View source for Reproduction

{{Short description|Biological process}}
{{Other uses}}
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[[File:Kalanchoe veg.jpg|thumb|350px|right|Production of new individuals along a leaf margin of the  miracle leaf plant (''[[Kalanchoe pinnata]]''). The small plant in front is about 1&nbsp;cm (0.4&nbsp;in) tall. The concept of "individual" is obviously stretched by this asexual reproductive process.]]
'''Reproduction''' (or '''procreation''' or '''breeding''') is the [[biological process]] by which new individual [[organism]]s – "[[offspring]]" – are produced from their "parent" or parents. There are two forms of reproduction: [[Asexual reproduction|asexual]] and [[Sexual reproduction|sexual]].

In asexual reproduction, an organism can reproduce without the involvement of another organism. Asexual reproduction is not limited to [[unicellular organism|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 sexual reproduction is that only 50% of organisms reproduce<ref>Ridley M (2004) Evolution, 3rd edition. Blackwell Publishing, p. 314.</ref> and organisms only pass on 50% of their [[gene]]s.<ref name="maynard">[[John Maynard Smith]] ''The Evolution of Sex'' 1978.</ref>

Sexual reproduction typically requires the sexual interaction of two specialized reproductive cells, called [[gamete]]s, which contain half the number of [[chromosome]]s of normal cells and are created by [[meiosis]], with typically a [[sperm cell]] [[fertilization|fertilizing]] an [[egg cell]] from the same [[species]] to create a fertilized [[zygote]]. This produces offspring organisms whose genetic characteristics are derived from those of the two parental organisms.

== Methods ==

=== Asexual ===
{{Main|Asexual reproduction}}
Asexual reproduction is a process by which organisms create genetically similar or identical copies of themselves without the contribution of genetic material from another organism. Bacteria divide asexually via [[binary fission]]; viruses take control of host cells to produce more viruses; [[Hydra (genus)|Hydras]] ([[invertebrate]]s of the [[Order (biology)|order]] ''Hydroidea'') and [[yeast]]s 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 (genus)|hydra]], [[yeast]] (See [[Mating of yeast]]s) and jellyfish, may also reproduce sexually. For instance, most plants are capable of [[vegetative reproduction]]{{snd}}reproduction without seeds or spores{{snd}}but can also reproduce sexually. Likewise, bacteria may exchange genetic information by [[bacterial conjugation|conjugation]].

Other ways of asexual reproduction include [[parthenogenesis]], [[Fragmentation (reproduction)|fragmentation]] and [[spore formation]] that involves only [[mitosis]]. Parthenogenesis is the growth and development of [[embryo]] or [[seed]] without [[fertilization]]. Parthenogenesis occurs naturally in some species, including lower plants (where it is called [[apomixis]]), [[invertebrate]]s (e.g. [[water flea]]s, [[aphid]]s, some [[bee]]s and [[parasitic wasp]]s), and [[vertebrate]]s (e.g. some
reptiles,<ref name="reptiles">{{Cite book |editor-last1=Halliday |editor-first1=Tim R. |editor-last2=Adler |editor-first2=Kraig |title=Reptiles & Amphibians |publisher=Torstar Books |year=1986 |page=101 |isbn=978-0-920269-81-7}}</ref> some fish,<ref>{{Cite news|last=Eilperin|first=Juliet|date=2007-05-23|title=Female Sharks Can Reproduce Alone, Researchers Find|newspaper=[[The Washington Post]] |language=en-US|url=http://www.washingtonpost.com/wp-dyn/content/article/2007/05/22/AR2007052201405.html|access-date=2023-03-27|issn=0190-8286 |archive-url=https://web.archive.org/web/20180428143803/http://www.washingtonpost.com/wp-dyn/content/article/2007/05/22/AR2007052201405.html |archive-date=2018-04-28}}</ref> and very rarely, domestic birds<ref>{{Cite web|last=Savage |first=Thomas F. |title=A Guide to the Recognition of Parthenogenesis in Incubated Turkey Eggs |work=Oregon State University |date=September 12, 2005 |url=http://oregonstate.edu/Dept/animal-sciences/poultry/index.html |access-date=2006-10-11 |archive-url=https://web.archive.org/web/20061115201713/http://oregonstate.edu/dept/animal-sciences/poultry/index.html |archive-date=November 15, 2006 }}</ref>).

=== Sexual ===
{{Main|Sexual reproduction}}
{{See also|Human reproduction}}
{{More citations needed section|date=August 2021}}
{{multiple image|perrow=1|image1=Hoverflies mating midair.jpg|caption1=[[Hoverfly|Hoverflies]] mate in midair flight|image2=Korean wolves mating (cropped).jpg|caption2=[[Bird]]s, [[reptile]]s, and [[Mammalian reproduction|mammals reproduce]] by [[internal fertilization]]. Male mammals [[ejaculation|ejaculate]] [[semen]] through the [[penis]] into the [[vagina]] during [[copulation (zoology)|copulation]].<ref name="Hyman1992">{{cite book |author=Libbie Henrietta Hyman |url=https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA583 |title=Hyman's Comparative Vertebrate Anatomy |date=15 September 1992 |publisher=University of Chicago Press |isbn=978-0-226-87013-7 |pages=583–}}</ref><ref name=":0">{{Cite book |last=Birkhead |first=Tim |url=https://books.google.com/books?id=3i4Q8SvohfEC&q=insemination |title=Promiscuity: An Evolutionary History of Sperm Competition |date=2000 |publisher=Harvard University Press |isbn=978-0-674-00666-9 |language=en}}</ref>
}}
Sexual reproduction is a [[biological process]] that creates a new [[organism]] by combining the [[Genetics|genetic]] material of two organisms in a process that starts with [[meiosis]], a specialized type of [[cell division]]. Each of two parent organisms contributes half of the offspring's genetic makeup by creating [[haploid]] [[gametes]].<ref>{{Citation|last1=Griswold|first1=M. D.|title=Meiosis|date=2013-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780123749840009165|encyclopedia=Brenner's Encyclopedia of Genetics (Second Edition)|pages=338–341|editor-last=Maloy|editor-first=Stanley|place=San Diego|publisher=Academic Press|language=en|isbn=978-0-08-096156-9|access-date=2020-10-05|last2=Hunt|first2=P. A.|editor2-last=Hughes|editor2-first=Kelly|archive-date=2021-04-20|archive-url=https://web.archive.org/web/20210420024023/https://www.sciencedirect.com/science/article/pii/B9780123749840009165|url-status=live}}</ref> Most organisms form two different types of gametes. In these '''''anisogamous''''' species, the two sexes are referred to as male (producing [[sperm]] or microspores) and female (producing [[ovum|ova]] or megaspores).<ref name="Kumar-201922">{{Cite encyclopedia|entry=Anisogamy|encyclopedia=Encyclopedia of Animal Cognition and Behavior|publisher=Springer International Publishing|place=Cham |date=2019|pages=1–5|doi=10.1007/978-3-319-47829-6_340-1|isbn=978-3-319-47829-6 |vauthors=Kumar R, Meena M, Swapnil P|title=Anisogamy |veditors=Vonk J, Shackelford T}}</ref> In '''''isogamous species''''', the gametes are similar or identical in form ([[isogamete]]s), but may have separable properties and then may be given other different names (see [[isogamy]]).<ref>{{Cite journal|last1=Lehtonen|first1=Jussi|last2=Kokko|first2=Hanna|last3=Parker|first3=Geoff A.|date=2016-10-19|title=What do isogamous organisms teach us about sex and the two sexes?|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|volume=371|issue=1706|doi=10.1098/rstb.2015.0532|issn=0962-8436|pmc=5031617|pmid=27619696}}</ref> Because both gametes look alike, they generally cannot be classified as male or female. For example, in the green alga, ''Chlamydomonas reinhardtii'', there are so-called "plus" and "minus" gametes. A few types of organisms, such as many fungi and the [[ciliate]] ''Paramecium aurelia'',<ref>{{cite journal |author=T.M. Sonneborn |title=Mating Types in Paramecium Aurelia: Diverse Conditions for Mating in Different Stocks; Occurrence, Number and Interrelations of the Types |journal=Proceedings of the American Philosophical Society |volume=79 |issue=3 |pages=411–434 |publisher=American Philosophical Society |jstor=984858 |year=1938}}</ref> have more than two  "sexes", called [[mating type]]s.
Most animals (including humans) and plants reproduce sexually. Sexually reproducing organisms have different sets of genes for every trait (called [[alleles]]). Offspring inherit one allele for each trait from each parent. Thus, offspring have a combination of the parents' genes. It is believed that "the masking of deleterious alleles favors the evolution of a dominant diploid phase in organisms that alternate between haploid and diploid phases" where recombination occurs freely.<ref>{{cite journal |last1=Otto |first1=S.P. |last2=Goldstein |first2=D.B. |year=1992 |title=Recombination and the Evolution of Diploidy |journal=[[Genetics (journal)|Genetics]] |volume=131 |issue=3 |pages=745–751 |doi=10.1093/genetics/131.3.745 |pmid=1628815 |pmc=1205045}}</ref><ref>{{cite book |last1=Bernstein |first1=H. |last2=Hopf |first2=F.A. |last3=Michod |first3=R.E. |chapter=The Molecular Basis of the Evolution of Sex |year=1987 |title=Molecular Genetics of Development |series=Advances in Genetics |volume=24 |pages=323–370 |pmid=3324702 |doi=10.1016/s0065-2660(08)60012-7 |isbn=978-0-12-017624-3}}</ref>

[[Bryophyte]]s reproduce sexually, but the larger and commonly-seen organisms are [[haploid]] and produce [[gametes]]. The gametes fuse to form a [[zygote]] which develops into a [[sporangium]], which in turn produces haploid spores. The [[diploid]] stage is relatively small and short-lived compared to the haploid stage, i.e. ''haploid dominance''. The advantage of diploidy, heterosis, only exists in the diploid life generation. Bryophytes retain sexual reproduction despite the fact that the haploid stage does not benefit from heterosis. This may be an indication that the sexual reproduction has advantages other than heterosis, such as [[genetic recombination]] between members of the species, allowing the expression of a wider range of traits and thus making the population more able to survive environmental variation.<ref>{{cite journal |last1=Haig |first1=David |title=Living together and living apart: the sexual lives of bryophytes |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |date=19 October 2016 |volume=371 |issue=1706 |article-number=20150535 |doi=10.1098/rstb.2015.0535 |pmid=27619699 |pmc=5031620 }}</ref>

==== Allogamy ====
{{Main|Allogamy}}

Allogamy is the fertilization of flowers through cross-pollination, this occurs when a flower's ovum is fertilized by spermatozoa from the pollen of a different plant's flower.<ref>{{cite web |title=Allogamy |url=https://www.biologyonline.com/dictionary/allogamy |website=Biology Online |access-date=20 August 2022 |date=7 October 2019 |archive-date=25 September 2021 |archive-url=https://web.archive.org/web/20210925025751/https://www.biologyonline.com/dictionary/allogamy |url-status=live }}</ref><ref name="plant systematics">{{cite book |last1=Simpson |first1=Michael G. |title=Plant systematics |date=2019 |publisher=Academic Press |location=Burlington, MA |isbn=978-0-12-812628-8 |pages=595–606 |edition=3rd |chapter-url=https://www.sciencedirect.com/science/article/pii/B9780128126288500134 |chapter=13 – Plant Reproductive Biology |access-date=2022-08-20 |archive-date=2022-08-20 |archive-url=https://web.archive.org/web/20220820183122/https://www.sciencedirect.com/science/article/pii/B9780128126288500134 |url-status=live }}</ref> Pollen may be transferred through [[pollinator|pollen vectors]] or abiotic carriers such as wind. Fertilization begins when the pollen is brought to a female gamete through the pollen tube. Allogamy is also known as cross fertilization, in contrast to autogamy or geitonogamy which are methods of self-fertilization.

==== Autogamy ====
{{main|Autogamy}}

Self-[[fertilization]], also known as autogamy, occurs in [[hermaphrodite|hermaphroditic]] organisms where the two [[gamete]]s fused in fertilization come from the same individual, e.g., many [[vascular plants]], some [[foraminifera]]ns, some [[ciliate]]s.<ref name="plant systematics" /> The term "autogamy" is sometimes substituted for autogamous pollination (not necessarily leading to successful fertilization) and describes [[self-pollination]] within the same flower, distinguished from [[geitonogamy|geitonogamous pollination]], transfer of pollen to a different flower on the same [[flowering plant]],<ref>{{cite journal |author=Eckert, C.G. |year=2000 |title=Contributions of autogamy and geitonogamy to self-fertilization in a mass-flowering, clonal plant |journal=Ecology |volume=81 |issue=2 |pages=532–542 |doi=10.1890/0012-9658(2000)081[0532:coaagt]2.0.co;2}}</ref> or within a single [[monoecious]] [[gymnosperm]] plant.

==== Mitosis and meiosis ====
[[Mitosis]] and [[meiosis]] are types of [[cell division]]. Mitosis occurs in [[somatic cells]], while meiosis occurs in [[gametes]].

'''Mitosis'''
The resultant number of cells in mitosis is twice the number of original cells. The number of [[chromosomes]] in the offspring cells is the same as that of the parent cell.
{{Clear}}<!-- Unsourced image removed: [[File:mitosis.jpg|center]] -->
'''Meiosis'''
The resultant number of cells is four times the number of original cells. This results in cells with half the number of [[chromosomes]] present in the parent cell. A [[diploid]] cell duplicates itself, then undergoes two divisions ([[tetraploid]] to diploid to haploid), in the process forming four [[haploid]] cells. This process occurs in two phases, meiosis I and meiosis II.
{{Clear}}<!-- Unsourced image removed: [[File:meiosis.jpg|center]] -->

==== Gametogenesis ====
Animals, including mammals, produce gametes (sperm and egg) by means of  [[Origin and function of meiosis|meiosis]] in gonads (testicles in males and ovaries in females).  Sperm are produced by  [[spermatogenesis]] and eggs are produced by [[oogenesis]].  During gametogenesis in mammals numerous genes encoding proteins that participate in DNA repair mechanisms exhibit enhanced or specialized expression.<ref name="pmid11226027">{{cite journal |vauthors=Baarends WM, van der Laan R, Grootegoed JA |title=DNA repair mechanisms and gametogenesis |journal=Reproduction |volume=121 |issue=1 |pages=31–9 |year=2001 |pmid=11226027 |doi= 10.1530/reprod/121.1.31|doi-access=free |hdl=1765/9599 |hdl-access=free }}</ref>  Male [[germ cell]]s produced in the testes of animals are capable of special [[DNA repair]] processes that function during meiosis to repair DNA damages and to maintain the integrity of the [[genome]]s that are to be passed on to progeny.<ref name = Garcia-Rodriguez2018>{{cite journal |vauthors=García-Rodríguez A, Gosálvez J, Agarwal A, Roy R, Johnston S |title=DNA Damage and Repair in Human Reproductive Cells |journal=Int J Mol Sci |volume=20 |issue=1 |date=December 2018 |page=31 |pmid=30577615 |doi=10.3390/ijms20010031 |doi-access=free |pmc=6337641 |url=}}</ref>  Such DNA repair processes include [[homologous recombination]]al repair as well as [[non-homologous end joining]].<ref name = Garcia-Rodriguez2018/>  [[Oocyte]]s located in the [[folliculogenesis|primordial follicle]] of the ovary are in a non-growing [[prophase]] arrested state, but are able to undergo highly efficient [[homologous recombination]]al repair of [[DNA damage (naturally occurring)|DNA damages]] including double-strand breaks.<ref name = Stringer2020>{{cite journal |vauthors=Stringer JM, Winship A, Zerafa N, Wakefield M, Hutt K |title=Oocytes can efficiently repair DNA double-strand breaks to restore genetic integrity and protect offspring health |journal=Proc Natl Acad Sci U S A |volume=117 |issue=21 |pages=11513–11522 |date=May 2020 |pmid=32381741 |pmc=7260990 |doi=10.1073/pnas.2001124117 |doi-access=free |bibcode=2020PNAS..11711513S |url=}}</ref>  These repair processes allow the integrity of the [[genome]] to be maintained and offspring health to be protected.<ref name = Stringer2020/>

=== Same-sex ===
{{main|LGBT reproduction|Reproductive technology}}
{{Original research section|date=April 2021}} Scientific research is currently investigating the possibility of same-sex procreation, which would produce offspring with equal genetic contributions from either two females or two males.<ref name = "Kissin_2019">{{cite book |url= https://books.google.com/books?id=dIOWDwAAQBAJ&pg=PR10 |title=Assisted Reproductive Technology Surveillance |last1=Kissin |first1=Dmitry M. |last2=Adamson |first2=G. David |last3=Chambers |first3=Georgina |last4= DeGeyter |first4=Christian | name-list-style = vanc |year=2019|publisher=Cambridge University Press |isbn=978-1-108-49858-6 }}</ref><ref>{{cite web |url=http://www.samesexprocreation.com/timeline.htm |title=Timeline of same-sex procreation scientific developments |publisher=samesexprocreation.com |access-date=2008-01-31 |archive-date=2011-02-08 |archive-url=https://web.archive.org/web/20110208053033/http://samesexprocreation.com/timeline.htm |url-status=live }}</ref><ref>{{cite journal |last1=Segers |first1=Seppe |last2=Mertes |first2=Heidi |last3=Pennings |first3=Guido |last4=de Wert |first4=Guido |last5=Dondorp |first5=Wybo |title=Using stem cell-derived gametes for same-sex reproduction: an alternative scenario |journal=Journal of Medical Ethics |date=2017 |volume=43 |issue=10 |pages=688–691 |doi=10.1136/medethics-2016-103863 |jstor=26535499 |pmid=28122990 |s2cid=35387886  |issn=0306-6800|url=https://cris.maastrichtuniversity.nl/en/publications/1d0a93d6-7a0f-462c-9532-fef16e1ea0f4 }}</ref> The obvious approaches, subject to a growing amount of activity, are [[female sperm]] and [[male egg]]s. In 2004, by altering the function of a few genes involved with imprinting, other Japanese scientists combined two mouse eggs to produce daughter mice<ref>{{Cite news |newspaper=[[The Washington Post]] |date=April 22, 2004 |title=Japanese scientists produce mice without using sperm |publisher=Sarasota Herald-Tribune |url=https://news.google.com/newspapers?id=nUIgAAAAIBAJ&pg=6950,1352704&dq=japanese+scientists+combine+two+mouse+eggs+to+produce+daughter+mice&hl=en |access-date=November 19, 2020 |archive-date=July 23, 2021 |archive-url=https://web.archive.org/web/20210723112403/https://news.google.com/newspapers?id=nUIgAAAAIBAJ&pg=6950,1352704&dq=japanese+scientists+combine+two+mouse+eggs+to+produce+daughter+mice&hl=en |url-status=live }}</ref> In 2010, American scientists used genetically manipulated stem cells to produce viable mouse offspring carrying genetic contributions from two fathers.<ref>{{cite web|url=https://www.wired.com/2010/12/two-dad-mice/?utm|title=Reproduction Hack Makes Mice From Two Dads|website=[[Wired (magazine)|Wired]]|date=9 December 2010}}</ref><ref>{{cite web|url=https://www.sciencedaily.com/releases/2010/12/101208151611.htm|title=Viable female and male mice from two fathers produced using stem cell technology|website=[[Science Daily]]|date=9 December 2010}}</ref> In 2018 Chinese scientists created 29 female mice from two mice mothers but were unable to produce viable offspring from two father mice. Researches noted that there is little chance these techniques would be applied to humans in the near future.<ref>{{Cite news|url=https://www.thetimes.com/article/no-father-necessary-as-mice-are-created-with-two-mothers-2vskf8w58|title=No father necessary as mice are created with two mothers|last=Blakely|first=Rhys|date=2018-10-12|work=The Times|access-date=2018-10-12|language=en|issn=0140-0460|url-access=subscription|archive-date=2018-10-12|archive-url=https://web.archive.org/web/20181012015608/https://www.thetimes.co.uk/article/no-father-necessary-as-mice-are-created-with-two-mothers-2vskf8w58|url-status=live}}</ref><ref>{{Cite journal|last1=Li|first1=Zhi-Kun|last2=Wang|first2=Le-Yun|last3=Wang|first3=Li-Bin|last4=Feng|first4=Gui-Hai|last5=Yuan|first5=Xue-Wei|last6=Liu|first6=Chao|last7=Xu|first7=Kai|last8=Li|first8=Yu-Huan|last9=Wan|first9=Hai-Feng|date=2018-10-01|title=Generation of Bimaternal and Bipaternal Mice from Hypomethylated Haploid ESCs with Imprinting Region Deletions|journal=Cell Stem Cell|volume=23|issue=5|pages=665–676.e4|doi=10.1016/j.stem.2018.09.004|pmid=30318303|issn=1934-5909|doi-access=free}}</ref> In 2023, Japanese scientists created mouse pups from two mice fathers which grew into adulthood.<ref>{{cite web|url=https://www.theguardian.com/science/2023/mar/08/scientists-create-mice-with-two-fathers-after-making-eggs-from-male-cells|title=Scientists create mice with two fathers after making eggs from male cells|website=[[The Guardian]]|date=8 March 2023}}</ref>

== Strategies ==
<!--[[Reproductive strategy]], [[Reproduction strategy]] and [[Reproduction strategies]] redirect here-->
{{further|Modes of reproduction|Life history theory}}

There are a wide range of reproductive strategies employed by different species. Some animals, such as the human and [[northern gannet]], do not reach [[sexual maturity]] for many years after birth and even then produce few offspring. Others reproduce quickly; but, under normal circumstances, most offspring do not survive to [[adult]]hood. For example, a rabbit (mature after 8 months) can produce 10–30 offspring per year, and a [[Drosophila melanogaster|fruit fly]] (mature after 10–14 days) can produce up to 900 offspring per year. These two main strategies are known as [[K-selection]] (few offspring) and [[r-selection]] (many offspring). Which strategy is favoured by [[evolution]] depends on a variety of circumstances. Animals with few offspring can devote more resources to the nurturing and protection of each individual offspring, thus reducing the need for many offspring. On the other hand, animals with many offspring may devote fewer resources to each individual offspring; for these types of animals it is common for many offspring to die soon after birth, but enough individuals typically survive to maintain the population.  Some organisms such as honey bees and fruit flies retain sperm in a process called [[Female sperm storage|sperm storage]] thereby increasing the duration of their fertility.

===Other types===
{{main|Semelparity and iteroparity}}
* '''Polycyclic animals''' reproduce intermittently throughout their lives.
* '''Semelparous organisms''' reproduce only once in their lifetime,<ref>{{cite journal|last=Cole|first=Lamont C.|title=The Population Consequences of Life History Phenomena|journal=The Quarterly Review of Biology|date=June 1954|volume=29|issue=2|pages=103–137|doi=10.1086/400074|pmid=13177850|jstor=2817654|s2cid=26986186}}</ref> such as [[annual plant]]s (including all grain crops), and certain species of salmon, spider, bamboo and century plant.<ref name=Young>
{{cite journal |last=Young |first=Truman P. |author2=Carol K. Augspurger |year=1991 |title=Ecology and evolution of long-lived semelparous plants |journal=Trends in Ecology and Evolution |volume=6 |pages=285–289 |doi=10.1016/0169-5347(91)90006-J |issue=9 |pmid=21232483|bibcode=1991TEcoE...6..285Y }}</ref> Often, they die shortly after reproduction. This is often associated with [[R/K selection theory|r-strategists]].
* '''Iteroparous organisms''' produce offspring in successive (e.g. annual or seasonal) cycles, such as [[perennial plant]]s. Iteroparous animals survive over multiple seasons (or periodic condition changes). This is more associated with [[R/K selection theory|K-strategists]].

==Asexual vs. sexual reproduction==

[[File:Evolsex-dia1a.svg|thumb|250px|right|Illustration of the ''twofold cost of sexual reproduction''. If each organism were to contribute to the same number of offspring (two), ''(a)'' the population remains the same size each generation, where the ''(b)'' asexual population doubles in size each generation.]]

Organisms that reproduce through asexual reproduction tend to grow in number exponentially. However, because they rely on mutation for variations in their DNA, all members of the species have similar vulnerabilities. Organisms that reproduce sexually yield a smaller number of offspring, but the large amount of variation in their genes makes them less susceptible to disease.

Many organisms can reproduce sexually as well as asexually. [[Aphid]]s, [[slime mold]]s, [[sea anemone]]s, some species of [[starfish]] (by [[fragmentation (reproduction)|fragmentation]]), and many plants are examples. When environmental factors are favorable, asexual reproduction is employed to exploit suitable conditions for survival such as an abundant food supply, adequate shelter, favorable climate, disease, optimum pH or a proper mix of other lifestyle requirements. Populations of these organisms increase exponentially via asexual reproductive strategies to take full advantage of the rich supply resources.<ref name=aphids>{{cite journal |last1=Yan |first1=Shuo |last2=Wang |first2=Wan-xing |last3=Shen |first3=Jie |title=Reproductive polyphenism and its advantages in aphids: Switching between sexual and asexual reproduction |journal=Journal of Integrative Agriculture |date=June 2020 |volume=19 |issue=6 |pages=1447–1457 |doi=10.1016/S2095-3119(19)62767-X|s2cid=218971234 |doi-access=free |bibcode=2020JIAgr..19.1447Y }}</ref>

When food sources have been depleted, the climate becomes hostile, or individual survival is jeopardized by some other adverse change in living conditions, these organisms switch to sexual forms of reproduction. Sexual reproduction ensures a mixing of the gene pool of the species. The variations found in offspring of sexual reproduction allow some individuals to be better suited for survival and provide a mechanism for selective adaptation to occur. The meiosis stage of the sexual cycle also allows especially effective repair of DNA damages (see [[Meiosis]]).<ref name=aphids />  In addition, sexual reproduction usually results in the formation of a life stage that is able to endure the conditions that threaten the offspring of an asexual parent. Thus, seeds, spores, eggs, pupae, cysts or other "over-wintering" stages of sexual reproduction ensure the survival during unfavorable times and the organism can "wait out" adverse situations until a swing back to suitability occurs.

==Life without==
The existence of life without reproduction is the subject of some speculation. The biological study of how the origin of life produced reproducing organisms from non-reproducing elements is called [[abiogenesis]]. Whether or not there were several independent abiogenetic events, biologists believe that the [[last universal ancestor]] to all present life on Earth lived about [[Timeline of evolution|3.5 billion years ago]].

Scientists have speculated about the possibility of creating life non-reproductively in the laboratory. Several scientists have succeeded in producing simple viruses from entirely non-living materials.<ref>{{Cite journal|last1=Cello|first1=Jeronimo|last2=Paul|first2=Aniko V.|last3=Wimmer|first3=Eckard|date=2002-08-09|title=Chemical synthesis of poliovirus cDNA: generation of infectious virus in the absence of natural template|journal=Science|volume=297|issue=5583|pages=1016–1018|doi=10.1126/science.1072266|issn=1095-9203|pmid=12114528|bibcode=2002Sci...297.1016C |s2cid=5810309 |doi-access=free}}</ref> However, viruses are often regarded as not alive. Being nothing more than a bit of RNA or DNA in a protein capsule, they have no [[metabolism]] and can only [[self-replication|replicate]] with the assistance of a hijacked [[cell (biology)|cell]]'s metabolic machinery.

The production of a truly living organism (e.g. a simple bacterium) with no ancestors would be a much more complex task, but may well be possible to some degree according to current biological knowledge. A [[Synthetic Genomics|synthetic genome]] has been transferred into an existing bacterium where it replaced the native DNA, resulting in the artificial production of a new ''[[M. mycoides]]'' organism.<ref>{{Cite journal |doi=10.1126/science.1190719 |pmid=20488990 |year=2010 |last1=Gibson |first1=D. |last2=Glass |first2=J. |last3=Lartigue |first3=C. |last4=Noskov |first4=V. |last5=Chuang |first5=R. |last6=Algire |first6=M. |last7=Benders |first7=G. |last8=Montague |first8=M. |last9=Ma |first9=L. |last10=Moodie |first10=M.M. |last11=Merryman |first11=C. |last12=Vashee |first12=S. |last13=Krishnakumar |first13=R. |last14=Assad-Garcia |first14=N. |last15=Andrews-Pfannkoch |first15=C. |last16=Denisova |first16=E.A. |last17=Young |first17=L. |last18=Qi |first18=Z.-Q. |last19=Segall-Shapiro |first19=T.H. |last20=Calvey |first20=C.H. |last21=Parmar |first21=P.P. |last22=Hutchison Ca |first22=C.A. |last23=Smith |first23=H.O. |last24=Venter |first24=J.C. |title=Creation of a Bacterial Cell Controlled by a Chemically Synthesized Genome |journal=Science |volume=329 |issue=5987 |pages=52–56 |bibcode=2010Sci...329...52G |s2cid=7320517 |doi-access=}}</ref>

There is some debate within the scientific community over whether this cell can be considered completely synthetic<ref name="venter">{{cite news|url=https://www.wsj.com/articles/SB10001424052748703559004575256470152341984|title=Scientists Create First Synthetic Cell|author=Robert Lee Hotz|newspaper=The Wall Street Journal|date=May 21, 2010|access-date=April 13, 2012|archive-date=January 29, 2015|archive-url=https://web.archive.org/web/20150129112052/http://www.wsj.com/articles/SB10001424052748703559004575256470152341984|url-status=live}}</ref> on the grounds that the chemically synthesized genome was an almost 1:1 copy of a naturally occurring genome and, the recipient cell was a naturally occurring bacterium. The Craig Venter Institute maintains the term "synthetic bacterial cell" but they also clarify "...we do not consider this to be "creating life from scratch" but rather we are creating new life out of already existing life using synthetic DNA".<ref>{{cite web |url=http://www.jcvi.org/cms/research/projects/first-self-replicating-synthetic-bacterial-cell/faq |title=FAQ |author=Craig Venter Institute |access-date=2011-04-24 |archive-date=2010-12-28 |archive-url=https://web.archive.org/web/20101228180155/http://www.jcvi.org/cms/research/projects/first-self-replicating-synthetic-bacterial-cell/faq/ }}</ref> Venter plans to patent his experimental cells, stating that "they are pretty clearly human inventions".<ref name="venter"/> Its creators suggests that building 'synthetic life' would allow researchers to learn about life by building it, rather than by tearing it apart. They also propose to stretch the boundaries between life and machines until the two overlap to yield "truly programmable organisms".<ref>{{Cite journal | author = W. Wayte Gibbs | title = Synthetic Life | journal = Scientific American | url = http://www.scientificamerican.com/article.cfm?id=synthetic-life | date = May 2004 | volume = 290 | issue = 5 | page = 75 | doi = 10.1038/scientificamerican0504-75 | bibcode = 2004SciAm.290e..75G | access-date = 2012-12-22 | archive-date = 2012-10-13 | archive-url = https://web.archive.org/web/20121013161012/http://www.scientificamerican.com/article.cfm?id=synthetic-life | url-access = subscription }}</ref> Researchers involved stated that the creation of "true synthetic biochemical life" is relatively close in reach with current technology and cheap compared to the effort needed to place man on the Moon.<ref>{{cite web |url= https://www.pbs.org/wgbh/nova/video/artificial-life/ |title= NOVA: Artificial life |website= [[PBS]] |date= 18 October 2005 |access-date= 2007-01-19 |archive-date= 2007-01-21 |archive-url= https://web.archive.org/web/20070121003935/http://www.pbs.org/wgbh/nova/sciencenow/3214/01.html |url-status= live }}</ref>

==Lottery principle==
Sexual reproduction has many drawbacks, since it requires far more energy than asexual reproduction and diverts the organisms from other pursuits, and there is some argument about why so many species use it. [[George C. Williams (biologist)|George C. Williams]] used [[lottery]] tickets as an [[analogy]] in one explanation for the widespread use of sexual reproduction.<ref>[[George C. Williams (biologist)|Williams G. C.]] 1975. Sex and Evolution. Princeton (NJ): Princeton University Press.</ref> He argued that asexual reproduction, which produces little or no genetic variety in offspring, was like buying many tickets that all have the same number, limiting the chance of "winning" – that is, producing surviving offspring. Sexual reproduction, he argued, was like purchasing fewer tickets but with a greater variety of numbers and therefore a greater chance of success. The point of this analogy is that since asexual reproduction does not produce genetic variations, there is little ability to quickly adapt to a changing environment. The lottery principle is less accepted these days because of evidence that asexual reproduction is more prevalent in unstable environments, the opposite of what it predicts.<ref>{{cite journal |last1=Lively |first1=C. M. |last2=Morran |first2=L. T. |title=The ecology of sexual reproduction |journal=Journal of Evolutionary Biology |date=July 2014 |volume=27 |issue=7 |pages=1292–1303 |doi=10.1111/jeb.12354 |pmid=24617324|pmc=4077903 }}</ref>

==See also==
{{col div|colwidth=30em}}
* [[Breeding season]]
* [[Masting]]
* [[Mating system]]
* [[Modes of reproduction]]
* [[Plant reproduction]]
* [[Reproductive system]]
{{colend}}

==Notes==
{{Reflist}}

==References==
* Tobler, M. & Schlupp, I. (2005) Parasites in sexual and asexual mollies (Poecilia, Poeciliidae, Teleostei): a case for the Red Queen? Biol. Lett. 1 (2): 166–168.
* [[Carl Zimmer|Zimmer, Carl]]. ''[[Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures]]'', New York: Touchstone, 2001.
* {{Cite encyclopedia
 |encyclopedia=GardenWeb Glossary of Botanical Terms
 |edition=2.1
 |year=2002
 |article=Allogamy, cross-fertilization, cross-pollination, hybridization
}}
* {{Cite encyclopedia
 |encyclopedia=Stedman's Online Medical Dictionary
 |edition=27
 |year=2004
 |article=Allogamy
}}

==Further reading==
* Judson, Olivia (2003). ''[[Dr Tatiana's Sex Advice to All Creation|Dr. Tatiana's Sex Advice to All Creation: Definitive Guide to the Evolutionary Biology of Sex.]]'' {{ISBN|978-0-09-928375-1}}
* Richard E. Michod and Bruce E. Levin, editors (1987). ''The Evolution of Sex: An Examination of Current Ideas''. Sinauer Associates Inc., Publishers, Sunderland, MA {{ISBN|978-0-87893-459-1}}
* Michod, R.E. (1994). ''Eros and Evolution: A natural philosophy of sex''.  Addison-Wesley Publishing Company, Reading, MA {{ISBN|978-0-201-44232-8}}

==External links==
{{Commons category|Reproduction}}
* [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AsexualReproduction.html Asexual Reproduction] {{Webarchive|url=https://web.archive.org/web/20180122112348/http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AsexualReproduction.html |date=2018-01-22 }}
* [http://www.biolreprod.org/ Journal of Biology of Reproduction]
* [http://www.andrologyjournal.org/ Journal of Andrology] {{Webarchive|url=https://web.archive.org/web/20051107234115/http://www.andrologyjournal.org/ |date=2005-11-07 }}
* {{Cite EB1911|wstitle=Reproduction|short=x}}
* [https://plato.stanford.edu/entries/replication/ "Replication and Reproduction." ''Stanford Encyclopedia of Philosophy'']
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[[Category:Reproduction| ]]