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Reproduction (or procreation) is the biological process by which new individual organisms – "offspring" – are produced from their "parents". Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction. There are two forms of reproduction: asexual and sexual. In asexual reproduction, an organism can reproduce without the involvement of another organism. Asexual reproduction is not limited to single-celled organisms. The cloning of an organism is a form of asexual reproduction. By asexual reproduction, an organism creates a genetically similar or identical copy of itself. The evolution of sexual reproduction is a major puzzle for biologists. The two-fold cost of sex is that only 50% of organisms reproduce〔Ridley M (2004) Evolution, 3rd edition. Blackwell Publishing, p. 314.〕 and organisms only pass on 50% of their genes.〔John Maynard Smith ''The Evolution of Sex'' 1978.〕 Sexual reproduction typically requires the sexual interaction of two specialized organisms, called gametes, which contain half the number of chromosomes of normal cells and are created by meiosis, with typically a male fertilizing a female of the same species to create a fertilized zygote. This produces offspring organisms whose genetic characteristics are derived from those of the two parental organisms. ==Asexual reproduction== (詳細はBacteria divide asexually via binary fission; viruses take control of host cells to produce more viruses; Hydras (invertebrates of the order ''Hydroidea'') and yeasts are able to reproduce by budding. These organisms often do not possess different sexes, and they are capable of "splitting" themselves into two or more copies of themselves. Most plants have the ability to reproduce asexually and the ant species Mycocepurus smithii is thought to reproduce entirely by asexual means. Some species that are capable of reproducing asexually, like hydra, yeast (See Mating of yeasts) and jellyfish, may also reproduce sexually. For instance, most plants are capable of vegetative reproduction—reproduction without seeds or spores—but can also reproduce sexually. Likewise, bacteria may exchange genetic information by conjugation. Other ways of asexual reproduction include parthenogenesis, fragmentation and spore formation that involves only mitosis. Parthenogenesis is the growth and development of embryo or seed without fertilization by a male. Parthenogenesis occurs naturally in some species, including lower plants (where it is called apomixis), invertebrates (e.g. water fleas, aphids, some bees and parasitic wasps), and vertebrates (e.g. some reptiles, fish, and, very rarely, birds and sharks〔("Female Sharks Can Reproduce Alone, Researchers Find" ), Washington Post, Wednesday, May 23, 2007; Page A02〕). It is sometimes also used to describe reproduction modes in hermaphroditic species which can self-fertilize. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「reproduction」の詳細全文を読む スポンサード リンク
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