Sex ratio
A sex ratio is the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. However, many species deviate from an even sex ratio, either periodically or permanently. These include parthenogenic and androgenetic species, periodically mating organisms such as aphids, some eusocial wasps, bees, ants, and termites.