Blackwell Publishing

Virtual Experiments

Directional selection Genetic drift Gene frequencies Mutational load Sex ratio Sexual reproduction

Why do most species maintain a 50:50 ratio between the sexes? Is this some accidental by-product of reproductive processes, or a feature of populations which is shaped by natural selection? The sex ratio is in fact one of the most successfully understood adaptations, due to R. A. Fisher. He explained that a 50:50 sex ratio as an equilibrium point: if a population ever comes to deviate from it, natural selection drives it back.

This Virtual Experiment simulates a population of males and females. There is no differential mortality due to natural selection and no mating preferences. The initial percentage of males can be given as an input value before the experiment is run. Try percentages far from the equilibrium point and observe how the 50:50 balance is restored.

Click to download the program. (PC)