Sexual reproduction is a process specific to eukaryotes, organisms whose cells contain a nucleus and mitochondria.
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Frequently, physical differences are 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.
Other animals have a sex-determination system as well, such as the ZW sex-determination system in birds, and the X0 sex-determination system in insects.