Muller's ratchet

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The pawl (2) prevents the ratchet wheel (1) from turning backwards. In the same way, the Muller ratchet can only be turned in the "direction" of additional, harmful gene mutations.

Muller's ratchet (after Hermann Joseph Muller ), German " Muller ratchet or ratchet ", is a concept in evolutionary biology . It describes the inevitable accumulation of disadvantageous mutations in natural populations of limited size, with moderately high to high mutation rates, in the absence of recombination . Since recombination is usually linked to sexual reproduction, the mechanism is considered to be an essential reason for the evolution of sexuality in general, and for the inevitable, mostly in evolutionarily short periods of time, extinction of species that have given up sexual reproduction secondarily; these become "evolutionary dead ends".

In brief, the mechanism can be described as follows: “Any population - whether sexually or asexually reproducing - will contain deleterious mutations, and individuals completely absent from such mutations are rare exceptions. In a small population, each individual in each generation may contain at least one of them. This is not a problem if the species reproduces sexually, because individuals can be rebuilt at any time without adverse mutations through the action of recombination. In an asexually reproducing population, however, disadvantageous mutations can no longer be eliminated (except through reverse mutations , which are always unlikely, but especially in small populations). In the next generation, the best ( fittest ) individual therefore also has a disadvantageous mutation. In a later generation there will at some point no longer be an individual who has only one disadvantageous mutation, so that the best individual now has two such mutations; and so on. The number of mutations increases in stages, a return is impossible. Thus, in a development corresponding to the turning of a ratchet wheel, in a small, asexually reproducing population, an ever increasing number of disadvantageous mutations accumulate until it eventually dies out because of it. "

The mechanism arises from the theory of recombination, which researchers such as Hermann Joseph Muller and Ronald Aylmer Fisher developed from the 1930s onwards. Muller introduced the ratchet mechanism in a work from 1932, the linguistic image of the "ratchet" first in 1964. The expression "Muller's ratchet" goes back to a work by Joseph Felsenstein , a mathematical formalization comes from John Haigh .

Of course, positive mutations, ie those with beneficial consequences for the individual, work against this mechanism. However, it is a realistic assumption that many natural populations with their long evolutionary history are primarily subject to stabilizing or purifying selection ; in this case it is realistic to assume that most of the mutations will be rather detrimental. But it could be made likely that if the rate of favorable mutations is high enough, the mechanism can be stopped. It should be noted that the mechanism strongly depends on the mutation rate (more precisely: on the mutation rate in relation to the population size). With this, the level of the mutation rate under strong selection can itself become a selection factor and possibly be reduced considerably in a short time. On the other hand, the mutation rate can be greatly accelerated by the mechanism itself, so that it ultimately amplifies itself.

Most prokaryotes and viruses probably escape the mechanism through their extremely high effective population size, and non-sexual horizontal gene transfer counteracts it here . Whether the mechanism could have been the cause of the development of horizontal gene transfer is controversial. Another protection is probably the possession of RNA polymerases with a relatively high error rate, which, with their relatively small genome, increase the likelihood of reverse mutations. Species that maintain recombination can escape the mechanism even if they have given up classic sexual reproduction with two sexes, as has also been shown experimentally with the fungus Cryptococcus neoformans .

Individual evidence

  1. ↑ James F. Crow (2005): Hermann Joseph Muller, Evolutionist. Nature Reviews Genetics 6: 941-945. doi : 10.1038 / nrg1728
  2. ↑ H. Muller (1932): Some genetic aspects of sex. American Naturalist 66: 118-138.
  3. ↑ H. Muller (1964): The relation of recombination to mutational advance. Mutation Research 1: 2-9.
  4. ↑ J. Felsenstein (1974): The evolutionary advantage of recombination. Genetics 78: 737-756.
  5. ^ J. Haigh (1978): The accumulation of deleterious genes in a population: Muller's ratchet. Theoretical Population Biology 14: 251-267. doi : 10.1016 / 0040-5809 (78) 90027-8
  6. ↑ Philip J. Gerrish, Alexandre Colato, Paul D. Sniegowski (2013): Genomic mutation rates that neutralize adaptive evolution and natural selection. Journal of the Royal Society Interface vol. 10 no.85, article 20130329. doi : 10.1098 / rsif.2013.0329 (open access)
  7. ^ R. Jonas Söderberg & Otto G. Berg (2011): Kick-Starting the Ratchet: The Fate of Mutators in an Asexual Population. Genetics vol. 187 no.4: 1129-1137. doi : 10.1534 / genetics.110.124818
  8. ↑ Etienne Simon-Loriere & Edward C. Holmes (2011): Why do RNA viruses recombine? Nature Reviews Microbiology 9 (8): 617-626. doi : 10.1038 / nrmicro2614 (open access)
  9. ↑ Lark L Coffey (2011): Arbovirus high fidelity variant loses fitness in mosquitoes and mice. Proceedings of the National Academy of Sciences USA 108 (38): 16038-16043. doi : 10.1073 / pnas.1111650108
  10. ↑ Kevin C. Roach & Joseph Heitman (2014): Unisexual Reproduction Reverses Muller's Ratchet. Genetics vol. 198 no.3: 1059-1069. doi : 10.1534 / genetics.114.170472