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{{Short description|Trait that determines an organism's sexually reproductive function}}
{{About|the distinguishing trait in sexually reproducing organisms|activities|Human sexual activity|and|Animal sexual behavior|other uses|Sex (disambiguation)}}
{{Redirect|Male and female}}
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{{Sex (biology) sidebar}}
'''Sex''' is the [[biological trait]] that determines whether a [[sexual reproduction|sexually reproducing]] organism produces [[male]] or [[female]] [[gamete]]s.<!-- NOTE: Per previous talk page discussion -- the "Which lead sentence should we go with?" RfC -- consensus is for mentioning the male and female aspect in the first sentence. If wanting to remove this or significantly change this sentence, please discuss on the article talk page first.--><ref name="Stevenson-2011">{{cite book| vauthors = Stevenson A, Waite M | chapter = Sex |title=Concise Oxford English Dictionary: Book & CD-ROM Set|publisher=[[OUP Oxford]]|isbn=978-0-19-960110-3|year=2011|page=1302|access-date=23 March 2018| chapter-url={{GBurl|id=4XycAQAAQBAJ|p=1320}}|quote=Sex: Either of the two main categories (male and female) into which humans and most other living things are divided on the basis of their reproductive functions. The fact of belonging to one of these categories. The group of all members of either sex.}}</ref><ref>{{cite book | vauthors = Mills A | chapter = Sex and Reproduction |title=Biology of Sex |date=1 January 2018 |publisher=University of Toronto Press |isbn=978-1-4875-9337-7 |pages=43–45 | chapter-url=https://books.google.com/books?id=bLhcDwAAQBAJ&pg=PA43 |access-date=3 October 2023 |language=en}}</ref><ref name="Purves-2000"/><ref>{{Cite journal |last=De Loof |first=Arnold |date=31 January 2018 |title=Only two sex forms but multiple gender variants: How to explain? |journal=Communicative & Integrative Biology |volume=11 |issue=1 |pages=e1427399 |doi=10.1080/19420889.2018.1427399 |issn=1942-0889 |pmc=5824932 |pmid=29497472}}</ref><ref name="Goymann-2022">{{cite journal |last1=Goymann |first1=Wolfgang |last2=Brumm |first2=Henrik |last3=Kappeler |first3=Peter M. |title=Biological sex is binary, even though there is a rainbow of sex roles: Denying biological sex is anthropocentric and promotes species chauvinism |journal=[[BioEssays]] |date=February 2023 |volume=45 |issue=2 |pages=e2200173 |doi=10.1002/bies.202200173 |pmid=36543364 |url=https://onlinelibrary.wiley.com/doi/10.1002/bies.202200173 |access-date=23 November 2024}}</ref> During sexual reproduction, a male and a female gamete fuse to form a [[zygote]], which develops into an [[offspring]] that inherits traits from each parent. By convention, [[organism]]s that produce smaller, more mobile gametes ([[spermatozoa]], [[sperm]]) are called ''male'', while organisms that produce larger, non-mobile gametes ([[ovum|ova]], often called egg cells) are called ''female''.<ref name="Royle-2012">{{cite book | vauthors = Kokko H, Jennions M | author-link1 = Hanna Kokko | chapter = Sex differences in parental care | veditors = Royle NJ, Smiseth PT, Kölliker M | chapter-url={{GBurl|id=K-EUDAAAQBAJ|q=Sex+differences+in+parental+care|pg=PR5}}|title=The Evolution of Parental Care|date=2012|publisher=Oxford University Press|isbn=978-0-19-969257-6|pages=103|language=en|quote=The answer is that there is an agreement by convention: individuals producing the smaller of the two gamete types{{snd}}sperm or pollen{{snd}}are males, and those producing larger gametes{{snd}}eggs or ovules{{snd}}are females.}}</ref> An organism that produces both types of gamete is a [[hermaphrodite]].<ref name="Purves-2000">{{cite book|url={{GBurl|id=kS-h84pMJw4C|p=736}}|title=Life: The Science of Biology|vauthors=Purves WK, Sadava DE, [[Gordon Orians|Orians GH]], Heller HC|publisher=[[Macmillan Publishers|Macmillan]]|year=2000|isbn=978-0-7167-3873-2|page=736|quote=A single body can function as both male and female. Sexual reproduction requires both male and female haploid gametes. In most species, these gametes are produced by individuals that are either male or female. Species that have male and female members are called dioecious (from the Greek for 'two houses'). In some species, a single individual may possess both female and male reproductive systems. Such species are called monoecious ("one house") or hermaphroditic.|access-date=23 March 2018}}</ref><ref name="Avise-2011">{{cite book | chapter = Two sexes in one | chapter-url={{GBurl|id=jqiR8C0lEckC|q=Hermaphrodite}}|title=Hermaphroditism: A Primer on the Biology, Ecology, and Evolution of Dual Sexuality|vauthors=Avise JC|date=2011|publisher=Columbia University Press|isbn=978-0-231-52715-6|pages=1–7|language=en|author-link=John Avise|access-date=18 September 2020}}</ref>
In non-hermaphroditic species, the sex of an individual is determined through one of several biological [[sex-determination system]]s. Most [[mammal|mammalian species]] have the [[XY sex-determination system]], where the male usually carries an X and a [[Y chromosome]] (XY), and the female usually carries two [[X chromosome]]s (XX). Other [[Sex-determination system#Chromosomal systems|chromosomal sex-determination system]]s in animals include the [[ZW sex-determination system|ZW system]] in birds, and the [[XO sex-determination system|XO system]] in some insects.<ref>{{cite journal | vauthors = Blackmon H, Ross L, Bachtrog D | title = Sex Determination, Sex Chromosomes, and Karyotype Evolution in Insects | journal = The Journal of Heredity | volume = 108 | issue = 1 | pages = 78–93 | date = January 2017 | pmid = 27543823 | pmc = 6281344 | doi = 10.1093/jhered/esw047 |issn = 0022-1503 }}</ref> Various [[environmental sex determination|environmental systems]] include [[temperature-dependent sex determination]] in reptiles and crustaceans.<ref name="Hake-2008">{{cite journal | vauthors = Hake L, O'Connor C | date = 2008 | title = Genetic Mechanisms of Sex Determination {{!}} Learn Science at Scitable | journal = Nature Education | volume = 1 | issue = 1 | page = 25 |url=https://www.nature.com/scitable/topicpage/genetic-mechanisms-of-sex-determination-314/ |url-status=live |archive-url=https://web.archive.org/web/20170819121941/http://www.nature.com/scitable/topicpage/genetic-mechanisms-of-sex-determination-314 |archive-date=19 August 2017 |access-date=13 April 2021 }}</ref>
The male and female of a species may be physically alike (sexual monomorphism) or have physical differences ([[sexual dimorphism]]). In sexually dimorphic species, including most birds and mammals, the sex of an individual is usually [[Sex identification|identified]] through observation of that individual's [[sexual characteristics]]. [[Sexual selection]] or [[mate choice]] can accelerate the evolution of differences between the sexes.
The terms ''male'' and ''female'' typically do not apply in sexually undifferentiated species in which the individuals are isomorphic (look the same) and the gametes are [[Isogamy|isogamous]] (indistinguishable in size and shape), such as the [[Chlorophyte|green alga]] ''[[Ulva lactuca]]''. Some kinds of functional differences between individuals, such as in [[Mating in fungi|fungi]],<ref name="Moore-2020">{{cite book |title=21st Century guidebook to fungi |vauthors=Moore D, Robson JD, [[Anthony P. J. Trinci |Trinci AP]] |date=2020 |publisher=Cambridge University Press |isbn=978-1-108-74568-0 |edition=2 |pages=211–228}}</ref> may be referred to as [[mating type]]s.<ref name="Kumar-2019">{{cite encyclopedia|title=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 |quote=Anisogamy can be defined as a mode of sexual reproduction in which fusing gametes, formed by participating parents, are dissimilar in size.|vauthors=Kumar R, Meena M, Swapnil P|veditors=Vonk J, Shackelford T|editor-link2=Todd K. Shackelford}}</ref>
==Sexual reproduction==
{{Main|Sexual reproduction}}
{{Further|Isogamy|Anisogamy}}
[[File:Sexual cycle.svg|thumb|left|The life cycle of a sexually reproducing species cycles through haploid and diploid stages]]
Sexual reproduction, in which two individuals produce an offspring that possesses a selection of the genetic traits of each parent, is exclusive to [[eukaryote]]s. Genetic traits are encoded in the [[DNA|deoxyribonucleic acid]] (DNA) of [[chromosome]]s. The eukaryote cell has a set of paired [[homologous chromosomes]], one from each parent, and this double-chromosome stage is called "[[diploid]]". During sexual reproduction, a diploid organism produces specialized [[haploid]] sex cells called [[gametes]] via [[meiosis]],<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|___location=New York | chapter = Meiosis | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK26840/ | archive-url = https://web.archive.org/web/20170125115052/https://www.ncbi.nlm.nih.gov/books/NBK26840/ | archive-date=25 January 2017}}</ref> each of which has a single set of chromosomes. Meiosis involves a stage of [[genetic recombination]] via [[chromosomal crossover]], in which regions of DNA are exchanged between matched pairs of chromosomes, to form new chromosomes, each with a new combination of the genes of the parents. Then the chromosomes are separated into single sets in the gametes. When gametes fuse during fertilization, the resulting zygote has half of the genetic material of the mother and half of the father.<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|___location=New York | chapter = The Benefits of Sex | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK26823/ }}</ref> The combination of chromosomal crossover and [[fertilization]], bringing the two single sets of chromosomes together to make a new diploid [[zygote]], results in a new organism that contains a different set of the genetic traits of each parent.
In [[animal]]s, the haploid stage only occurs in the gametes, the sex cells that fuse to form a zygote that develops directly into a new diploid organism. In a [[plant]] species, the diploid organism produces a type of haploid [[spore]] by meiosis that is capable of undergoing repeated [[mitosis|cell division]] to produce a [[multicellular]] haploid organism. In either case, the gametes may be externally similar ([[isogamy]]) as in the green alga ''Ulva'' or may be different in size and other aspects ([[anisogamy]]).<ref>{{cite book |title=Developmental Biology |vauthors=[[Scott F. Gilbert | Gilbert SF]] |date=2000 |publisher=Sinauer Associates |isbn=978-0-87893-243-6 |edition=6th |___location=Sunderland (MA) |chapter=Multicellularity: Evolution of Differentiation |access-date=17 April 2021 |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK10031/ |archive-url=https://web.archive.org/web/20210308143406/https://www.ncbi.nlm.nih.gov/books/NBK10031/ |archive-date=8 March 2021 |url-status=live}}</ref> The size difference is greatest in [[oogamy]], a type of anisogamy in which a small, [[motile]] gamete combines with a much larger, non-motile gamete.<ref>{{cite book|url={{GBurl|id=wzZGQOmcjqAC|q=a%20dictionary%20of%20plant%20sciences%20oogamy|p=350}}|title=A Dictionary of Plant Sciences|vauthors=Allaby M|date=2012|publisher=OUP Oxford|isbn=978-0-19-960057-1|page=350|language=en|author-link=Michael Allaby}}</ref>
In anisogamic organisms, by convention, the larger gamete (called an [[ovum]], or egg cell) is considered female, while the smaller gamete (called a spermatozoon, or sperm cell) is considered male. An individual that produces large gametes is female, and one that produces small gametes is male.<ref>{{cite news | vauthors = Gee H |author-link=Henry Gee |date=22 November 1999 |title=Size and the single sex cell |url=https://www.nature.com/news/1999/991122/full/news991125-4.html |url-status=live |archive-url=https://web.archive.org/web/20171011062235/http://www.nature.com/news/1999/991122/full/news991125-4.html |archive-date=11 October 2017 |access-date=4 June 2018 |work=Nature |ref=10.1038/news991125-4}}</ref> An individual that produces both types of gamete is a [[hermaphrodite]]. In some species, a hermaphrodite can [[self-fertilization|self-fertilize]] and produce an offspring on its own.
===Animals===
{{Main|Sexual reproduction#Animals}}
[[File:Hoverflies mating midair.jpg|thumb|left|''[[Simosyrphus grandicornis]]'' mating]]
Most sexually reproducing animals spend their lives as diploid, with the haploid stage reduced to single-cell gametes.<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|___location=New York | chapter = Mendelian genetics in eukaryotic life cycles | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK21836 | archive-url = https://web.archive.org/web/20170402185423/https://www.ncbi.nlm.nih.gov/books/NBK21836/ | archive-date = 2 April 2017}}</ref> The gametes of animals have male and female forms—[[spermatozoa]] and egg cells, respectively. These gametes combine to form [[embryos]] which develop into new organisms.
The male gamete, a [[spermatozoon]] (produced in vertebrates within the [[testes]]), is a small cell containing a single long [[flagellum]] which propels it.<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|___location=New York | chapter = Sperm | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK26914/ |url-status=live| archive-url = https://web.archive.org/web/20090629222617/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3729 | archive-date= 29 June 2009 }}</ref> Spermatozoa are extremely reduced cells, lacking many cellular components that would be necessary for embryonic development. They are specialized for motility, seeking out an egg cell and fusing with it in a process called [[Fertilization#Fertilisation in animals|fertilization]].
Female gametes are egg cells. In vertebrates, they are produced within the [[ovary|ovaries]]. They are large, immobile cells that contain the nutrients and cellular components necessary for a developing embryo.<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|___location=New York | chapter = Eggs | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK26842/ |url-status=live| archive-url = https://web.archive.org/web/20090629074430/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3718 | archive-date= 29 June 2009}}</ref> Egg cells are often associated with other cells which support the development of the embryo, forming an [[Egg (biology)|egg]]. In mammals, the fertilized embryo instead develops within the female, receiving nutrition directly from its mother.
Animals are usually<!--h'm. but not Sponges, Corals, Bryozoa, Barnacles, ...--> mobile and seek out a partner of the opposite sex for [[mating]]. Animals which live in the water can mate using [[external fertilization]], where the eggs and sperm are released into and combine within the surrounding water.<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|___location=New York | chapter = Fertilization | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK26843/ |url-status=live| archive-url = https://web.archive.org/web/20081219005819/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3738 | archive-date= 19 December 2008 }}</ref> Most animals that live outside of water, however, use [[internal fertilization]], transferring sperm directly into the female to prevent the gametes from drying up.<!--Dragonflies use indirect method, transferring a spermatheca.-->
In most birds, both excretion and reproduction are done through a single posterior opening, called the [[cloaca]]—male and female birds touch cloaca to transfer sperm, a process called "cloacal kissing".<ref>{{cite web |title=Avian Reproduction |url=http://people.eku.edu/ritchisong/avianreproduction.html |publisher=Eastern Kentucky University | vauthors = Ritchison G |access-date=3 April 2008 |archive-date=12 April 2008 |archive-url=https://web.archive.org/web/20080412231002/http://people.eku.edu/ritchisong/avianreproduction.html |url-status=live }}</ref> In many other terrestrial animals, males use specialized [[sex organ]]s to assist the transport of sperm—these male sex organs are called [[intromittent organ]]s. In humans and other mammals, this male organ is known as the [[penis]], which enters the female reproductive tract (called the [[vagina]]) to achieve [[insemination]]—a process called [[sexual intercourse]]. The penis contains a tube through which [[semen]] (a fluid containing sperm) travels. In female mammals, the vagina connects with the [[uterus]], an organ which directly supports the development of a fertilized embryo within (a process called [[gestation]]).
Because of their motility, [[animal sexual behavior]] can involve coercive sex. [[Traumatic insemination]], for example, is used by some insect species to inseminate females through a wound in the abdominal cavity—a process detrimental to the female's health.
===Plants===
{{Main|Plant reproduction}}
[[File:Mature flower diagram.svg|thumb|left|Flowers contain the sexual organs of flowering plants. They are usually hermaphrodite, containing both male and female parts.]]
Like animals, [[land plants]] have specialized male and female gametes.<ref>{{cite book |title=Developmental Biology |vauthors=[[ Scott F. Gilbert | Gilbert SF]] |date=2000 |publisher=Sinauer Associates |isbn=978-0-87893-243-6 |edition=6th |___location=Sunderland (MA) |chapter=Gamete Production in Angiosperms |access-date=17 April 2021 |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK10129/ |archive-url=https://web.archive.org/web/20210421143911/https://www.ncbi.nlm.nih.gov/books/NBK10129/ |archive-date=21 April 2021 |url-status=live}}</ref><ref name="Dusenbery-2009">{{cite book|url={{GBurl|id=QCrimQJu1RAC|q=living+at+micro+scale+reproduction|p=308}}|title=Living at Micro Scale: The Unexpected Physics of Being Small|vauthors=Dusenbery DB|date=2009|publisher=Harvard University Press|isbn=978-0-674-03116-6|pages=308–326|language=en|author-link=David B. Dusenbery}}</ref> In [[Spermatophyte|seed plants]], male gametes are produced by reduced male [[gametophytes]] that are contained within [[pollen]] which have hard coats that protect the male gamete forming cells during transport from the [[Stamen|anther]]s to the [[Stigma (botany)|stigma]]. The female gametes of seed plants are contained within [[ovule]]s. Once fertilized, these form [[seed]]s which, like eggs, contain the nutrients necessary for the initial development of the embryonic plant.
{{multiple image
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| image1 = Pinus nigra cone.jpg
| image2 = Pine cones, immature male.jpg
| footer = Female (left) and male (right) cones contain the sex organs of pines and other conifers.
}}
The [[flower]]s of [[flowering plant]]s contain their sexual organs. Most flowering plants are hermaphroditic, with both male and female parts in the same flower or on the same plant in single sex flowers. About 5% of plant species have individual plants that are one sex or the other.<ref>{{cite web | vauthors = Wilton P | date = 12 March 2009 |title=Plants, sex & Darwin | work = OxSciBlog | publisher = University of Oxford |url=https://www.ox.ac.uk/news/science-blog/plants-sex-darwin |access-date=10 January 2024 |language=en}}</ref> The female parts, in the center of a hermaphroditic or female flower, are the [[pistils]], each unit consisting of a [[carpel]], a [[Style (botany)|style]] and a [[stigma (botany)|stigma]]. Two or more of these reproductive units may be merged to form a single compound [[pistil]], the fused carpels forming an [[Ovary (botany)|ovary]]. Within the carpels are [[ovules]] which develop into seeds after fertilization. The male parts of the flower are the [[stamen]]s: these consist of long filaments arranged between the pistil and the petals that produce pollen in [[Glossary of botanical terms#anther|anthers]] at their tips. When a pollen grain lands upon the stigma on top of a carpel's style, it germinates to produce a [[pollen tube]] that grows down through the tissues of the style into the carpel, where it delivers male gamete nuclei to fertilize an ovule that eventually develops into a seed.
Some hermaphroditic plants are self-fertile, but plants have evolved multiple different [[self-incompatibility]] mechanisms to avoid self-fertilization, involving [[sequential hermaphroditism]], molecular recognition systems and morphological mechanisms such as [[heterostyly]].<ref name="Judd-2002">{{cite book | vauthors = Judd WS, Campbell CS, Kellogg EA, Stevens PF, Donoghue MJ |author-link1=Walter Stephen Judd |author-link3=Elizabeth Anne Kellogg |author-link4=Peter F. Stevens |author-link5=Michael Donoghue |title=Plant systematics, a phylogenetic approach |date=2002 |publisher=Sinauer Associates Inc. |isbn=0-87893-403-0 |edition=2nd |___location=Sunderland MA}}</ref>{{rp|73, 74}}
In [[pine]]s and other [[conifer]]s, the sex organs are produced within [[conifer cone|cones]] that have male and female forms. Male cones are smaller than female ones and produce pollen, which is transported by wind to land in female cones. The larger and longer-lived female cones are typically more durable, and contain ovules within them that develop into seeds after fertilization.
Because [[Spermatophyte|seed plants]] are immobile, they depend upon passive methods for transporting pollen grains to other plants. Many, including conifers and grasses, produce lightweight pollen which is carried by wind to neighboring plants. Some flowering plants have heavier, sticky pollen that is specialized for transportation by insects or larger animals such as [[hummingbird]]s and [[bat]]s, which may be attracted to flowers containing rewards of nectar and pollen. These animals transport the pollen as they move to other flowers, which also contain female reproductive organs, resulting in [[pollination]].
===Fungi===
{{Main|Mating in fungi}}
[[File:Shiitake mushroom.jpg|thumb|Mushrooms are produced as part of fungal sexual reproduction.]]
Most species of [[fungus]] can reproduce sexually and have life cycles with both haploid and diploid phases. These species of fungus are typically [[isogamy|isogamous]], i.e. lacking male and female specialization. One haploid fungus grows into contact with another, and then they fuse their cells. In some cases, the fusion is asymmetric, and the cell which donates only a nucleus (and no accompanying cellular material) could arguably be considered male.<ref>{{cite book |author=Nick Lane |title=Power, Sex, Suicide: Mitochondria and the Meaning of Life |url=https://archive.org/details/powersexsuicidem0000lane |url-access=registration |pages=[https://archive.org/details/powersexsuicidem0000lane/page/236 236–237] |isbn=978-0-19-280481-5 |year=2005 |publisher=Oxford University Press}}</ref> Fungi may also have more complex allelic mating systems, with other sexes not accurately described as male, female, or hermaphroditic.<ref name="Watkinson-2015">{{cite book|url={{GBurl|id=x8qcBAAAQBAJ|p=115}}|title=The Fungi|vauthors=Watkinson SC, [[Lynne Boddy|Boddy L]], Money N|publisher=Elsevier Science|year=2015|isbn=978-0-12-382035-8|page=115|access-date=18 February 2018}}</ref>
Some fungi, including [[baker's yeast]], have [[mating type]]s that determine compatibility. Yeasts with the same mating types will not fuse with each other to form diploid cells, only with yeast carrying another mating type.<ref>{{cite book | vauthors = Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D, Darnell J |year=2000 |title=Molecular Cell Biology |edition=Fourth |publisher=W.H. Freeman and Co |isbn=978-0-7167-4366-8 | chapter = Cell-Type Specification and Mating-Type Conversion in Yeast | chapter-url = https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mcb.section.3752 |archive-url=https://web.archive.org/web/20090701170903/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mcb.section.3752 | archive-date=1 July 2009 }}</ref>
Many species of [[Dikarya|higher fungi]] produce [[mushroom]]s as part of their [[Fungus#Reproduction|sexual reproduction]]. Within the mushroom, diploid cells are formed, later dividing into haploid [[spore]]s.
== Sexual systems ==
{{Main|Sexual system}}A sexual system is a distribution of male and female functions across organisms in a species.<ref name="Leonard-2013" />
=== Animals ===
Approximately 95% of [[animal]] species have separate male and female individuals, and are said to be [[gonochoric]]. About 5% of animal species are hermaphroditic.<ref name="Leonard-2013">{{cite journal| vauthors = Leonard JL |date=22 August 2013|title=Williams' Paradox and the Role of Phenotypic Plasticity in Sexual Systems|journal=Integrative and Comparative Biology|volume=53|issue=4|pages=671–688|doi=10.1093/icb/ict088|pmid=23970358|issn=1540-7063|doi-access=free}}</ref> This low percentage is partially attributable to the very large number of [[insect]] species, in which hermaphroditism is absent.<ref name="Bachtrog-2014">{{cite journal |vauthors=Bachtrog D, [[Judith Mank | Mank JE]], Peichel CL, [[Mark Kirkpatrick | Kirkpatrick M]], [[Sarah Otto | Otto SP]], Ashman TL, Hahn MW, Kitano J, Mayrose I, Ming R, Perrin N, Ross L, Valenzuela N, Vamosi JC |date=July 2014 |title=Sex determination: why so many ways of doing it? |journal=PLOS Biology |volume=12 |issue=7 |pages=e1001899 |doi=10.1371/journal.pbio.1001899 |pmc=4077654 |pmid=24983465 |doi-access=free}}</ref> About 99% of [[vertebrate]]s are gonochoric, and the remaining 1% that are hermaphroditic are almost all fishes.<ref>{{cite journal|vauthors=Kuwamura T, Sunobe T, Sakai Y, Kadota T, Sawada K|date=1 July 2020|title=Hermaphroditism in fishes: an annotated list of species, phylogeny, and mating system|journal=Ichthyological Research|language=en|volume=67|issue=3|pages=341–360|doi=10.1007/s10228-020-00754-6|bibcode=2020IchtR..67..341K |issn=1616-3915|doi-access=free|s2cid=218527927}}</ref>
=== Plants ===
The majority of plants are [[Glossary of botanical terms#bisexual|bisexual]],<ref name="Kliman-2016">{{cite book | vauthors = Kliman RM |url={{GBurl|id=_r4OCAAAQBAJ}} |title=Encyclopedia of Evolutionary Biology |date=2016 |publisher=Academic Press |isbn=978-0-12-800426-5 |volume=2 |___location= |pages=212–224 |archive-url=https://web.archive.org/web/20210506205920/https://www.google.com/books/edition/Encyclopedia_of_Evolutionary_Biology/_r4OCAAAQBAJ?hl=en&gbpv=0&kptab=overview |archive-date=6 May 2021 |url-status=live |access-date=14 April 2021 }}</ref>{{Rp|page=212}} either hermaphrodite (with both stamens and pistil in the same flower) or [[Monoecious (botany)|monoecious]].<ref name="Sabath-2016">{{cite journal | vauthors = Sabath N, Goldberg EE, Glick L, Einhorn M, Ashman TL, Ming R, Otto SP, Vamosi JC, Mayrose I | title = Dioecy does not consistently accelerate or slow lineage diversification across multiple genera of angiosperms | journal = The New Phytologist | volume = 209 | issue = 3 | pages = 1290–1300 | date = February 2016 | pmid = 26467174 | doi = 10.1111/nph.13696 | doi-access = free | bibcode = 2016NewPh.209.1290S }}</ref><ref name="Beentje-2016">{{cite book|title=The Kew plant glossary|vauthors=[[Henk Jaap Beentje|Beentje H]]|date=2016|publisher=Kew Publishing|isbn=978-1-84246-604-9|edition=2|___location=Royal Botanic Gardens, Kew}}</ref> In [[Dioecy|dioecious]] species male and female sexes are on separate plants.<ref>{{cite journal | vauthors = Leite Montalvão AP, Kersten B, Fladung M, Müller NA | title = The Diversity and Dynamics of Sex Determination in Dioecious Plants | language = English | journal = Frontiers in Plant Science | volume = 11 | pages = 580488 | date = 2021 | pmid = 33519840 | pmc = 7843427 | doi = 10.3389/fpls.2020.580488 | doi-access = free | bibcode = 2021FrPS...1180488L }}</ref> About 5% of flowering plants are dioecious, resulting from as many as 5000 independent origins.<ref name="Renner-2014">{{cite journal | vauthors = Renner SS | title = The relative and absolute frequencies of angiosperm sexual systems: dioecy, monoecy, gynodioecy, and an updated online database | journal = American Journal of Botany | volume = 101 | issue = 10 | pages = 1588–1596 | date = October 2014 | pmid = 25326608 | doi = 10.3732/ajb.1400196 | doi-access = free }}</ref> Dioecy is common in [[gymnosperm]]s, in which about 65% of species are dioecious, but most [[conifers]] are monoecious.<ref name="Walas-2018">{{cite journal|vauthors=Walas Ł, Mandryk W, Thomas PA, Tyrała-Wierucka Ż, Iszkuło G|date=2018|title=Sexual systems in gymnosperms: A review|url=http://eprints.keele.ac.uk/4961/1/29052018_1-s2.0-S1439179117304498-main.pdf|journal=Basic and Applied Ecology|volume=31|pages=1–9|doi=10.1016/j.baae.2018.05.009|bibcode=2018BApEc..31....1W |s2cid=90740232|access-date=7 June 2021|archive-date=27 January 2022|archive-url=https://web.archive.org/web/20220127084144/https://eprints.keele.ac.uk/4961/1/29052018_1-s2.0-S1439179117304498-main.pdf|url-status=live}}</ref>
==Evolution of sex==
{{Main|Evolution of sexual reproduction}}
{{multiple image
| total_width = 220
| image1 = Anisogamy.svg
| caption1 = Different forms of [[anisogamy]]:<br />A) anisogamy of motile cells, B) oogamy (egg cell and sperm cell), C) anisogamy of non-motile cells (egg cell and spermatia).
| image2 = Isogamy.svg
| caption2 = Different forms of isogamy:<br />A) isogamy of [[motile cell]]s, B) isogamy of non-motile cells, C) conjugation.
}}
It is generally accepted that [[Isogamy#Evolution|isogamy was ancestral]] to [[anisogamy]]<ref name="Awasthi-2015">{{cite book | vauthors = Awasthi AK |url={{GBurl|id=r0h1DwAAQBAJ|q=isogamy+generally+accepted|p=363}} |title=Textbook of Algae |publisher=Vikas Publishing House |isbn=978-93-259-9022-7 |page=363 |language=en}}</ref> and that anisogamy [[Evolved independently|evolved several times independently]] in different groups of eukaryotes, including protists, algae, plants, and animals.<ref name="Bachtrog-2014" /> The [[Anisogamy#Evolution|evolution of anisogamy]] is synonymous with the [[Male#Evolution|origin of male]] and the [[Female#Evolution|origin of female]].<ref name="Lehtonen-2016" /> It is also the [[Sexual dimorphism#Evolution|first step towards sexual dimorphism]]<ref name="Togashi-2012">{{cite journal | vauthors = Togashi T, Bartelt JL, Yoshimura J, Tainaka K, Cox PA | title = Evolutionary trajectories explain the diversified evolution of isogamy and anisogamy in marine green algae | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 109 | issue = 34 | pages = 13692–13697 | date = August 2012 | pmid = 22869736 | pmc = 3427103 | doi = 10.1073/pnas.1203495109 | bibcode = 2012PNAS..10913692T | doi-access = free | author-link5 = Paul Alan Cox }}</ref> and influenced the evolution of various sex differences.<ref name="Székely-2007">{{cite book | vauthors = Székely T, Fairbairn DJ, Blanckenhorn WU |author-link1 = Tamás Székely (biologist) |url={{GBurl|id=IDoTDAAAQBAJ|q=anisogamy+lead+to+sex+differences}} |title=Sex, Size and Gender Roles: Evolutionary Studies of Sexual Size Dimorphism |date=2007|publisher=OUP Oxford |isbn=978-0-19-920878-4 |pages=167–169, 176, 185 |language=en }}</ref>
It is unclear whether anisogamy first led to [[Hermaphroditism#Evolution|the evolution of hermaphroditism]] or the [[Gonochorism#Evolution|evolution of gonochorism]],<ref name="Kliman-2016" />{{Rp|page=213}} and [[Gamete#Evolution|the evolution of sperm and eggs]] has left no fossil evidence.<ref name="Pitnick-2008">{{cite book |url={{GBurl|id=kctYNbO1fE0C|q=isogamy+in+multicellular+organisms|p=44}} |title=Sperm Biology: An Evolutionary Perspective |vauthors=Pitnick SS, Hosken DJ, Birkhead TR |author-link3=Tim Birkhead |date=2008 |publisher=Academic Press |isbn=978-0-08-091987-4 |pages=43–44 |language=en}}</ref>
A 1.2 billion year old fossil from ''[[Bangiomorpha pubescens]]'' has provided the oldest fossil record for the differentiation of male and female reproductive types and shown that sexes evolved early in eukaryotes.<ref name="Hörandl-2020">{{cite journal | vauthors = Hörandl E, Hadacek F | title = Oxygen, life forms, and the evolution of sexes in multicellular eukaryotes | journal = Heredity | volume = 125 | issue = 1–2 | pages = 1–14 | date = August 2020 | pmid = 32415185 | pmc = 7413252 | doi = 10.1038/s41437-020-0317-9 | bibcode = 2020Hered.125....1H }}</ref> Studies on [[green algae]] have provided genetic evidence for the [[Mating type#Evolution|evolutionary link between sexes and mating types]].<ref name="Sawada-2014">{{cite book | veditors = Sawada H, Inoue N, Iwano M |url=https://directory.doabooks.org/handle/20.500.12854/29412 |title=Sexual Reproduction in Animals and Plants |date=2014 |publisher=Springer |isbn=978-4-431-54589-7 |pages=215–227 |doi=10.1007/978-4-431-54589-7 |language=en}}</ref>
The original form of sex was [[external fertilization]]. [[Internal fertilization]], or sex as we know it, evolved later<ref>{{cite web | vauthors = Black R |date=19 October 2014 |title=Armored Fish Pioneered Sex As You Know It |url=https://www.nationalgeographic.com/animals/article/141019-fossil-fish-evolution-sex-fertilization |archive-url=https://web.archive.org/web/20210302234743/https://www.nationalgeographic.com/animals/article/141019-fossil-fish-evolution-sex-fertilization |url-status=live |url-access=registration |archive-date=2 March 2021 |access-date=10 July 2023 | work = Animals | publisher = National Geographic |language=en}}</ref> and became dominant for vertebrates after their [[vertebrate land invasion|emergence on land]].<ref>{{cite web|date=17 July 2018|title=43.2A: External and Internal Fertilization|url=https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.2%3A_Fertilization/43.2A%3A_External_and_Internal_Fertilization|access-date=9 November 2020|website=Biology LibreTexts|language=en|archive-date=24 May 2022|archive-url=https://web.archive.org/web/20220524084927/https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.2%3A_Fertilization/43.2A%3A_External_and_Internal_Fertilization|url-status=live}}</ref>
===Adaptive function of sex===
The most basic role of [[meiosis]] appears to be conservation of the integrity of the [[genome]] that is passed on to progeny by parents.<ref name="Brandeis-2018">{{cite journal | vauthors = Brandeis M | title = New-age ideas about age-old sex: separating meiosis from mating could solve a century-old conundrum | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 93 | issue = 2 | pages = 801–810 | date = May 2018 | pmid = 28913952 | doi = 10.1111/brv.12367 }}</ref><ref name="Hörandl-2024">{{cite journal | vauthors = Hörandl E | title = Apomixis and the paradox of sex in plants | journal = Annals of Botany | volume = 134 | issue = 1 | pages = 1–18 | date = June 2024 | pmid = 38497809 | doi = 10.1093/aob/mcae044 | pmc = 11161571 }}</ref> The two most fundamental aspects of [[sexual reproduction]], [[genetic recombination|meiotic recombination]] and [[outcrossing]], are likely maintained respectively by the adaptive advantages of recombinational repair of genomic [[DNA damage (naturally occurring)|DNA damage]] and genetic [[complementation (genetics)|complementation]] which masks the expression of deleterious recessive [[mutation]]s.<ref name="Bernstein-1985">{{cite journal | vauthors = Bernstein H, Byerly HC, Hopf FA, Michod RE | title = Genetic damage, mutation, and the evolution of sex | journal = Science | ___location = New York, N.Y. | volume = 229 | issue = 4719 | pages = 1277–81 | date = September 1985 | pmid = 3898363 | doi = 10.1126/science.3898363 | bibcode = 1985Sci...229.1277B }}</ref> [[Genetic variation]], often produced as a byproduct of these processes, may provide long-term advantages in those sexual lineages that favor [[outcrossing]].<ref name="Bernstein-1985"/>
== Sex-determination systems ==
{{Main|Sex-determination system}}
[[File:Evolsex-dia2a.svg|thumb|left|Sex helps the spread of advantageous traits through recombination. The diagrams compare the evolution of [[allele frequency]] in a sexual population (top) and an asexual population (bottom). The vertical axis shows frequency and the horizontal axis shows time. The alleles a/A and b/B occur at random. The advantageous alleles A and B, arising independently, can be rapidly combined by sexual reproduction into the most advantageous combination AB. Asexual reproduction takes longer to achieve this combination because it can only produce AB if A arises in an individual which already has B or vice versa.]]
The biological cause of an organism developing into one sex or the other is called ''sex determination''. The cause may be genetic, environmental, [[haplodiploidy]], or multiple factors.<ref name="Bachtrog-2014" /> Within animals and other organisms that have genetic sex-determination systems, the determining factor may be the presence of a [[sex chromosome]]. In plants that are sexually dimorphic, such as ''Ginkgo biloba'',<ref name="Judd-2002"/>{{rp|203}} the liverwort ''[[Marchantia polymorpha]]'' or the dioecious species in the flowering plant genus ''[[Silene]]'', sex may also be determined by sex chromosomes.<ref name="Tanurdzic-2004">{{cite journal | vauthors = Tanurdzic M, Banks JA | title = Sex-determining mechanisms in land plants | journal = The Plant Cell | volume = 16 | issue = Suppl | pages = S61–S71 | date = 2004 | pmid = 15084718 | pmc = 2643385 | doi = 10.1105/tpc.016667 | bibcode = 2004PlanC..16S..61T }}</ref> Non-genetic systems may use environmental cues, such as the [[Temperature-dependent sex determination|temperature]] during early development in [[crocodile]]s, to determine the sex of the offspring.<ref name="Warner-2008">{{cite journal | vauthors = Warner DA, Shine R | title = The adaptive significance of temperature-dependent sex determination in a reptile | journal = Nature | volume = 451 | issue = 7178 | pages = 566–568 | date = January 2008 | pmid = 18204437 | doi = 10.1038/nature06519 | bibcode = 2008Natur.451..566W | s2cid = 967516 }}</ref>
[[Sex-determination system|Sex determination]] is often distinct from [[Sexual differentiation|sex differentiation]]. Sex determination is the designation for the development stage towards either male or female while sex differentiation is the pathway towards the development of the [[phenotype]].<ref>{{cite book| vauthors = Beukeboom LW, Perrin N |url={{GBurl|id=7yrnAwAAQBAJ|q=the+evolution+of+sex+determination}}|title=The Evolution of Sex Determination |date=2014|publisher=Oxford University Press|isbn=978-0-19-965714-8|pages=16|language=en}}</ref>
=== Genetic ===
==== XY sex determination ====
[[File:Drosophila XY sex-determination.svg|thumb|The common fruit fly has an [[XY sex-determination system]], as do humans and most mammals.]]
Humans and most other [[mammal]]s have an [[XY sex-determination system]]: the [[Y chromosome]] carries factors responsible for triggering male development, making XY sex determination mostly based on the presence or absence of the [[Y chromosome]]. It is the male [[gamete]] that determines the sex of the offspring.<ref name="Wallis-2008">{{cite journal|vauthors=Wallis MC, Waters PD, Graves JA|date=October 2008|title=Sex determination in mammals – before and after the evolution of SRY|journal=Cellular and Molecular Life Sciences|volume=65|issue=20|pages=3182–95|doi=10.1007/s00018-008-8109-z|pmid=18581056|s2cid=31675679|pmc=11131626}}</ref> In this system XX mammals typically are female and XY typically are male.<ref name="Bachtrog-2014" /> However, individuals with [[Klinefelter syndrome|XXY]] or [[XYY syndrome|XYY]] are males, while individuals with [[Turner syndrome|X]] and [[Triple X syndrome|XXX]] are females.<ref name="Hake-2008" /> Unusually, the [[platypus]], a [[monotreme]] mammal, has ten sex chromosomes; females have ten X chromosomes, and males have five X chromosomes and five Y chromosomes. Platypus egg cells all have five X chromosomes, whereas sperm cells can either have five X chromosomes or five Y chromosomes.<ref>{{cite book | vauthors = Pierce BA |title=Genetics: a conceptual approach |date=2012 |publisher=W.H. Freeman |isbn=978-1-4292-3250-0 |edition=4th |___location=New York |pages=73–74 |oclc=703739906}}</ref>
XY sex determination is found in other organisms, including insects like the [[Drosophila melanogaster|common fruit fly]],<ref name="Kaiser-2010">{{cite journal|vauthors=Kaiser VB, Bachtrog D|year=2010|title=Evolution of sex chromosomes in insects|journal=Annual Review of Genetics|volume=44|pages=91–112|doi=10.1146/annurev-genet-102209-163600|pmc=4105922|pmid=21047257}}</ref> and some plants.<ref name="Dellaporta-1993">{{cite journal | vauthors = Dellaporta SL, Calderon-Urrea A | title = Sex determination in flowering plants | journal = The Plant Cell | volume = 5 | issue = 10 | pages = 1241–1251 | date = October 1993 | pmid = 8281039 | pmc = 160357 | doi = 10.1105/tpc.5.10.1241 | jstor = 3869777 }}</ref> In some cases, it is the number of X chromosomes that determines sex rather than the presence of a Y chromosome.<ref name="Hake-2008" /> In the fruit fly individuals with XY are male and individuals with XX are female; however, individuals with XXY or XXX can also be female, and individuals with X can be males.<ref>{{cite book| vauthors = Fusco G, Minelli A |author-link2=Alessandro Minelli (biologist) |url={{GBurl|id=AKGsDwAAQBAJ}}|title=The Biology of Reproduction|year=2019|publisher=Cambridge University Press|isbn=978-1-108-49985-9|pages=306–308 }}</ref>
==== ZW sex determination ====
In birds, which have a [[ZW sex-determination system]], the W chromosome carries factors responsible for female development, and default development is male.<ref>{{cite journal | vauthors = Smith CA, Katz M, Sinclair AH | title = DMRT1 is upregulated in the gonads during female-to-male sex reversal in ZW chicken embryos | journal = Biology of Reproduction | volume = 68 | issue = 2 | pages = 560–570 | date = February 2003 | pmid = 12533420 | doi = 10.1095/biolreprod.102.007294 | doi-access = free }}</ref> In this case, ZZ individuals are male and ZW are female. It is the female [[gamete]] that determines the sex of the offspring. This system is used by birds, some fish, and some [[crustacean]]s.<ref name="Hake-2008" />
The majority of [[Lepidoptera|butterflies and moths]] also have a ZW sex-determination system. Females can have Z, ZZW, and even ZZWW.<ref>{{cite book| vauthors = Majerus ME | author-link = Michael Majerus |url= {{GBurl|id=vDHOYPQ2mmYC|q=ZW+sex+determination}}|title=Sex Wars: Genes, Bacteria, and Biased Sex Ratios|date=2003|publisher=Princeton University Press|isbn=978-0-691-00981-0|page=59|language=en}}</ref>
==== XO sex determination ====
In the [[XO sex-determination system]], males have one X chromosome (XO) while females have two (XX). All other chromosomes in these diploid organisms are paired, but organisms may inherit one or two X chromosomes. This system is found in most [[arachnid]]s, insects such as [[silverfish]] ([[Apterygota]]), [[dragonfly|dragonflies]] ([[Paleoptera]]) and [[grasshopper]]s ([[Exopterygota]]), and some nematodes, crustaceans, and gastropods.<ref name="Bull-1983">{{cite book|title=Evolution of sex determining mechanisms|vauthors=Bull JJ|year=1983|isbn=0-8053-0400-2|page=17|publisher=Benjamin/Cummings Publishing Company, Advanced Book Program |author-link=James J. Bull}}</ref><ref>{{cite journal|vauthors=Thiriot-Quiévreux C|date=2003|title=Advances in chromosomal studies of gastropod molluscs.|journal=Journal of Molluscan Studies|volume=69|issue=3|pages=187–202|doi=10.1093/mollus/69.3.187|doi-access=free}}</ref>
In [[Gryllus|field cricket]]s, for example, insects with a single X chromosome develop as male, while those with two develop as female.<ref>{{cite journal |title=Karyotypes of two American field crickets: Gryllus rubens and Gryllus sp. (Orthoptera: Gryllidae) | vauthors = Yoshimura A |journal=Entomological Science |volume=8 |issue=3 |pages=219–222 |year=2005 |doi=10.1111/j.1479-8298.2005.00118.x| s2cid = 84908090 }}</ref>
In the nematode ''[[Caenorhabditis elegans]]'', most worms are self-fertilizing hermaphrodites with an XX karyotype, but occasional abnormalities in chromosome inheritance can give rise to individuals with only one X chromosome—these XO individuals are fertile males (and half their offspring are male).<ref>{{cite book|title=''C. elegans'' II|vauthors=[[Barbara J. Meyer|Meyer BJ]]|publisher=Cold Spring Harbor Laboratory Press|year=1997|isbn=978-0-87969-532-3|veditors=Riddle DL, Blumenthal T, Meyer BJ, Priess JR|chapter=Sex Determination and X Chromosome Dosage Compensation: Sexual Dimorphism|chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK20094/|access-date=17 April 2021|archive-date=6 May 2021|archive-url=https://web.archive.org/web/20210506154914/http://www.ncbi.nlm.nih.gov/books/NBK20094/|url-status=live}}</ref>
==== ZO sex determination ====
In the [[ZO sex-determination system]], males have two Z chromosomes whereas females have one. This system is found in several species of moths.<ref>{{cite book | vauthors = De Prins J, Saithoh K | chapter = Karyology and Sex Determination |chapter-url={{GBurl|id=5w8FgSGuH34C|q=ZO+sex-determination+system+moth|p=461}} | veditors = Kristensen N |title=Handbuch Der Zoologie / Handbook of Zoology | volume = Arthropoda: Insecta: Lepidoptera, Moths and Butterflies |publisher=Walter de Gruyter|year=2003 | pages = 449–468 | doi = 10.1515/9783110893724.449 |isbn=978-3-11-016210-3|access-date=29 September 2020|via=Google Books}}</ref>
===Environmental===
{{main|Environmental sex determination}}
For many species, sex is not determined by inherited traits, but instead by environmental factors such as temperature experienced during development or later in life.<ref>{{cite journal | vauthors = Janzen FJ, Phillips PC | title = Exploring the evolution of environmental sex determination, especially in reptiles | journal = Journal of Evolutionary Biology | volume = 19 | issue = 6 | pages = 1775–1784 | date = November 2006 | pmid = 17040374 | doi = 10.1111/j.1420-9101.2006.01138.x }}</ref>
In the [[fern]] ''[[Ceratopteris]]'' and other [[Glossary of botanical terms#homospory|homosporous]] fern species, the default sex is hermaphrodite, but individuals which grow in soil that has previously supported hermaphrodites are influenced by the [[pheromone]] [[antheridiogen]] to develop as male.<ref name="Tanurdzic-2004" /> The [[bonelliidae|bonelliidae larvae]] can only develop as males when they encounter a female.<ref name="Bachtrog-2014" />
==== Sequential hermaphroditism ====
[[File:Ocellaris clownfish.JPG|thumb|[[Clownfish]]es are initially male; the largest fish in a group becomes female.]]
Some species can change sex over the course of their lifespan, a phenomenon called [[sequential hermaphroditism]].<ref name="Fusco-2019">{{cite book| vauthors = Fusco G, Minelli A |author-link2=Alessandro Minelli (biologist) |url={{GBurl|id=AKGsDwAAQBAJ|q=sequential+hermaphroditism+in+plants}}|title=The Biology of Reproduction |date=2019|publisher=Cambridge University Press|isbn=978-1-108-49985-9|pages=124|language=en}}</ref>
[[Teleost|Teleost fishes]] are the only vertebrate [[Lineage (evolution)|lineage]] where sequential hermaphroditism occurs. In [[clownfish]], smaller fish are male, and the dominant and largest fish in a group becomes female; when a dominant female is absent, then her partner changes sex from male to female. In many [[wrasse]]s the opposite is true: the fish are initially female and become male when they reach a certain size.<ref>{{cite journal|author-link4=Neil Gemmell|vauthors=Todd EV, Liu H, Muncaster S, Gemmell NJ|date=2016|title=Bending Genders: The Biology of Natural Sex Change in Fish|journal=Sexual Development|language=english|volume=10|issue=5–6|pages=223–2241|doi=10.1159/000449297|pmid=27820936|s2cid=41652893|doi-access=free|hdl=10536/DRO/DU:30153787|hdl-access=free}}</ref>
Sequential hermaphroditism also occurs in plants such as ''[[Arisaema triphyllum]]''.
==== Temperature-dependent sex determination ====
[[File:Crocoparc Naissance des bébés.jpg|alt=pile of eggs in the sand, with a newly hatched crocodile looking over the top|thumb|Crocodiles do not have [[sex chromosomes]].<ref>{{cite journal | vauthors = González EJ, Martínez-López M, Morales-Garduza MA, García-Morales R, Charruau P, Gallardo-Cruz JA | title = The sex-determination pattern in crocodilians: A systematic review of three decades of research | journal = The Journal of Animal Ecology | volume = 88 | issue = 9 | pages = 1417–1427 | date = September 2019 | pmid = 31286510 | doi = 10.1111/1365-2656.13037 | bibcode = 2019JAnEc..88.1417G }}</ref> Instead, whether these eggs will produce male or female crocodiles depends on the temperature of the eggs.]]
Many [[reptile]]s, including all [[crocodile]]s and most [[turtles]], have [[temperature-dependent sex determination]]. In these species, the temperature experienced by the embryos during their development determines their sex.<ref name="Bachtrog-2014" />
In some turtles, for example, males are produced at lower temperatures than females; but ''[[Alligator snapping turtle|Macroclemys]]'' females are produced at temperatures lower than 22 °C or above 28 °C, while males are produced in between those temperatures.<ref>{{cite journal|author-link=Scott F. Gilbert|vauthors=Gilbert SF|date=2000|title=Environmental Sex Determination|url=https://www.ncbi.nlm.nih.gov/books/NBK9989/|journal=Developmental Biology. 6th Edition|language=en|access-date=19 May 2021|archive-date=12 June 2021|archive-url=https://web.archive.org/web/20210612123300/https://www.ncbi.nlm.nih.gov/books/NBK9989/|url-status=live}}</ref>
====Haplodiploidy====
Certain insects, such as [[honey bee]]s and [[ant]]s, use a [[Haplodiploidy|haplodiploid sex-determination system]].<ref>{{cite journal|author-link=Brian Charlesworth|vauthors=Charlesworth B|date=August 2003|title=Sex determination in the honeybee|journal=Cell|volume=114|issue=4|pages=397–398|doi=10.1016/S0092-8674(03)00610-X|pmid=12941267|doi-access=free}}</ref> Diploid bees and ants are generally female, and haploid individuals (which develop from unfertilized eggs) are male. This sex-determination system results in highly biased [[sex ratio]]s, as the sex of offspring is determined by fertilization ([[arrhenotoky]] or [[pseudo-arrhenotoky]] resulting in males) rather than the assortment of chromosomes during meiosis.<ref>{{cite journal |vauthors=de la Filia A, Bain S, Ross L |title=Haplodiploidy and the reproductive ecology of Arthropods |journal=Current Opinion in Insect Science |date=June 2015 |volume=9 |pages=36–43 |doi=10.1016/j.cois.2015.04.018 |pmid=32846706 |bibcode=2015COIS....9...36D |url=https://www.pure.ed.ac.uk/ws/files/20137629/1_s2.0_S221457451500084X_main.pdf |hdl=20.500.11820/b540f12f-846d-4a5a-9120-7b2c45615be6 |s2cid=83988416 |hdl-access=free |access-date=25 June 2021 |archive-date=25 June 2021 |archive-url=https://web.archive.org/web/20210625230005/https://www.pure.ed.ac.uk/ws/files/20137629/1_s2.0_S221457451500084X_main.pdf |url-status=live }}</ref>
==Sex ratio==
{{Excerpt|Sex ratio|files=-Sex ratio total population 2020.svg}}
==Sex differences==
{{See also|Sex differences in medicine|Sex differences in intelligence|Neuroscience of sex differences|Sex differences in human physiology}}
[[Anisogamy]] is the fundamental difference between male and female.<ref name="Whitfield-2004">{{cite journal|vauthors=Whitfield J|date=June 2004|title=Everything you always wanted to know about sexes|journal=PLOS Biology|volume=2|issue=6|pages=e183|doi=10.1371/journal.pbio.0020183|pmc=423151|pmid=15208728|quote=One thing biologists do agree on is that males and females count as different sexes. And they also agree that the main difference between the two is gamete size: males make lots of small gametes—sperm in animals, pollen in plants—and females produce a few big eggs. |doi-access=free }}</ref><ref>{{cite book| vauthors = Pierce BA |url={{GBurl|id=z4pXRaZAkdkC|q=Gamete+size}}|title=Genetics: A Conceptual Approach|date=2012|publisher=W.H. Freeman|isbn=978-1-4292-3252-4|pages=74|language=en}}</ref> [[Richard Dawkins]] has stated that it is possible to interpret all the differences between the sexes as stemming from this.<ref>{{cite book| vauthors = Dawkins R |url={{GBurl|id=ekonDAAAQBAJ|q=the+selfish+gene+battle+of+the+sexes+2016}}|title=The Selfish Gene|date=2016|publisher=Oxford University Press|isbn=978-0-19-878860-7|pages=183–184|language=en|quote=However, there is one fundamental feature of the sexes which can be used to label males as males, and females as females, throughout animals and plants. This is that the sex cells or 'gametes' of males are much smaller and more numerous than the gametes of females. This is true whether we are dealing with animals or plants. One group of individuals has large sex cells, and it is convenient to use the word female for them. The other group, which it is convenient to call male, has small sex cells. The difference is especially pronounced in reptiles and in birds, where a single egg cell is big enough and nutritious enough to feed a developing baby for. Even in humans, where the egg is microscopic, it is still many times larger than the sperm. As we shall see, it is possible to interpret all the other differences between the sexes as stemming from this one basic difference.|author-link=Richard Dawkins}}</ref>
=== Sexual characteristics ===
{{excerpt|Sexual characteristics|templates=-more citations needed}}
=== Sexual dimorphism ===
{{Main|Sexual dimorphism}}
[[File:Male and female pheasant.jpg|thumb|[[Common pheasant]]s are [[sexual dimorphism|sexually dimorphic]] in both size and appearance.]]
[[File:Beo-2.jpg|thumb|The [[common hill myna]] is sexually [[Monomorphism (biology)|monomorphic]], meaning that the external appearance of males and females is very similar.<ref>{{cite journal | vauthors = Robin VV, Sinha A, Ramakrishnan U |date=2011 |title=Determining the sex of a monomorphic threatened, endemic passerine in the sky islands of southern India using molecular and morphometric methods |url=https://www.jstor.org/stable/24078632 |journal=Current Science |volume=101 |issue=5 |pages=676–679 |jstor=24078632 |issn=0011-3891 |quote=Many species of birds are, however, monomorphic and difficult to sex visually, particularly in the field and some even in hand. Some examples are the Hill Mynah, ''Gracula religiosa'' and the Black-capped Chickadee, ''Parus atricapillus''.}}</ref>]]
In many animals and some plants, individuals of male and female sex differ in size and appearance, a phenomenon called [[sexual dimorphism]].<ref name="Choe-2019">{{cite book|title=Encyclopedia of Animal Behavior|vauthors=Choe J|date=2019|publisher=Academic Press|isbn=978-0-12-813252-4|veditors=Cox R|volume=2|pages=7–11|language=en|chapter=Body Size and Sexual Dimorphism|chapter-url={{GBurl|id=O5lnDwAAQBAJ|pg=RA1-PA7}}}}</ref> Sexual dimorphism in animals is often associated with [[sexual selection]]: the mating competition between individuals of one sex vis-à-vis the opposite sex.<ref name="Mori-2017">{{cite encyclopedia | vauthors = Mori E, Mazza G, Lovari S |title=Sexual Dimorphism | veditors = Vonk J, Shackelford T |encyclopedia=Encyclopedia of Animal Cognition and Behavior |publisher=Springer International Publishing |place=Cham |url=https://link.springer.com/referenceworkentry/10.1007%2F978-3-319-47829-6_433-1 |access-date=5 June 2021 |date=2017 |pages=1–7 |language=en |doi=10.1007/978-3-319-47829-6_433-1 |isbn=978-3-319-47829-6 |url-access=subscription }}</ref> Other examples demonstrate that it is the preference of females that drives sexual dimorphism, such as in the case of the [[stalk-eyed fly]].<ref>{{cite journal| vauthors = Wilkinson GS, Reillo PR |date=22 January 1994|title=Female choice response to artificial selection on an exaggerated male trait in a stalk-eyed fly|url=http://www.indiana.edu/~curtweb/L567/readings/Wilkinson_%26_Reillo_1994.pdf|journal=Proceedings of the Royal Society B|volume=225|issue=1342|pages=1–6|bibcode=1994RSPSB.255....1W|citeseerx=10.1.1.574.2822|doi=10.1098/rspb.1994.0001|s2cid=5769457|archive-url=https://web.archive.org/web/20060910164858/http://www.indiana.edu/~curtweb/L567/readings/Wilkinson_%26_Reillo_1994.pdf|archive-date=10 September 2006}}</ref>
[[Sex differences in humans]] include a generally larger size and more body hair in men, while women have larger breasts, wider hips, and a higher body fat percentage. In other species, there may be differences in coloration or other features, and may be so pronounced that the different sexes may be mistaken for two entirely different taxa.<ref name="Mori-2017" />
Females are the larger sex in a majority of animals.<ref name="Choe-2019" /> For instance, female [[southern black widow]] spiders are typically twice as long as the males.<ref>{{cite book | vauthors = Drees BM, Jackman J | date = 1999 | publisher = Gulf Publishing Company | ___location = Houston, Texas | chapter = Southern black widow spider | title = Field Guide to Texas Insects | chapter-url=http://insects.tamu.edu/fieldguide/cimg368.html|archive-url=https://web.archive.org/web/20030831114452/http://insects.tamu.edu/fieldguide/cimg368.html|archive-date=31 August 2003|access-date=8 August 2012| via = Extension Entomology, Insects.tamu.edu, Texas A&M University }}</ref> This size disparity may be associated with the cost of producing egg cells, which requires more nutrition than producing sperm: larger females are able to produce more eggs.<ref>{{cite journal |title=Is fecundity the ultimate cause of female-biased size dimorphism in a dragon lizard? |vauthors=Stuart-Smith J, Swain R, Stuart-Smith R, Wapstra E |journal=Journal of Zoology |volume=273 |issue=3 |year=2007 |pages=266–272 |doi=10.1111/j.1469-7998.2007.00324.x }}</ref><ref name="Choe-2019" /> In many other cases, the male of a species is larger than the female. Mammal species with extreme sexual size dimorphism, such as [[elephant seal]]s, tend to have highly [[Polygyny in animals|polygynous]] mating systems, presumably due to selection for success in [[intraspecific competition|competition]] with other males.
Sexual dimorphism can be extreme, with males, such as some [[anglerfish]], living [[parasite|parasitically]] on the female. Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the moss genus ''[[Dicranum]]''<ref>{{cite book | vauthors = Shaw AJ |year=2000 |chapter=Population ecology, population genetics, and microevolution |pages=379–380 | veditors = Shaw AJ, Goffinet B |title=Bryophyte Biology |___location=Cambridge |publisher=Cambridge University Press |isbn=978-0-521-66097-6}}</ref> and the liverwort genus ''[[Sphaerocarpos]]''.<ref name="Schuster-1984">{{cite book | vauthors = Schuster RM |year=1984 |chapter=Comparative Anatomy and Morphology of the Hepaticae |title=New Manual of Bryology |___location=Nichinan, Miyazaki, Japan |publisher=The Hattori botanical Laboratory |volume=2 |page=891}}</ref> There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome,<ref name="Schuster-1984" /><ref name="Crum-1980">{{cite book | vauthors = Crum HA, Anderson LE |year=1980 |title=Mosses of Eastern North America |volume=1 |page=196 |___location=New York |publisher=Columbia University Press |isbn=978-0-231-04516-2}}</ref> or to chemical signaling from females.<ref>{{cite journal | vauthors = Briggs DA |year=1965 |title=Experimental taxonomy of some British species of genus ''Dicranum'' |journal=New Phytologist |volume=64 |pages=366–386 |doi=10.1111/j.1469-8137.1965.tb07546.x |issue=3|doi-access=free |bibcode=1965NewPh..64..366B }}</ref>
In birds, males often have a more [[Animal coloration|colorful]] appearance and may have features (like the long tail of male peacocks) that would seem to put them at a disadvantage (e.g. bright colors would seem to make a bird more visible to predators). One proposed explanation for this is the [[handicap principle]].<ref>{{cite book |author-link=Amotz Zahavi | vauthors = Zahavi A, Zahavi A |year=1997 |title=The handicap principle: a missing piece of Darwin's puzzle |publisher=Oxford University Press |isbn=978-0-19-510035-8 |url=https://archive.org/details/handicapprincipl0000zeha }}</ref> This hypothesis argues that, by demonstrating he can survive with such handicaps, the male is advertising his [[Fitness (biology)|genetic fitness]] to females—traits that will benefit daughters as well, who will not be encumbered with such handicaps.
=== Sex differences in behavior ===
{{See also|Sex differences in psychology|Animal sexual behaviour|Non-reproductive sexual behavior in animals|Sex differences in cognition}}
The sexes across gonochoric species usually differ in behavior. In most animal species, females invest more in parental care,<ref>{{cite book| vauthors = Kliman R |url={{GBurl|id=_r4OCAAAQBAJ|pg=RA1}} | veditors = Herridge EJ, Murray RL, Gwynne DT, Bussiere L |title=Encyclopedia of Evolutionary Biology|year=2016|publisher=Academic Press|isbn=978-0-12-800426-5|volume=2|pages=453–454|language=en }}</ref> although in some species, such as some [[coucal]]s, the males invest more [[parental care]].<ref>{{cite journal | vauthors = Henshaw JM, Fromhage L, Jones AG | title = Sex roles and the evolution of parental care specialization | journal = Proceedings. Biological Sciences | volume = 286 | issue = 1909 | pages = 20191312 | date = August 2019 | pmid = 31455191 | pmc = 6732396 | doi = 10.1098/rspb.2019.1312 }}</ref> Females also tend to be more choosy for who they mate with,<ref>{{cite journal | vauthors = Brennan P | date = 2010 | title = Sexual Selection {{!}} Learn Science at Scitable | journal = Nature Education Knowledge | volume = 3 | issue = 10 | page = 79 |url= https://www.nature.com/scitable/knowledge/library/sexual-selection-13255240/|access-date=25 July 2021 |archive-date=9 October 2021|archive-url= https://web.archive.org/web/20211009163133/https://www.nature.com/scitable/knowledge/library/sexual-selection-13255240/|url-status=live}}</ref> such as most bird species.<ref>{{cite book| vauthors = Macedo RH, Manica LT | chapter = Sexual Selection and Mating Systems: Contributions from a Neotropical Passerine Model | veditors = Reboreda JC, Fiorini VD, Tuero DT |url={{GBurl|id=ItmUDwAAQBAJ|q=behavioral+ecology+choosy+female|p=75}}|title=Behavioral Ecology of Neotropical Birds |date=2019|publisher=Springer|isbn=978-3-030-14280-3|pages=75|language=en}}</ref> Males tend to be more competitive for mating than females.<ref name="Lehtonen-2016">{{cite journal | vauthors = Lehtonen J, Kokko H, Parker GA | title = What do isogamous organisms teach us about sex and the two sexes? | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 371 | issue = 1706 | date = October 2016 | pmid = 27619696 | pmc = 5031617 | doi = 10.1098/rstb.2015.0532 | author-link2 = Hanna Kokko | author-link3 = Geoff Parker }}</ref>
== Distinction from gender ==
{{excerpt|Sex–gender distinction|paragraphs=1}}
== See also ==
{{div col|colwidth=22}}
* [[Sex allocation]]
* [[Sex assignment]]
* [[Sex–gender distinction]]
* [[Sexing]]
{{div col end}}
== References ==
{{reflist}}
{{sfn whitelist|CITEREFKimmel2017}}
== Further reading ==
{{refbegin|30em}}
* {{cite book | vauthors=Arnqvist G, Rowe L|year=2005|title=Sexual conflict|publisher=Princeton University Press|isbn=978-0-691-12217-5}}
* {{cite book | vauthors=Ellis H|year=1933|title=Psychology of Sex |url= https://archive.org/details/b2044249x |___location=London |publisher=W. Heinemann Medical Books}} ''N.B''.: One of many books by this pioneering authority on aspects of human sexuality.
* {{cite book |vauthors=[[Scott F. Gilbert|Gilbert SF]] |title=Developmental Biology |edition=6th |publisher=Sinauer Associates, Inc. |year=2000 |isbn=978-0-87893-243-6 |url=https://archive.org/details/developmentalbio00gilb}}
* {{cite book|vauthors=[[John Maynard Smith|Maynard-Smith J]]|title=The Evolution of Sex|url=https://archive.org/details/evolutionofsex0000mayn|url-access=registration|publisher=Cambridge University Press|year=1978|isbn=978-0-521-29302-0}}
* {{cite journal | vauthors = Otto S | date = 2008 | title = Sexual Reproduction and the Evolution of Sex {{!}} Learn Science at Scitable | journal = Nature Education | volume = 1 | issue = 1 | page = 182 | url = https://www.nature.com/scitable/topicpage/sexual-reproduction-and-the-evolution-of-sex-824/ | archive-url = https://archive.today/20231008141323/https://www.nature.com/scitable/topicpage/sexual-reproduction-and-the-evolution-of-sex-824/ | archive-date = 8 October 2023 }}
{{refend}}
== External links ==
{{Spoken Wikipedia|date=29 December 2022|En-Sex-article.ogg}}
{{Sister project links|Sex}}
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* {{cite web | vauthors = Sizonenko PC | url = http://www.gfmer.ch/Books/Reproductive_health/Human_sexual_differentiation.html | title = Human Sexual Differentiation | archive-url = https://web.archive.org/web/20100209011913/http://www.gfmer.ch/Books/Reproductive_health/Human_sexual_differentiation.html | archive-date = 9 February 2010 | work = Geneva Foundation for Medical Education and Research (GFMER) }}
{{Sex}}
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