The sex ratio is the number of males per female. The classical sex ratio theory that arose from Fisher (1930) suggests that the sex ratio evolves by differences in the rates whereby male and female offspring copy genes into future generations through sexual reproduction. According to this process an even sex ratio should evolve because the rare sex has an advantage in sexual reproduction over the common sex and, thus, the success of the two sexes in sexual reproduction is even when the sex ratio is even. This process depends on the mating pattern, where random mating results in an even sex ratio (Fisher, 1930), while local mating results in a female biased sex ratio (Hamilton, 1967).

The classical sex ratio theory seems to hold on a local scale where the organism has a fixed mating pattern and a fixed degree of sexual reproduction. But on a longer time-scale, where the mating pattern and degree of sexual reproduction evolve by natural selection, the theory predicts the evolution of asexual variants with no males and no sex ratios. This failure is connected to the two-fold cost of the male and the two-fold cost of meiosis, where the classical sex ratio theory provides no benefit that will outweigh the costs. In Malthusian Relativity the paradox of an evolutionary unstable sex ratio theory is avoided. This is because the density dependent competitive interactions of high-energy organisms provides a fitness advantage that can outweigh both the two-fold cost of the male and the two-fold cost of meiosis.

References

• Fisher, R. A. (1930). The genetical theory of natural selection. Oxford: Clarendon.
• Hamilton, W. D. (1967). Extraordinary sex ratios. Science 156, 477--488.