性选择或性择是一个进化生物学的理论。此理论解释同一性别的个体（通常是雄性）对交配机会的竞争如何促进性状的演化。同一物种的两个性别之间，通常有至少一个性别必须竞争取得有限的交配机会。由于个体间存在可遗传的差异，造成有的个体在竞争中较为成功，此较成功的个体将此差异给后代，便造成性择演化。通常雌性在生殖过程中投资较多，因此对交配对象较挑剔，所以性择是作用在雄性的性状上，但在性别角色相反的海马等海龙科鱼类上，则是作用在雌性（参见：贝特曼原理）。

Sexual selection is a mode of natural selection where members of one biological sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (intr****ual selection). These two forms of selection mean that some individuals have better reproductive success than others within a population, either from being more attractive or preferring more attractive partners to produce offspring. For instance in the breeding season sexual selection in frogs occurs with the males first gathering at the water's edge and making their mating calls: croaking. The females then arrive and choose the males with the deepest croaks and best territories. Generalizing, males benefit from frequent mating and monopolizing access to a group of fertile females. Females have a limited number of offspring they can have and they maximize the return on the energy they invest in reproduction.