In evolutionary genetics, Muller's 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. This article is about evolution in biology. ... DNA, the molecular basis for inheritance. ... 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. ... A ratchet lever hoist. ... In biology the genome of an organism is the whole hereditary information of an organism that is encoded in the DNA (or, for some viruses, RNA). ... It has been suggested that Parthenogenesis be merged into this article or section. ... A genetic deletion is a genetic aberration in which part of a chromosome is missing. ...

A ratchet illustration

Muller proposed this mechanism as a theory to explain the evolution of sex. One must note, that although Muller's ratchet is proposed to explain the success of sexual reproduction over asexual reproduction, the effect may not be prevalent in organisms that reproduce asexually but undergo other forms of recombination and may be observed in the portions of the genomes of organisms that reproduce sexually which do not undergo recombination. Image File history File links Sperrklinke_Schema. ... Image File history File links Sperrklinke_Schema. ... A ratchet lever hoist. ... The evolution of sex is a major puzzle in modern evolutionary biology. ... Sexual reproduction is a union that results in increasing genetic diversity of the offspring. ... It has been suggested that Parthenogenesis be merged into this article or section. ... Genetic recombination is the process by which a strand of DNA is broken and then joined to the end of a different DNA molecule. ...

Explanation

Asexual reproduction compels genomes to be inherited as indivisible blocks so that once the least mutated genomes in an asexual population begin to carry at least one (additional) bad mutation, no genomes with fewer mutations can be expected to be found in future generations (except as a result of highly unlikely back mutation). In sexual populations, the process of genetic recombination allows the genomes of the progeny to be different from the genomes of the parents. In particular, progeny genomes with fewer mutations can be generated from more highly mutated parental genomes by putting together in progeny genomes mutation-free parental chromosomes or mutation-free pieces thereof. Back mutation is a change in a nucleotide pair, in a mutated gene, that restores the original sequence and hence the original phenotype. ...

Among protists and prokaryotes there is a plethora of supposedly asexual organisms. More and more are being shown to exchange genetic information through a variety of mechanisms. It is instead quite clear that the genomes of mitochondria and chloroplasts do not recombine and would undergo Muller's ratchet were they not as small as they are. Indeed, the probability that the least mutated genomes in an asexual population end up carrying at least one (additional) mutation depends heavily on the genomic mutation rate and this increases more or less linearly with the size of the genome (more accurately, with the number of base pairs present in active genes). However, reductions in genome size specially in parasites and symbionts can also be caused by direct selection to get rid of genes that have become unnecessary. Therefore a smaller genome is not a sure indication of the action of Muller's Ratchet. Typical phyla Rhodophyta (red algae) Chromista Heterokontophyta (heterokonts) Haptophyta Cryptophyta (cryptomonads) Alveolates Pyrrhophyta (dinoflagellates) Apicomplexa Ciliophora (ciliates) Excavates Euglenozoa Percolozoa Metamonada Rhizaria Radiolaria Foraminifera Cercozoa Amoebozoa Choanozoa Many others; classification varies The Kingdom Protista or Protoctista is one of the commonly recognized biological kingdoms, including all the eukaryotes except for... Prokaryotes are unicellular (in rare cases, multicellular) organisms without a nucleus. ... In cell biology, a mitochondrion is an organelle found in the cells of most eukaryotes. ... Chloroplasts are organelles found in plant cells and eukaryotic algae that conduct photosynthesis. ...

In sexually reproducing organisms non-recombining chromosomes or chromosomal regions like, e.g., the mammalian Y chromosome, should also undergo Muller's Ratchet. (The Y chromosome appears to repair double-strand breaks by means of template-assisted recombinational repair at its mirror-like sequences but this "self-recombination" does not neutralize this chromosome's tendency to undergo Muller's Ratchet). And indeed such non-recombining sequences tend to shrink and evolve quickly. However, such fast evolution can also be due to these sequences' inability to repair DNA damage via template-assisted repair which de facto equates to an increase in the mutation rate. Therefore it is not easy to ascribe such cases of genome shrinkage and/or fast evolution only to Muller's Ratchet sensu stricto, i.e., to an accelerated accumulation of bad mutations that is caused by an inability to generate recombinant progeny. The human Y chromosome is one of two sex chromosomes, it contains the genes that cause testis development, thus determining maleness. ...

Muller's Ratchet turns faster in smaller populations and it is thought to set limits to the maximum size of asexual genomes and to the long-term evolutionary continuity of asexual lineages (but some asexual lineages are thought to be quite ancient: Bdelloid rotifers, e.g., appear to have been asexual for nearly 40 million years). The bdelloids (Bdelloidea) are a class of rotifers, found in freshwater and moist soil. ...

Note, furthermore, that a battery of additional population-genetic and ecological processes have been proposed to explain the success of sexual forms, and it is still unclear which one is truly crucial (see evolution of sex, the Red Queen hypothesis). The evolution of sex is a major puzzle in modern evolutionary biology. ... The Red Queens Hypothesis, Red Queen, Red Queens race or Red Queen Effect is an evolutionary hypothesis to explain the advantage of sex at the level of individuals, and the constant evolutionary arms race between competing species. ...

Origin of the term

Although Muller discussed the advantages of sexual reproduction in his 1932 talk, it does not contain the word "ratchet". Muller first introduced the term "ratchet" in his 1964 paper, and the phrase "Muller's ratchet" was coined by Joe Felsenstein in his 1974 paper, "The Evolutionary Advantage of Recombination". Joseph Joe Felsenstein is Professor of Genome Sciences and Biology and Adjunct Professor of Computer Science and Statistics at the Department of Genome Sciences of the University of Washington in Seattle. ...

See also

• Genetic hitchhiking
• Mutational meltdown
• Hill-Robertson effect

The process by which an evolutionary neutral or in some cases deleterious allele or mutation may spread through the gene pool by virtue of being linked to a beneficial mutation. ... Mutational meltdown refers to the process by which a small population accumulates deleterious mutations, which leads to loss of fitness and decline of the population size, which leads to further accumulation of deleterious mutations. ... Hill and Robertson (1966) found an effect that explains why recombination is evolutionarily selected for. ...

References

• Muller, H.J. (1932). "Some Genetic Aspects of Sex". American Naturalist 66 (703): 118-138.  (Muller's original paper)
• Muller, H.J. (1964). "The Relation of Recombination to Mutational Advance". Mutat Res 106: 2-9. PMID 14195748.  (original paper as cited by, e.g.: Maynard Smith J; Szathmary E (1997). The major transitions in evolution. Oxford, New York, Tokyo: Oxford University Press.  ; Futuyma DJ (1998). Evolutionary biology, 3rd edn, Sunderland, Mass.: Sinauer Associates. )
• Nancy A. Moran (Apr 1996). "Accelerated evolution and Muller's ratchet in endosymbiotic bacteria". Proceedings of the National Academy of Sciences USA 93 (7): 2873-8. PMID 8610134.  (An article that discusses Muller's ratchet in the context of endosymbiotic bacteria.)
• Felsenstein, J (Oct 1974). "The Evolutionary Advantage of Recombination". Genetics 78 (2): 737-756. PMID 4448362.