Darwin recognized that different selective forces act upon the sexes, especially in regard to competition for mates. It is a struggle between individuals of one sex, generally the males, for the possession of the other sex. The result is not death to the unsuccessful competitor, but few or no offspring. More modern treatments usually consider sexual selection to be a sub-category of natural selection. Sexual selection operates on females as well as males.
Sexual selection operates in monogamous species as well as polygamous ones.
In mammals the corresponding limiting factors are uterine nutrition and milk production, which together may be termed the capacity for rearing young. This would explain why in unisexual organisms there is nearly always a combination of an undiscriminating eagerness in the males and a discriminating passivity in the females. Bateman conducted a simple experiment demonstrating the different reproductive strategies of the sexes. Equal numbers of each sex of drosophila were placed together, and the number of matings by each individual recorded. Whereas most females mated at least once but fewer than four times, some males were able to mate many times, and some others were unable to mate at all.