Asexual reproduction, such as binary fission, can be successful in an unchanging environment. Sexual reproduction mixes traits and therefore provides species' protection in a changing environment. Sexual reproduction also increases the number of organisms. It occurs when haploid gametes fuse, restoring the diploid state.
Some species reproduce both asexually and sexually, including yeast and plants.
Plant sexual reproduction involves an alternation of generations and both haploid and diploid phases. Chlamydomonas undergoes cell fusion, which may have been a forerunner of sexual reproduction. In humans, gametes are produced in the male and female reproductive systems. These systems include paired gonads, where sperm and oocytes are manufactured, networks of tubes, and glands. Gametogenesis includes meiosis and maturation and produces sperm and oocytes.