In order to mate, males need to gain access to females and vice versa, and not all individuals will be equally successful at this task. If there are genetically based differences in the ability of one sex to insure successful mating with the other sex, sexual selection will occur. The patterns and processes of sexual selection are best understood in the context of parental investment . Sexual selection occurs because there is a correlation between the gender of an individual and its parental investment in each offspring. By investing in the production of current offspring a parent will reduce the likelihood that it will be able to invest in future offspring.
Costs can be the energetic demands of parenting, increased predation risk, etc.
One important component of parental investment is the investment in gametes . This serves as one means of distinguishing the sexes. Females are the sex with a large parental investment per gamete . Example are eggs which have the nutrients to promote the development once fertilization occurs. This difference in investment in gametes has important consequences for how these resources are invested and hence how selection might act differently in the two sexes.