Evolution of sexual reproduction



Das Thema Biodiversität wird transversal aufgegriffen. As discussed in the earlier part of this article, sexual reproduction is conventionally explained as an adaptation for producing genetic variation through allelic recombination.

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Auch das Gegenteil ist der Fall: Sie traten aus verschiedenen Gründen auf und entfalteten sich auf unterschiedliche Weise. Ihr Einfluss auf die Lebensformen war nicht absolut, sondern relativ. Einige Gruppen litten mehr, andere weniger. Dies ist das Zeitalter des ältesten bekannten Fossils. Am Ende der Kreidezeit waren die Säugetierarten sehr vielfältig. Sie teilten den Planeten mit Monotromen eierlegende Säugetiere und Multituberkulaten ein ausgestorbenes Taxon von nagetierähnlichen Säugetieren, das nach der spezifischen Form ihrer Zähne benannt ist, die mehrere Warzen hatten.

Die Studie von Pires and Quental betont, dass Säugetiere vom Massensterben in der Kreide besonders stark betroffen waren. Das Massensterben war für einige schwerer als für andere. Während der Kreidezeit, vor bis 66 Millionen Jahren, waren die Multituberkulaten die dominante und vielfältigste Gruppe von Säugetieren. Wir wissen das, weil Multituberkulate die überwiegende Mehrheit in der fossilen Aufzeichnung vor dem K-Pg-Ereignis darstellen.

Fossilien von Plazentatieren und Beuteltieren sind weniger zahlreich, aber auch reichlich vorhanden. Monotreme bilden eine Ausnahme. Heute sind sie selten aber weit verbreitet. Tatsächlich bestehen sie nur aus zwei Familien: Eine umfasst das Schnabeltier, die andere betrifft Echidnas. Monotreme sind auch in der fossilen Aufzeichnung sowohl vor als auch nach der Kreide selten, was darauf hindeutet, dass die Gruppe immer relativ marginal war.

Welche hatten die am besten erhaltenen Gattungen? Welche Gruppe konnte sich von der Katastrophe nicht erholen? Quental und Pires wählten Nordamerika als Schwerpunkt ihrer Studie. Einhundertfünfzig Jahre kontinuierlicher paläontologischer Prospektion in der Region haben ein detailliertes Bild der Vielfalt der Säugetiere vor, während und nach dem K-Pg-Ereignis erhalten. Die Wissenschaftler verwendeten einen Datensatz mit aktuellen fossilen Assemblagen aus der Kreide und dem Paläozän vor 69,9 Millionen bis 55 Millionen Jahren im westlichen Landesinneren Nordamerikas.

Fossile Vorkommen sind relativ gut abgegrenzt, wodurch die taxonomische Unsicherheit minimiert werden kann. Man verwendete mehrere fortschrittliche statistische Methoden, um Ursprungs-, Aussterbe- und Diversifikationsmuster vor, während und nach dem K-Pg-Ereignis zu ermitteln.

Die Ergebnisse zeigten, dass sich die drei Gruppen nach dem Massenaussterben sehr unterschiedlich entwickelten. Die Entstehungsrate für Methateria Beuteltiere zum Beispiel blieb während des untersuchten Intervalls annähernd konstant.

Allerdings wurde während des K-Pg Ereignisses ein deutlicher Aussterbe-Spitzenwert festgestellt, der einen Puls negativer Nettodiversifikation erzeugt. Nach dem K-Pg-Event nahm die Aussterberate allmählich ab, aber die negative Nettodiversifikation hielt mehr als 2 Millionen Jahre lang an, bis vor etwa 64 Millionen Jahren. Multituberkulate diversifizierten gegen Ende der Kreidezeit vor der K-Pg-Grenze und zeigten hohe Ursprungsraten und relativ niedrige Aussterberaten.

Mit anderen Worten, während der K-Pg war die Diversifikationsrate im Gleichgewicht, da etwa die gleiche Anzahl von Gattungen gebildet wurden und ausstarben. Der Studie zufolge ist die Extinktionsrate für Multituberkulate nach der K-Pg-Grenze weiter gesunken; der Rückgang der Entstehungsrate für Multituberkulate war jedoch noch stärker, was zu einer negativen Diversifizierung führte.

Der Rückgang scheint seit langem andauert zu haben, da die Multituberkulate stetig weiter verschwanden. Der Stamm endet vor etwa 35 Millionen Jahren. Wissenschaftler glauben, dass der Grund für das Verschwinden der Multituberkulate in der zunehmenden Konkurrenz mit den Nagetieren lag, einer neuen eutherischen Linie, die kurz nach dem Massenaussterben entstand.

Kurz danach gab es einen zweiten Entstehungsimpuls, der von einem Rückgang der Extinktionsrate begleitet wurde, was auf einen kurzen Sprung in der Diversifikation hinweist. 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. 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 [42] 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: 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. 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, [54] [60] 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 [65] and reovirus. Another theory is that sexual reproduction originated from selfish parasitic genetic elements that exchange genetic material that is: In some organisms, sexual reproduction has been shown to enhance the spread of parasitic genetic elements e.

A similar origin of sexual reproduction is proposed to have evolved in ancient haloarchaea as a combination of two independent processes: A third theory is that sex evolved as a form of cannibalism: Sex may also be derived from another prokaryotic process. 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.

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. 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. The homologous nature of the two viral centromeres would incite the grouping of both sets into tetrads. It is speculated that this grouping may be the origin of crossing over, characteristic of the first division in modern meiosis.

The partitioning apparatus of the mitotic-like cell cycle the cells used to replicate independently would then pull each set of chromosomes to one side of the cell, still bound by centromeres. These centromeres would prevent their replication in subsequent division, resulting in four daughter cells with one copy of one of the two original pox-like viruses. The process resulting from combination of two similar pox viruses within the same host closely mimics meiosis.

An alternative theory, proposed by Thomas Cavalier-Smith , was labeled the Neomuran revolution. The designation "Neomuran revolution" refers to the appearances of the common ancestors of eukaryotes and archaea.

Cavalier-Smith proposes that the first neomurans emerged million years ago. Other molecular biologists assume that this group appeared much earlier, but Cavalier-Smith dismisses these claims because they are based on the "theoretically and empirically" unsound model of molecular clocks. Cavalier-Smith's theory of the Neomuran revolution has implications for the evolutionary history of the cellular machinery for recombination and sex.

It suggests that this machinery evolved in two distinct bouts separated by a long period of stasis; first the appearance of recombination machinery in a bacterial ancestor which was maintained for 3 Gy, [ clarification needed ] until the neomuran revolution when the mechanics were adapted to the presence of nucleosomes. The archaeal products of the revolution maintained recombination machinery that was essentially bacterial, whereas the eukaryotic products broke with this bacterial continuity.

They introduced cell fusion and ploidy cycles into cell life histories. Cavalier-Smith argues that both bouts of mechanical evolution were motivated by similar selective forces: From Wikipedia, the free encyclopedia. Redirected from Evolution of sex. How sexually reproducing multicellular organisms could have evolved from a common ancestor species. Introduction to evolution Evidence of evolution Common descent Evidence of common descent. History of evolutionary theory.

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