The Evolution Of Sex

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DNA can be damaged in at least two ways. First, ionizing radiation or mutagenic chemicals can alter the genetic code. Or, second, a mutation can occur via errors during the replication process itself. Most mutations are deleterious see Cartwright [ 22 ]. In an asexual organism, by definition, any mutation that occurs in one generation will be passed on automatically to the next. In his book, The Red Queen, [ 23 ] Matt Ridley compared it to what occurs when you photocopy a document, then photocopy the photocopy, and then photocopy that photocopy, etc.

Eventually, the quality deteriorates severely. Asexual organisms, as they continue to accumulate mutations, face the unpleasant prospect of eventually becoming both unable to reproduce and unviable-neither of which would be at all helpful to evolution. But sexual reproduction allows most plants and animals to create offspring with good copies of two genes via crossover and would thus, help eliminate this downward ratchet since mutations, although they might still be passed on from one generation to the next, would not necessarily be expressed in the next generation a mutation must appear in the genes of both parents before it is expressed in the offspring.

As Cartwright put it:. We must not overlook an important fact throughout all of this: These theories valiantly attempt to explain why sex exists now, but they do not explain the origin of sex. In addressing this very issue, Maddox asked quizzically:. Exactly our point! It is one thing to develop a theory or hypothesis to explain something that already exists, but it is entirely another to develop a theory or hypothesis to explain why that something in this case, sex does exist.

But we would suggest that there is no naturalistic explanation at all for the origin or maintenance of sex. Why, then, does sex exist? It is the complexity of this process, and the manner in which it is copied from generation to generation, which practically drove Mark Ridley to distraction in The Cooperative Gene. Yet there is an even more important question than why sex exists. How did sex come to exist? Quite obviously, if everything remained the same, there would be no evolution. Evolutionists believe that the driving forces behind evolution are natural selection and genetic mutations occurring over lengthy spans of geologic time see Peter Ward [ 33 ].

Mutations are primarily the result of mistakes that occur during DNA replication. There are three different types of mutations: beneficial, deleterious, and neutral see Mayr [ 34 ]. But what does all of this have to do with the origin of sex? Evolutionists adhere to the view that the first organisms on Earth were asexual, and thus they believe that, during billions of years of Earth history, asexual organisms experienced numerous beneficial mutations that caused them to evolve into sexual organisms.

Sexual Reproduction and the Evolution of Sex

In fact, quite the opposite would be true. Why so? Beneficial mutations viz. In addressing the complete ineffectiveness of mutations as an alleged evolutionary mechanism, Dr. In a speech presented at Hobart College several years ago, the late Harvard paleontologist Stephen Jay Gould spoke out in a somewhat militant fashion about the subject when he said:. There is the added problem related to the two different types of cell division we mentioned earlier-mitosis and meiosis. During mitosis, all of the chromosomes are copied and passed on from the parent cell to the daughter cells.

Meiosis from the Greek meaning to split , on the other hand, occurs only in sex cells i. And the critical nature of meiosis to life as we know it has been acknowledged albeit perhaps begrudgingly even by evolutionists. Margulis and Sagan, for example, wrote:. These two evolutionists have admitted that meiosis is critical for sexual reproduction. At some point authors must be questioned as to the rationale in their thinking?

To answer this question, Levitis compared the viability of offspring produced by three different kinds of reproduction. Sexual reproduction, where two players make a genetic contribution, always requires meiosis. On the other hand, asexual reproduction—where the offspring are clones of their parents—usually uses the much simpler mitosis, a comparatively easy cloning of cells, no genetic reshuffling required. When asexual reproduction does use meiosis, it is even more complicated than sex.

In this three-way comparison, Levitis found that more complex reproduction resulted in lower offspring survival. For example, asexual lizards that use meiosis had lower viability than sexual lizards that also use meiosis because asexual meiosis was more complicated. Yet the organisms that used the simpler mitosis, like palm trees and damselflies, produced healthier offspring. This pattern held true in 42 of 44 species. But even after a second look, the data checked out.

Something about meiosis, seemingly its complexity, kills offspring. Regarding the evolution of sex, Levitis' findings suggest that the advantages of going through meiosis must be significant enough to balance that tally sheet. The reshuffling of genes between two parents during sex might provide even more of an advantage than previously thought.

The other takeaway, says Levitis, is that although it's easy to think that natural selection can solve every problem—and that we might wish it had, such as for high rates of pregnancy loss—sometimes it comes up against fundamental constraints. Meiosis seems to be one of those insurmountable barriers. Yet the tradeoff, offspring that are truly unique, with novel genetic combinations to face a challenging world, must be worth it.

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Evolution of sexual reproduction - Wikipedia

Thank you for taking your time to send in your valued opinion to Science X editors. You can be assured our editors closely monitor every feedback sent and will take appropriate actions. Your opinions are important to us. We do not guarantee individual replies due to extremely high volume of correspondence. E-mail the story Pregnancy loss and the evolution of sex are linked by cellular line dance Your friend's email Your email I would like to subscribe to Science X Newsletter. Learn more Your name Note Your email address is used only to let the recipient know who sent the email.

Neither your address nor the recipient's address will be used for any other purpose. The information you enter will appear in your e-mail message and is not retained by Phys. You can unsubscribe at any time and we'll never share your details to third parties. More information Privacy policy. Then they exposed those populations to the S. It was found that the self-fertilizing populations of C. 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.

In particular, Otto and Nuismer [28] presented results showing that species interactions e. They concluded that, although the Red Queen hypothesis favors sex under certain circumstances, it alone does not account for the ubiquity of sex. Otto and Gerstein [29] further stated that "it seems doubtful to us that strong selection per gene is sufficiently commonplace for the Red Queen hypothesis to explain the ubiquity of sex". Parker [30] reviewed numerous genetic studies on plant disease resistance and failed to uncover a single example consistent with the assumptions of the Red Queen hypothesis.

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

For instance, double-strand breaks in DNA occur about 50 times per cell cycle in human cells [see DNA damage naturally occurring ]. 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. 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 i.

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 , [36] 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 fertilised offspring from either one of the same parents.

However, outcrossing may be abandoned in favor of parthenogenesis 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.

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.

The Queen theory: parasites?

Sexual reproduction is believed to be more efficient than asexual reproduction in removing those mutations from the genome. There are two main hypotheses which explain how sex may act to remove deleterious genes from the genome. While DNA is able to recombine to modify alleles, DNA is also susceptible to mutations within the sequence that can affect an organism in a negative manner. Asexual organisms do not have the ability to recombine their genetic information to form new and differing alleles. Once a mutation occurs in the DNA or other genetic carrying sequence, there is no way for the mutation to be removed from the population until another mutation occurs that ultimately deletes the primary mutation.

This is rare among organisms. Hermann Joseph Muller introduced the idea that mutations build up in asexual reproducing organisms. Muller described this occurrence by comparing the mutations that accumulate as a ratchet. Each mutation that arises in asexually reproducing organisms turns the ratchet once. The ratchet is unable to be rotated backwards, only forwards. The next mutation that occurs turns the ratchet once more. Additional mutations in a population continually turn the ratchet and the mutations, mostly deleterious, continually accumulate without recombination.

The genetic load of organisms and their populations will increase due to the addition of multiple deleterious mutations and decrease the overall reproductive success and fitness. For sexually reproducing populations, studies have shown that single-celled bottlenecks are beneficial for resisting mutation build-up. Passaging a population through a single-celled bottleneck involves the fertilization event occurring with haploid sets of DNA, forming one fertilized cell.

For example, humans undergo a single-celled bottleneck in that the haploid sperm fertilizes the haploid egg, forming the diploid zygote, which is unicellular. This passage through a single cell is beneficial in that it lowers the chance of mutations from being passed on through multiple individuals. Highly related individuals are more closely related, and more clonal, whereas less related individuals are less so, increasing the likelihood that an individual in a population of low relatedness may have a detrimental mutation.

Highly related populations also tend to thrive better than lowly related because the cost of sacrificing an individual is greatly offset by the benefit gained by its relatives and in turn, its genes, according to kin selection. The studies with D. This hypothesis was proposed by Alexey Kondrashov , and is sometimes known as the deterministic mutation hypothesis.

The Evolution of Sex Determination

This relationship between number of mutations and fitness is known as synergistic epistasis. 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. 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 some individuals with fewer deleterious mutations, and some with more. Because there is a major selective disadvantage to individuals with more mutations, these individuals die out. In essence, sex compartmentalises the deleterious mutations. 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 [44] and E. Thus, for instance, for the sexual species Saccharomyces cerevisiae yeast and Neurospora crassa fungus , the mutation rate per genome per replication are 0. For the nematode worm Caenorhabditis elegans , the mutation rate per effective genome per sexual generation is 0. Geodakyan suggested that sexual dimorphism provides a partitioning of a species' phenotypes into at least two functional partitions: a female partition that secures beneficial features of the species and a male partition that emerged in species with more variable and unpredictable environments.

The male partition is suggested to be an "experimental" part of the species that allows the species to expand their ecological niche, and to have alternative configurations. This theory underlines the higher variability and higher mortality in males, in comparison to females. This functional partitioning also explains the higher susceptibility to disease in males, in comparison to females and therefore includes the idea of "protection against parasites" as another functionality of male sex. Geodakyan's evolutionary theory of sex was developed in Russia in — and was not known to the West till the era of the Internet.

Trofimova, who analysed psychological sex differences, hypothesised that the male sex might also provide a "redundancy pruning" function. 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 is longer-term than in asexual populations — so the population is less affected by short-term changes.

It has recently been shown in experiments with Chlamydomonas algae that sex can remove the speed limit [ clarification needed ] on evolution. The evolution of sex can alternatively be described as a kind of gene exchange that is independent from reproduction. That interactions between two organisms be in balance appear to be a sufficient condition to make these interactions evolutionarily efficient, i.

The "libertine bubble theory" proposes that meiotic sex evolved in proto-eukaryotes to solve a problem that bacteria did not have, namely a large amount of DNA material, occurring in an archaic step of proto-cell formation and genetic exchanges. So that, rather than providing selective advantages through reproduction, sex could be thought of as a series of separate events which combines step-by-step some very weak benefits of recombination, meiosis, gametogenesis and syngamy.

Many protists reproduce sexually, as do the multicellular plants , animals , and fungi. In the eukaryotic fossil record, sexual reproduction first appeared by 1. Organisms need to replicate their genetic material in an efficient and reliable manner.

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The necessity to repair genetic damage is one of the leading theories explaining the origin of sexual reproduction. Diploid individuals can repair a damaged section of their DNA via homologous recombination , since there are two copies of the gene in the cell and if one copy is damaged , the other copy is unlikely to be damaged at the same site.

A harmful mutation in a haploid individual, on the other hand, is more likely to become fixed i. If, as evidence indicates, sexual reproduction arose very early in eukaryotic evolution, the essential features of meiosis may have already been present in the prokaryotic ancestors of eukaryotes. Natural transformation in bacteria, DNA transfer in archaea, and meiosis in eukaryotic microorganisms are induced by stressful circumstances such as overcrowding, resource depletion, and DNA damaging conditions.

If environmental stresses leading to DNA damage were a persistent challenge to the survival of early microorganisms, then selection would likely have been continuous through the prokaryote to eukaryote transition, [56] [62] and adaptative adjustments would have followed a course in which bacterial transformation or archaeal DNA transfer naturally gave rise to sexual reproduction in eukaryotes.

Exposure to conditions that cause RNA damage could have led to blockage of replication and death of these early RNA life forms. Sex would have allowed re-assortment of segments between two individuals with damaged RNA, permitting undamaged combinations of RNA segments to come together, thus allowing survival.

Such a regeneration phenomenon, known as multiplicity reactivation, occurs in influenza virus [67] and reovirus. 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. Bacterial conjugation is a form of genetic exchange that some sources describe as "sex", but technically is not a form of reproduction, even though it is a form of horizontal gene transfer.

However, it does support the "selfish gene" part theory, since the gene itself is propagated through the F-plasmid. A similar origin of sexual reproduction is proposed to have evolved in ancient haloarchaea as a combination of two independent processes: jumping genes and plasmid swapping. A third theory is that sex evolved as a form of cannibalism : One primitive organism ate another one, but instead of completely digesting it, some of the eaten organism's DNA was incorporated into the DNA of the eater.

Sex may also be derived from another prokaryotic process. A comprehensive theory called "origin of sex as vaccination" proposes that eukaryan sex-as- syngamy fusion sex arose from prokaryan unilateral sex-as-infection, when infected hosts began swapping nuclearised genomes containing coevolved, vertically transmitted symbionts that provided protection against horizontal superinfection by other, more virulent symbionts.

While theories positing fitness benefits that led to the origin of sex are often problematic, [ citation needed ] several theories addressing the emergence of the mechanisms of sexual reproduction have been proposed. The viral eukaryogenesis VE theory proposes that eukaryotic cells arose from a combination of a lysogenic virus, an archaean , and a bacterium.

This model suggests that the nucleus originated when the lysogenic virus incorporated genetic material from the archaean and the bacterium and took over the role of information storage for the amalgam. The archaeal host transferred much of its functional genome to the virus during the evolution of cytoplasm, but retained the function of gene translation and general metabolism.

The bacterium transferred most of its functional genome to the virus as it transitioned into a mitochondrion. For these transformations to lead to the eukaryotic cell cycle, the VE hypothesis specifies a pox-like virus as the lysogenic virus. A pox-like virus is a likely ancestor because of its fundamental similarities with eukaryotic nuclei.

These include a double stranded DNA genome, a linear chromosome with short telomeric repeats, a complex membrane bound capsid, the ability to produce capped mRNA, and the ability to export the capped mRNA across the viral membrane into the cytoplasm. The presence of a lysogenic pox-like virus ancestor explains the development of meiotic division, an essential component of sexual reproduction. Meiotic division in the VE hypothesis arose because of the evolutionary pressures placed on the lysogenic virus as a result of its inability to enter into the lytic cycle.

This selective pressure resulted in the development of processes allowing the viruses to spread horizontally throughout the population.

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The outcome of this selection was cell-to-cell fusion. This is distinct from the conjugation methods used by bacterial plasmids under evolutionary pressure, with important consequences.

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These proteins could have been transferred to the cell membrane during viral reproduction, enabling cell-to-cell fusion between the virus host and an uninfected cell. The theory proposes meiosis originated from the fusion between two cells infected with related but different viruses which recognised each other as uninfected. After the fusion of the two cells, incompatibilities between the two viruses result in a meiotic-like cell division.

The two viruses established in the cell would initiate replication in response to signals from the host cell. A mitosis-like cell cycle would proceed until the viral membranes dissolved, at which point linear chromosomes would be bound together with centromeres.