Organisms of many species are specialized into male and female varieties, each known as a sex. The gametes produced by an organism define its sex: males produce small gametes e. Individual organisms which produce both male and female gametes are termed hermaphroditic. Physical differences are often associated with the different sexes of an organism; these sexual dimorphisms can reflect the different reproductive pressures the sexes experience. For instance, mate choice and sexual selection can accelerate the evolution of physical differences between the sexes. Among humans and other mammals , males typically carry an X and a Y chromosome XY , whereas females typically carry two X chromosomes XX , which are a part of the XY sex-determination system.