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=== Sex and recombination === {{further|Sexual reproduction|Genetic recombination|Evolution of sexual reproduction}} In [[Asexual reproduction|asexual]] organisms, genes are inherited together, or ''linked'', as they cannot mix with genes of other organisms during reproduction. In contrast, the offspring of sexual organisms contain random mixtures of their parents' chromosomes that are produced through independent assortment. In a related process called [[homologous recombination]], sexual organisms exchange DNA between two matching chromosomes.<ref>{{cite journal |last=Radding |first=Charles M. |date=December 1982 |title=Homologous Pairing and Strand Exchange in Genetic Recombination |journal=[[Annual Review of Genetics]] |volume=16 |pages=405β437 |doi=10.1146/annurev.ge.16.120182.002201 |issn=0066-4197 |pmid=6297377}}</ref> Recombination and reassortment do not alter allele frequencies, but instead change which alleles are associated with each other, producing offspring with new combinations of alleles.<ref name="Agrawal-2006">{{cite journal |last=Agrawal |first=Aneil F. |s2cid=14739487 |date=5 September 2006 |title=Evolution of Sex: Why Do Organisms Shuffle Their Genotypes? |journal=[[Current Biology]] |volume=16 |issue=17 |pages=R696βR704 |doi=10.1016/j.cub.2006.07.063 |issn=0960-9822 |pmid=16950096 |bibcode=2006CBio...16.R696A |citeseerx=10.1.1.475.9645}}</ref> Sex usually increases genetic variation and may increase the rate of evolution.<ref>{{cite journal |last1=Peters |first1=Andrew D. |last2=Otto |first2=Sarah P. |date=June 2003 |title=Liberating genetic variance through sex |journal=[[BioEssays]] |volume=25 |issue=6 |pages=533β537 |doi=10.1002/bies.10291 |issn=0265-9247 |pmid=12766942}}</ref><ref>{{cite journal |last1=Goddard |first1=Matthew R. |last2=Godfray |first2=H. Charles J. |author-link2=Charles Godfray |last3=Burt |first3=Austin |date=31 March 2005 |title=Sex increases the efficacy of natural selection in experimental yeast populations |url=https://archive.org/details/sim_nature-uk_2005-03-31_434_7033/page/636 |journal=Nature |volume=434 |issue=7033 |pages=636β640 |bibcode=2005Natur.434..636G |doi=10.1038/nature03405 |issn=0028-0836 |pmid=15800622 |s2cid=4397491}}</ref> [[File:Evolsex-dia1a.svg|thumb|upright=1.15|This diagram illustrates the ''twofold cost of sex''. If each individual were to contribute to the same number of offspring (two), ''(a)'' the sexual population remains the same size each generation, where the ''(b)'' [[Asexual reproduction]] population doubles in size each generation.{{image reference needed|date=December 2022}}]] The two-fold cost of sex was first described by [[John Maynard Smith]].<ref name="maynard">{{harvnb|Maynard Smith|1978}}{{page needed|date=December 2014}}</ref> The first cost is that in sexually dimorphic species only one of the two sexes can bear young. This cost does not apply to hermaphroditic species, like most plants and many [[invertebrate]]s. The second cost is that any individual who reproduces sexually can only pass on 50% of its genes to any individual offspring, with even less passed on as each new generation passes.<ref name="ridley">{{harvnb|Ridley|2004|p=314}}</ref> Yet sexual reproduction is the more common means of reproduction among eukaryotes and multicellular organisms. The [[Red Queen hypothesis]] has been used to explain the significance of sexual reproduction as a means to enable continual evolution and adaptation in response to [[coevolution]] with other species in an ever-changing environment.<ref name="ridley" /><ref name="Van Valen-1973">{{cite journal |last=Van Valen |first=Leigh |author-link=Leigh Van Valen |year=1973 |title=A New Evolutionary Law |url=https://dl.dropboxusercontent.com/u/18310184/evolutionary-theory/vol-01/Vol.1%2CNo.1%2C1-30%2CL.%20Van%20Valen%2C%20A%20new%20evolutionary%20law..pdf |journal=Evolutionary Theory |volume=1 |pages=1β30 |issn=0093-4755 |access-date=24 December 2014 |archive-url=https://web.archive.org/web/20141222094258/https://dl.dropboxusercontent.com/u/18310184/evolutionary-theory/vol-01/Vol.1%2CNo.1%2C1-30%2CL.%20Van%20Valen%2C%20A%20new%20evolutionary%20law..pdf |archive-date=22 December 2014}}</ref><ref name="Hamilton-1990">{{cite journal |last1=Hamilton |first1=W. D. |author-link1=W. D. Hamilton |last2=Axelrod |first2=Robert |author-link2=Robert Axelrod (political scientist) |last3=Tanese |first3=Reiko |date=1 May 1990 |title=Sexual reproduction as an adaptation to resist parasites (a review) |journal=PNAS |volume=87 |issue=9 |pages=3566β3573 |bibcode=1990PNAS...87.3566H |doi=10.1073/pnas.87.9.3566 |issn=0027-8424 |pmid=2185476 |pmc=53943 |doi-access=free}}</ref><ref name="Birdsell">{{harvnb|Birdsell|Wills|2003|pp=113β117}}</ref> Another hypothesis is that sexual reproduction is primarily an adaptation for promoting accurate recombinational repair of damage in germline DNA, and that increased diversity is a byproduct of this process that may sometimes be adaptively beneficial.<ref>Bernstein H, Byerly HC, Hopf FA, Michod RE. Genetic damage, mutation, and the evolution of sex. Science. 1985 Sep 20;229(4719):1277β81. {{doi|10.1126/science.3898363}}. PMID 3898363</ref><ref>Bernstein H, Hopf FA, Michod RE. The molecular basis of the evolution of sex. Adv Genet. 1987;24:323-70. {{doi|10.1016/s0065-2660(08)60012-7}}. PMID 3324702</ref>
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