The evolution of sex is a major puzzle in modern evolutionary biology. Many groups of organisms, notably the majority of animals and plants, reproduce sexually. The evolution of sex contains two related, yet distinct, themes: its origin and its maintenance. However, since the hypotheses for the origins of sex are difficult to test experimentally, most current work has been focused on the maintenance of sexual reproduction. Several explanations have been suggested by biologists including W. D. Hamilton, Alexey Kondrashov, George C. Williams, Harris Bernstein, Carol Bernstein, Michael M. Cox, Frederic A. Hopf and Richard E. Michod to explain how sexual reproduction is maintained in a vast array of different living organism. FIRST SEX WAS DONE BY SAURABH MUKKAWAR AT DELHI IN 1993 WITH SHINA. ADDRESS-731 It seems that a sexual cycle is maintained because it improves the quality of progeny (fitness), despite reducing the overall number of offspring (twofold cost of sex). In order for sex to be evolutionarily advantageous, it must be associated with a significant increase in the fitness of offspring. One of the most widely accepted explanations for the advantage of sex lies in the creation of genetic variation. Another explanation is based on two molecular advantages. First is the advantage of recombinational DNA repair (promoted during meiosis because homologous chromosomes pair at that time), and second is the advantage of complementation (also known as hybrid vigor, heterosis or masking of mutations). This article or section does not cite any references or sources. ... For other uses, see Animal (disambiguation). ... For other uses, see Plant (disambiguation). ... Sexual reproduction is a union that results in increasing genetic diversity of the offspring. ... This article or section does not cite its references or sources. ... The evolution of sex is a major puzzle in modern evolutionary biology. ... W. D. Hamilton William Donald Bill Hamilton, F.R.S. (1 August 1936 — 7 March 2000) was a British evolutionary biologist, considered one of the greatest evolutionary theorists of the 20th century. ... Alexey S. Kondrashov is a senior investigator at the NCBI in Bethesda, MD. He has worked on a variety of subjects in evolutionary genetics. ... George Williams Professor George Christopher Williams (b. ... Fitness (often denoted in population genetics models) is a central concept in evolutionary theory. ... DNA damage resulting in multiple broken chromosomes DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. ... For the figure of speech, see meiosis (figure of speech). ... Homologous chromosomes are chromosomes in a biological cell that pair (synapse) during meiosis, or alternatively, non-identical chromosomes that contain information for the same biological features and contain the same genes at the same loci but possibly different genetic information, called alleles, at those genes. ... A complementation test (sometimes called a cis-trans test) is used in genetics to decide if two recessive mutant phenotypes are determined by mutations in the same gene or two different genes. ... Heterosis is increased strength of different characteristics in hybrids; the possibility to obtain a better individual by combining the virtues of its parents. ...

For the advantage due to creation of genetic variation, there are three possible reasons this might happen. First, sexual reproduction can bring together mutations that are beneficial into the same individual (sex aids in the spread of advantageous traits). Second, sex acts to bring together currently deleterious mutations to create severely unfit individuals that are then eliminated from the population (sex aids in the removal of deleterious genes). Last, sex creates new gene combinations that may be more fit than previously existing ones, or may simply lead to reduced competition among relatives. For linguistic mutation, see Apophony. ...

For the advantage due to DNA repair, there is an immediate large benefit to removal of DNA damage by recombinational DNA repair during meiosis, since this removal allows greater survival of progeny with undamaged DNA. The advantage of complementation to each sexual partner is avoidance of the bad effects of their deleterious recessive genes in progeny by the masking effect of normal dominant genes contributed by the other partner. DNA damage resulting in multiple broken chromosomes DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. ... For the figure of speech, see meiosis (figure of speech). ... A complementation test (sometimes called a cis-trans test) is used in genetics to decide if two recessive mutant phenotypes are determined by mutations in the same gene or two different genes. ...

The classes of hypotheses based on the creation of variation are further broken down below. It is important to realise that any number of these hypotheses may be true in any given species (they are not mutually exclusive), and that different hypotheses may apply in different species. However, a research framework based on creation of variation has yet to be found that allows one to determine whether the reason for sex is universal for all sexual species, and, if not, which mechanism is acting in each species. In logic, two mutually exclusive (or mutual exclusive according to some sources) propositions are propositions that logically cannot both be true. ...

On the other hand, the maintenance of sex based on DNA repair and complementation applies widely to all sexual species. This explanation for the maintenance of sex is explored further as Section 6.2 Sex is maintained by DNA repair and complementation, under “Other explanations” below.

Early history of study

Modern philosophical-scientific thinking on the problem can be traced back to Erasmus Darwin in the 18th century; it also features in Aristotle's writings. The thread was later picked up by August Weismann in 1889, who understood that the purpose of sex was to generate genetic variation, as is detailed in the majority of the explanations below. On the other hand, Charles Darwin, also in 1889, concluded that the effects of hybrid vigor (complementation) “is amply sufficient to account for the ... genesis of the two sexes.” This is consistent with the repair and complementation hypothesis, given below under “Other explanations.” This article is about Erasmus Darwin who lived 1731–1802; for his descendants with the same name see Erasmus Darwin (disambiguation). ... For other uses, see Aristotle (disambiguation). ... August Weismann Friedrich Leopold August Weismann (b. ... Genetic variation is the variation in the genetic material of a population, and includes the nuclear, mitochodrial, ribosomal genomes as well as the genomes of other organelles. ...

The two-fold cost of sex

This diagram illustrates the two-fold 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 population doubles in size each generation.

In all sexual species, the population consists of two sexes, only one of which is capable of bearing young (with the exception of simultaneous hermaphrodites). In an asexual species, each member of the population is capable of bearing young. This implies that an asexual population has an intrinsic capacity to grow more rapidly each generation. The cost was first described in mathematical terms by John Maynard Smith ^[1]. He imagined an asexual mutant arising in a sexual population, half of which comprises males that cannot themselves produce offspring. With female-only offspring, the asexual lineage doubles its representation in the population each generation, all else being equal. Often all else is not equal, however, in which case the realized fitness cost to sex may be much less than this intrinsic twofold cost of producing males. For example, an asexual mutant arising in a sexual population occupies a niche frozen to that of its parental genotype because the asexual descendants are genetically self-identical. Analysis of competitive Lotka-Volterra equations suggests that the asexual lineage may never realize its full twofold advantage in population growth capacity, if the broader niche of the sexual population confers even a small competitive edge ^[2]. Diagram for Evolution of sex. ... Diagram for Evolution of sex. ... The 1st-century BC sculpture The Reclining Hermaphrodite, in the Museo Nazionale Romano, Palazzo Massimo Alle Terme in Rome A hermaphrodite is an organism that possesses both male and female sex organs during its life. ... Professor John Maynard Smith[1], F.R.S. (6 January 1920 – 19 April 2004) was a British evolutionary biologist and geneticist. ... The competitive Lotka-Volterra equations are a simple model of the population dynamics of species competing for some common resource. ...

An additional cost is that males and females must search for each other in order to mate, and sexual selection often favours traits that reduce the survival of individuals ^[1]. Illustration from The Descent of Man and Selection in Relation to Sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right. ...

Evidence that the cost is not insurmountable comes from George C. Williams, who noted the existence of species which are capable of both asexual and sexual reproduction. These species time their sexual reproduction with periods of environmental uncertainty, and reproduce asexually when conditions are more favourable. The important point is that these species are observed to reproduce sexually when they could choose not to, implying that there is a selective advantage to sexual reproduction ^[3]. George Williams Professor George Christopher Williams (b. ... It has been suggested that Parthenogenesis be merged into this article or section. ... Sexual reproduction is a union that results in increasing genetic diversity of the offspring. ...

Sex creates genetic variation among siblings

August Weismann proposed in 1889 ^[4] an explanation for the evolution of sex, where the advantage of sex is the creation of variation among siblings. It was then subsequently explained in genetics terms by Fisher ^[5] and Muller ^[6] and has been recently summarized by Burt in 2000 ^[7]. August Weismann Friedrich Leopold August Weismann (b. ... Sir Ronald Aylmer Fisher, FRS (17 February 1890 – 29 July 1962) was an English statistician, evolutionary biologist, and geneticist. ... Hermann Joseph H. J. Muller (December 21, 1890 – April 5, 1967) was a Nobel Prize-winning American geneticist and educator, best known for his work on the physiological and genetic effects of radiation (X-ray mutagenesis) as well as his outspoken political beliefs. ...

George C. Williams gave an example based around the elm tree. In the forest of this example, empty patches between trees can support one individual each. When a patch becomes available because of the death of a tree, other trees' seeds will compete to fill the patch. Since the chance of a seed's success in occupying the patch depends upon its genotype, and a parent cannot anticipate which genotype is most successful, each parent will send many seeds, creating competition between siblings. Natural selection therefore favours parents which can produce a variety of offspring. George Williams Professor George Christopher Williams (b. ... Species See Elm species, varieties, cultivars and hybrids Elms are deciduous and semi-deciduous trees making up the genus Ulmus, family Ulmaceae, found throughout the Northern Hemisphere from Siberia to Indonesia, Mexico to Japan. ... This article is about a community of trees. ... For other uses, see Natural selection (disambiguation). ...

This hypothesis applies to some situations, but it is not sufficiently general by itself to explain the evolution of sex in a wide variety of species.

A similar hypothesis is named the tangled bank hypothesis after a passage in Charles Darwin's The Origin of Species: For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ... Charles Darwins Origin of Species (publ. ...

"It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us."

The hypothesis, proposed by Michael Ghiselin in his 1974 book, The Economy of Nature and the Evolution of Sex, suggests that a diverse set of siblings may be able to extract more food from its environment than a clone, because each sibling uses a slightly different niche. One of the main proponents of this hypothesis is Graham Bell of McGill University. The hypothesis has been criticized for failing to explain how asexual species developed sexes. In his book, Evolution and Human Behavior (MIT Press, 2000), John Cartwright comments: Michael T. Ghiselin is an American biologist, philosopher/historian of biology currently at the California Academy of Sciences. ... For other uses, see clone. ... Graham Bell is a contemporary evolutionary biologist with interests in the evolution of sexual reproduction and the maintenance of variation. ... McGill University. ...

"Although once popular, the tangled bank hypothesis now seems to face many problems, and former adherents are falling away. The theory would predict a greater interest in sex among animals that produce lots of small offspring that compete with each other. In fact, sex is invariably associated with organisms that produce a few large offspring, whereas organisms producing small offspring frequently engage in parthenogenesis [asexual reproduction]. In addition, the evidence from fossils suggests that species go for vast periods of [geologic] time without changing much."

Sex helps the spread of advantageous traits

Sex creates novel genotypes more rapidly

This diagram illustrates how sex might create novel genotypes more rapidly. Two advantageous alleles A and B occur at random. The two alleles are recombined rapidly in (a), a sexual population, but in (b), an asexual population, the two alleles must independently arise.

Sex could be a method by which novel genotypes are created. Since sex combines genes from two individuals, sexually reproducing populations can more easily combine advantageous genes than can asexual populations. If, in a sexual population, two different advantageous alleles arise at different loci on a chromosome in different members of the population, a chromosome containing the two advantageous alleles can be produced within a few generations by recombination. However, should the same two alleles arise in different members of an asexual population, the only way that one chromosome can develop the other allele is to independently gain the same mutation, which would take much longer. Diagram for Evolution of sex. ... Diagram for Evolution of sex. ... An allele is any one of a number of alternative forms of the same gene occupying a given locus (position) on a chromosome. ... Recombination usually refers to the biological process of genetic recombination and meiosis, a genetic event that occurs during the formation of sperm and egg cells. ...

Ronald Fisher also suggested that sex might facilitate the spread of advantageous genes by allowing them to escape their genetic surroundings, if they should arise on a chromosome with deleterious genes. Sir Ronald Aylmer Fisher, FRS (17 February 1890 – 29 July 1962) was an English statistician, evolutionary biologist, and geneticist. ...

But these explanations depend upon the rate of mutation. If favourable mutations are so rare that each will become fixed in the population before the next arises (bearing in mind that mutation is a Poisson process), then sexual and asexual populations would evolve at the same rate. It has been suggested that this article be split into multiple articles. ...

Additionally, these explanations depend upon group selection, which is a weak selective force relative to natural selection – sex is still disadvantageous to the individual due to the twofold cost of sex. Therefore, these explanations do not explain why heterogonic species choose to adopt sexual reproduction, as George C. Williams pointed out in his balance argument, and hence are insufficient to explain the evolution of sex. In evolutionary biology, group selection refers to the idea that alleles can become fixed or spread in a population because of the benefits they bestow on groups, regardless of the fitness of individuals within that group. ... For other uses, see Natural selection (disambiguation). ... George Williams Professor George Christopher Williams (b. ...

Supporters of these theories respond to the balance argument that the individuals produced by sexual and asexual reproduction may differ in other respects too – which may influence the persistence of sexuality. For example, in water fleas of the genus Cladocera, sexual offspring form eggs which are better able to survive the winter. Water flea is a generic term for a number of small aquatic crustacea characterised by their jumping or jerky mode of swimming. ...

Sex increases resistance to parasites

One of the most widely accepted theories to explain the evolution of sex is that it evolved as an adaptation to assist sexual individuals in resisting parasites, also known as the Red Queen hypothesis ^[8][9][10]. A parasite is an organism that spends a significant portion of its life in or on the living tissue of a host organism and which causes harm to the host without immediately killing it. ... This article is about the evolutionary hypothesis. ...

When an environment changes, previously neutral or deleterious alleles can become favourable. If the environment changed sufficiently rapidly (i.e. between generations), these changes in the environment can make sex advantageous for the individual. Such rapid changes in environment are caused by the co-evolution between hosts and parasites.

Imagine, for example that there is one gene in parasites with two alleles p and P conferring two types of parasitic ability, and one gene in hosts with two alleles h and H, conferring two types of parasite resistance, such that parasites with allele p can attach themselves to hosts with the allele h, and P to H. Such a situation will lead to cyclic changes in allele frequency - as p increases in frequency, h will be disfavoured.

In reality, there will be several genes involved in the relationship between hosts and parasites. In an asexual population of hosts, offspring will only have the different parasitic resistance if a mutation arises. In a sexual population of hosts, however, offspring will have a new combination of parasitic resistance alleles.

In other words, like Lewis Carroll's Red Queen, sexual hosts are continually adapting in order to stay ahead of their parasites. Charles Lutwidge Dodgson (Lewis Carroll) Charles Lutwidge Dodgson (IPA: ) (January 27, 1832 – January 14, 1898), better known by the pen name Lewis Carroll (), was an English author, mathematician, logician, Anglican clergyman and photographer. ...

Evidence for this explanation for the evolution of sex is provided by comparison of the rate of molecular evolution of genes for kinases and immunoglobulins in the immune system with genes coding other proteins. The genes coding for immune system proteins evolve considerably faster^[11][12]. In biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific target molecules (substrates); the process is termed phosphorylation. ... Schematic of antibody binding to an antigen An antibody is a protein complex used by the immune system to identify and neutralize foreign objects like bacteria and viruses. ... A scanning electron microscope image of a single neutrophil (yellow), engulfing anthrax bacteria (orange). ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ...

Critics of the Red Queen hypothesis question whether the constantly-changing environment of hosts and parasites is sufficiently common to explain the evolution of sex.

Sex helps the removal of deleterious genes

Mutations can have many different effects upon an organism. It is generally believed that the majority of non-neutral mutations are deleterious, which means that they will cause a decrease in the organism's overall fitness.^[13] If a mutation has a deleterious effect, it will then usually be removed from the population by the process of natural selection. Sexual reproduction is believed to be more efficient than asexual reproduction in removing those mutations from the genome.^[14] For linguistic mutation, see Apophony. ... For other uses, see Natural selection (disambiguation). ...

There are two main hypotheses which explain how sex may act to remove deleterious genes from the genome. For other uses, see Gene (disambiguation). ...

Sex allows reconstruction of mutation-free individuals

Main article: Muller's ratchet

In a finite asexual population under the pressure of deleterious mutations, the random loss of individuals without such mutations is inevitable. This is known as Muller's ratchet. In a sexual population, however, mutation-free genotypes can be restored by recombination of genotypes containing deleterious mutations. In evolutionary genetics, Mullers ratchet (named after Hermann Joseph Muller and a mechanical device) is the name given to the process by which the genomes of an asexual population accumulate deleterious mutations in an irreversible manner. ... In evolutionary genetics, Mullers ratchet (named after Hermann Joseph Muller and a mechanical device) is the name given to the process by which the genomes of an asexual population accumulate deleterious mutations in an irreversible manner. ... Recombination usually refers to the biological process of genetic recombination and meiosis, a genetic event that occurs during the formation of sperm and egg cells. ...

This comparison will only work for a small population - in a large population, random loss of the most fit genotype becomes unlikely even in an asexual population.

Sex acts to encourage removal of deleterious genes

Diagram illustrating different relationships between numbers of mutations and fitness. Kondrashov's model requires synergistic epistasis, which is represented by the red line - each mutation has a disproproportionately large effect on the organism's fitness.

This hypothesis was proposed by Alexey Kondrashov, and is sometimes known as the deterministic mutation hypothesis^[14]. It assumes that the majority of deleterious mutations are only slightly deleterious, and affect the individual such that the introduction of each additional mutation has an increasingly large effect on the fitness of the organism. This relationship between number of mutations and fitness is known as synergistic epistasis. Diagram for Evolution of sex. ... Diagram for Evolution of sex. ... Alexey S. Kondrashov is a senior investigator at the NCBI in Bethesda, MD. He has worked on a variety of subjects in evolutionary genetics. ... Epistasis takes place when the action of one gene is modified by one or more others that assort somewhat independently. ...

By way of analogy, think of a car with several minor faults. Each is not sufficient alone to prevent the car from running, but in combination, the faults combine to prevent the car from functioning. Analogy is both the cognitive process of transferring information from a particular subject (the analogue or source) to another particular subject (the target), and a linguistic expression corresponding to such a process. ... “Car” and “Cars” redirect here. ...

Similarly, an organism may be able to cope with a few defects, but the presence of many mutations could overwhelm its backup mechanisms.

Kondrashov argues that the slightly deleterious nature of mutations means that the population will tend to be composed of individuals with a small number of mutations. Sex will act to recombine these genotypes, creating individuals with fewer and more deleterious mutations, and since there is a major selective disadvantage to individuals with more mutations, these individuals die out. In essence, sex compartmentalizes the deleterious mutations. Recombination usually refers to the biological process of genetic recombination and meiosis, a genetic event that occurs during the formation of sperm and egg cells. ...

There has been much criticism of Kondrashov's theory, since it relies on two key restrictive conditions. The first requires that the rate of deleterious mutation should exceed one per genome per generation in order to provide a substantial advantage for sex. While there is some empirical evidence for it (for example in Drosophila^[15] and E. coli^[16]), there is also strong evidence against it. Secondly, there should be strong interactions among loci (synergistic epistasis), a mutation-fitness relation for which there is only limited evidence. Conversely, there is also the same amount of evidence that mutations show no epistasis (purely additive model) or antagonistic interactions (each additional mutation has a disproportionally small effect). Type species Drosophila funebris (Fabricius, 1787) Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called fruit flies, or more appropriately vinegar flies, wine flies, pomace flies, grape flies, and picked fruit-flies, a reference to the characteristic of many species to linger... See also Entamoeba coli. ...

Other explanations

Sex and the speed of evolution

Ilan Eshel suggested that sex prevents rapid evolution. He suggests that recombination breaks up favourable gene combinations more often than it creates them, and sex is maintained because it ensures selection in longer-term than in asexual populations - so the population is less affected by short-term changes. This explanation is not widely accepted, as its assumptions are very restrictive. Recombination usually refers to the biological process of genetic recombination and meiosis, a genetic event that occurs during the formation of sperm and egg cells. ...

Nevertheless, it has recently been shown in experiments with Chlamydomonas algae that sex can remove the speed limit on evolution. ^[17] Species See text. ...

Sex is maintained by DNA repair and complementation

As discussed in the earlier part of this article, conventionally, sexual reproduction is explained as an adaptation for producing genetic variation through allelic recombination. As acknowledged above, however, serious problems with this explanation have led many workers to conclude that the benefit of sex is a major unsolved problem in evolutionary biology.

An alternative “informational” approach to this problem led to the view that the two fundamental aspects of sex, genetic recombination and outcrossing, are adaptive responses to the two major sources of “noise” in transmitting genetic information. Genetic noise can occur as either physical damage to the genome (e.g. chemically altered bases of DNA or breaks in the chromosome) or replication errors (mutations)^{[18] [19] [20]} This alternative view is referred to as the repair and complementation hypothesis to distinguish it from the traditional variation hypothesis. Genetic recombination is the process by which a strand of DNA is broken and then joined to the end of a different DNA molecule. ... Outcrossing is the practice of introducing unrelated genetic material into a breeding line. ...

The repair and complementation hypothesis assumes that genetic recombination is fundamentally a DNA repair process, and that when it occurs during meiosis it is an adaptation for repairing the genomic DNA that is passed on to progeny. Recombinational repair is the only repair process known which can accurately remove double-strand damages in DNA, and such damages are both common in nature and ordinarily lethal if not repaired. Recombinational repair is prevalent from the simplest viruses to the most complex multicellular eukaryotes. It is effective against many different types of genomic damage, and, in particular is highly efficient at overcoming double-strand damages. Studies of the mechanism of meiotic recombination indicate that meiosis is an adaptation for repairing DNA.^[21] These considerations form the basis for the first part of the repair and complementation hypothesis. Genetic recombination is the process by which a strand of DNA is broken and then joined to the end of a different DNA molecule. ...

In some lines of descent from the earliest organisms, the diploid stage of the sexual cycle, which was at first transient, became the predominant stage because it allowed complementation, the masking of deleterious recessive mutations (hybrid vigor or heterosis). Outcrossing, the second fundamental aspect of sex, is maintained by the advantage of masking mutations and the disadvantage of inbreeding (mating with a close relative) which allows expression of recessive mutations (commonly observed as inbreeding depression). This is in accord with Charles Darwin ^[22] who concluded that the adaptive advantage of sex is hybrid vigor, or as he put it, “the offspring of two individuals, especially if their progenitors have been subjected to very different conditions, have a great advantage in height, weight, constitutional vigor and fertility over the self fertilized offspring from either one of the same parents.” Ploidy is the number of homologous sets of chromosomes in a biological cell. ... A complementation test (sometimes called a cis-trans test) is used in genetics to decide if two recessive mutant phenotypes are determined by mutations in the same gene or two different genes. ... Heterosis is increased strength of different characteristics in hybrids; the possibility to obtain a better individual by combining the virtues of its parents. ... Outcrossing is the practice of introducing unrelated genetic material into a breeding line. ... It has been suggested that inbreeding depression be merged into this article or section. ... It has been suggested that this article or section be merged into inbreeding. ...

However, outcrossing may be abandoned in favor of parthogenesis or selfing (which retain the advantage of meiotic recombinational repair) under conditions in which the costs of mating are very high. For instance, costs of mating are high when individuals are rare in a geographic area, such as when there has been a forest fire and the individuals entering the burned area are the initial ones to arrive. At such times mates are hard to find, and this favors parthenogenic species.

In the view of the repair and complementation hypothesis, the removal of DNA damage by recombinational repair produces a new, less deleterious form of informational noise, allelic recombination, as a by-product. This lesser informational noise generates genetic variation, viewed by some as the major effect of sex, as discussed in the earlier parts of this article.

Origin of sexual reproduction

Please help improve this article by expanding this section. See talk page for details. Please remove this message once the section has been expanded.

The most primitive organisms known to undergo meiosis and to reproduce sexually are protists (primitive unicellular eukaryotes) such as those that cause malaria. Image File history File links This is a lossless scalable vector image. ... Typical phyla Chromalveolata Chromista Heterokontophyta Haptophyta Cryptophyta (cryptomonads) Alveolata Dinoflagellata Apicomplexa Ciliophora (ciliates) Cabozoa Excavata Euglenozoa Percolozoa Metamonada Rhizaria Radiolaria Foraminifera Cercozoa Archaeplastida (in part) Rhodophyta (red algae) Glaucophyta (basal archaeplastids) Amoebozoa Choanozoa Many others; classification varies Protists (IPA: (RP); (GenAm)), Greek protiston -a meaning the (most) first of all... Kingdoms Eukaryotes are organisms with complex cells, in which the genetic material is organized into membrane-bound nuclei. ... Malaria is a vector-borne infectious disease caused by protozoan parasites. ...

Organisms need to replicate their genetic material in an efficient and reliable manner. The necessity to repair genetic damage is one of the leading theories explaining the origin of sexual reproduction. Diploid individuals can repair a mutated section of its DNA via homologous recombination, since there are two copies of the gene in the cell and one copy is presumed to be undamaged. A mutation in an haploid individual, on the other hand, is more likely to become resident, as the DNA repair machinery has no way of knowing what the original undamaged sequence was.^[18] The most primitive form of sex may have been one organism with damaged DNA replicating an undamaged strand from a similar organism in order to repair itself.^[23] Diploid (meaning double in Greek) cells have two copies (homologs) of each chromosome (both sex- and non-sex determining chromosomes), usually one from the mother and one from the father. ... Chromosomal crossover is the process by which two chromosomes, paired up during Prophase I of meiosis, exchange some distal portion of their DNA. Crossover occurs when two chromosomes, normally two homologous instances of the same chromosome, break and then reconnect but to the different end piece. ... Haploid (meaning simple in Greek) cells have only one copy of each chromosome. ... DNA damage resulting in multiple broken chromosomes DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. ...

Another theory is that sexual reproduction originated from selfish parasitic genetic elements that exchange genetic material (that is: copies of their own genome) for their transmission and propagation. In some organisms, sexual reproduction has been shown to enhance the spread of parasitic genetic elements (e.g.: yeast, filamentous fungi).^[24] Bacterial conjugation, a form of genetic exchange that some sources describe as sex, is not a form of reproduction. However, it does support the selfish genetic element theory, as it is propagated through such a "selfish gene", the F-plasmid.^[23] Selfish DNA is DNA which is prevalent in the genome, not because its phenotypic effect is beneficial, but because it has properties which cause the number of copies of it within the genome to increase with time. ... Bacterial conjugation is the transfer of genetic material between bacteria through direct cell-to-cell contact. ... To meet Wikipedias quality standards, this article or section may require cleanup. ...

A third theory is that sex evolved as a form of cannibalism. One primitive organism ate another one, but rather than completely digesting it, some of the 'eaten' organism's DNA was incorporated into the 'eater' organism.^[23]

A theory states that sexual reproduction evolved from ancient haloarchaea through a combination of jumping genes, and swapping plasmids. ^[25] Haloarchaea are a member of the halophile community, in that they require high salt concentrations to grow. ... Transposons are sequences of DNA that can move around to different positions within the genome of a single cell, a process called Transposition. ... Figure 1: Illustration of a bacterium with plasmids enclosed showing chromosomal DNA and plasmids. ...

A comprehensive 'origin of sex as vaccination' theory proposes that eukaryan sex-as-syngamy (fusion sex) arose from prokaryan unilateral sex-as-infection when infected hosts began swapping nuclearized genomes containing coevolved, vertically transmitted symbionts that provided protection against horizontal superinfection by more virulent symbionts. Sex-as-meiosis (fission sex) then evolved as a host strategy to uncouple (and thereby emasculate) the acquired symbiont genomes.^[26]

See also

• Asexual reproduction
• Biological reproduction
• Epistasis
• Genetic recombination
• Mutation
• Sexual reproduction

It has been suggested that Parthenogenesis be merged into this article or section. ... Biological reproduction is the biological process by which new individual organisms are produced. ... Epistasis takes place when the action of one gene is modified by one or more others that assort somewhat independently. ... Genetic recombination is the process by which a strand of DNA is broken and then joined to the end of a different DNA molecule. ... For linguistic mutation, see Apophony. ... Sexual reproduction is a union that results in increasing genetic diversity of the offspring. ...

References

Professor John Maynard Smith[1], F.R.S. (6 January 1920 – 19 April 2004) was a British evolutionary biologist and geneticist. ... George Williams Professor George Christopher Williams (b. ... August Weismann Friedrich Leopold August Weismann (b. ... Sir Ronald Fisher Sir Ronald Aylmer Fisher, FRS (February 17, 1890–July 29, 1962) was an extraordinarily talented evolutionary biologist, geneticist and statistician. ... W. D. Hamilton William Donald Bill Hamilton, F.R.S. (1 August 1936 — 7 March 2000) was a British evolutionary biologist, considered one of the greatest evolutionary theorists of the 20th century. ... The Proceedings of the National Academy of Sciences of the United States of America, usually referred to as PNAS, is the official journal of the United States National Academy of Sciences. ... Matthew (Matt) Ridley (born February 7, 1958 at Newcastle upon Tyne) (not to be confused with Mark Ridley) is an English science writer. ... Nature is a prominent scientific journal, first published on 4 November 1869. ... For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ...

Further reading

• Bernstein, Carol; Bernstein, Harris. 1991. Aging, Sex and DNA Repair. Academic Press. ISBN-10: 0120928604 ISBN-13: 978-0120928606
• Michod, Richard E. 1994. Eros and Evolution: A natural history of sex. Addison-Wesley. ISBN-10: 020140754X ISBN-13: 978-0201407549
• Graham Bell 1982. The masterpiece of nature. Croom Helm, London.
• George C. Williams Sex and Evolution 1975, Princeton University Press, ISBN 0-691-08152-2
• Maynard Smith, J. (1978) The Evolution of Sex. Cambridge University Press. ISBN 0-521-29302-2.
• Mark Ridley Evolution 1993, ISBN 0-632-03481-5
• Matt Ridley The Red Queen: Sex and the Evolution of Human Nature 1995, Penguin Reprint, ISBN 0-14-024548-0
• Michod, Richard; Levin, Bruce. 1987. The Evolution of Sex: An Examination of Current Ideas. Sinauer Associates. ISBN 0878934596
• John Maynard Smith & Szathmary, E. The Major Transitions in Evolution 1995
• Mark Ridley (2004) Mendel's Daemon: Evolution and the Complexity of Life
• Hurst, L. D. and Peck, J. R. "Recent advances in the understanding of the evolution and maintenance of sex" 1996. Trends in Ecology and Evolution 11:46-52.

Graham Bell is a contemporary evolutionary biologist with interests in the evolution of sexual reproduction and the maintenance of variation. ... George Williams Professor George Christopher Williams (b. ... Professor John Maynard Smith[1], F.R.S. (6 January 1920 – 19 April 2004) was a British evolutionary biologist and geneticist. ... Mark Ridley (born 1956) is a United Kingdom zoologist and writer on evolution. ... Matthew (Matt) Ridley (born February 7, 1958 at Newcastle upon Tyne) (not to be confused with Mark Ridley) is an English science writer. ... Professor John Maynard Smith[1], F.R.S. (6 January 1920 – 19 April 2004) was a British evolutionary biologist and geneticist. ... The Major Transitions in Evolution is a book written by John Maynard Smith and Eörs Szathmáry (Oxford University Press, 1995). ... Mark Ridley (born 1956) is a United Kingdom zoologist and writer on evolution. ... Trends in Ecology & Evolution (TREE; ISSN 0169-5347) is a monthly review journal that is one of fourteen Trends titles in the life sciences published by Elsevier. ...

External links

• An essay summarising the different theories, dating from around 2001
• "Pas de Deux: Why are there only two sexes?" Shaffer, Amanda (writer). Article in Slate, September 27, 2007.