Evolutionary explanations for partner preferences focus on sexual selection as the driving force of human reproductive behaviour, as those who manage to reproduce successfully will pass on their genes. Sexual selection operates in two main ways: 1) intrasexual selection, where members of one sex have to compete with other members of their sex in order to gain mates and reproduce; 2) intersexual selection, where members of one sex evolve with preferences for particular characteristics in the opposite sex, because these characteristics will increase the chance of passing on genes. For example, males will be attracted to females who display signs of fertility and females will be attracted to males who can provide resources and protection.