The existence of two sexes seems to have been selected independently across different evolutionary lineages see convergent evolution.
Other species, such as some snails , practice sex change: adults start out male, then become female.
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In seed plants, which exhibit alternation of generations , the female and male parts are both included within the sporophyte sex organ of a single organism.
Gender roles Intersex Sex and gender distinction Sexuality and gender identity-based cultures Social construction of gender.
Description: In those species with two sexes, males may differ from females in ways other than the production of spermatozoa. Most male mammals , including male humans, have a Y chromosome , which codes for the production of larger amounts of testosterone to develop male reproductive organs.