Sexual selection is natural selection caused by heritable differences in success at obtaining mates or achieving fertilization.

An individual can survive well yet leave few descendants if it rarely mates. Conversely, a trait that carries a survival cost can still spread if it increases mating success enough to raise total fitness.

Two common mechanisms are:

• intrasexual competition, in which individuals compete with others for access to mates;
• intersexual choice, in which traits influence how potential mates choose among individuals.

For example, suppose males with a conspicuous display survive to mating age at only 80% the rate of less conspicuous males, but each surviving displaying male fathers twice as many offspring. Relative reproductive contribution is proportional to

$$0.80\times2=1.60,$$

compared with

$$1.00\times1=1.00$$

for the less conspicuous phenotype. The display can therefore be favored despite its survival cost.

Sexual selection can produce sexual dimorphism, systematic phenotypic differences between sexes, when the reproductive advantages and costs of traits differ between them.

Sexual selection is not separate from evolutionary fitness. It highlights one component of fitness—successful reproduction—that can produce strong selection even when a trait is neutral or costly for survival.

Mate choice itself can also be nonrandom. The distinction is that nonrandom mating describes who pairs with whom, whereas sexual selection requires those differences to cause unequal reproductive success among heritable variants.