies spread, and male births become more common.
- As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
- The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.
In modern language, the 1:1 ratio is the evolutionarily stable strategy (ESS).
Read more about this topic: Sex Ratio
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