The principle of Muller’s ratchet
The main effect of Muller’s ratchet is the accumulation of slightly harmful DNA changes in a population over many generations. This can lead to the extinction of species and is thus of considerable biological interest.
Name. "Muller’s ratchet" is the name of a particular population genetical model that describes a situation, where many slightly harmful DNA changes occur repeatedly in a population of individuals. It's mechanism was first proposed by Hermann J. Muller, the Nobel-prize winning geneticist that discovered the mutagenic effects of high levels of radiation. The name "Muller's ratchet" comes from Joe Felsenstein (1974).
The main effect of Muller’s ratchet is the accumulation of slightly deleterious mutations, despite the fact that selection opposes their fixation in the population. Such mutation accumulation can sometimes lead to eventual extinction. This occurs if
- recombination is absent,
- population size is finite,
- almost no back mutations occur,
- slightly deleterious mutation rates are high and
- purifying selection is too weak to remove all new deleterious mutations.
A click of the ratchet. In clonally reproducing populations there is a substantial probability that all fittest individuals ("the fittest class") will eventually acquire a slightly deleterious mutation and therefore go extinct, so that only "second fittest" individuals survive. This event is called a ‘click’ of the ratchet. After each click all previously second fittest individuals become the new class of fittest individuals, because they carry only one additional deleterious mutation, while all other individuals carry more deleterious mutations.
The rate of the ratchet. A population will accumulate mutations with a characteristic rate that depends on effective population size, deleterious mutation rate and selection coefficient, which characterizes the size of mutational effects on fitness related traits like survival.
Extinction. If the mutations that accumulate decrease the effective reproductive capacity, they can lead to the extinction of the population. Muller (1963) was the first to consider the possibility of extinction exclusively due to the ratchet, but his later main paper dismissed this possibility, largely emphasizing the disadvantage of asexuals that compete with sexual species (Muller, 1964). Consequently, extinction was frequently considered only in the context of competing lines. However there is no reason, why mutation accumulation might not lead to the extinction of a whole species, even in the absence of competition, if deleterious mutations frequently decrease the absolute reproductive capacity as stated by the mutational meltdown theory (Lynch & Gabriel, 1990; Lynch et al., 1993).
Significance. If Muller’s ratchet can lead to extinctions, then it might help explaining the features that we observe in surviving species today. For example, asexual species are extremely rare and Muller's ratchet might contribute to this. This potential biological importance continues to attract attention. Any theory that contributes to a better understanding of extinction of species also has obvious applications in conservation biology.
- Loewe L (2006) "Quantifying the genomic decay paradox due to Muller's ratchet in human mitochondrial DNA" Genetics Research 87:133-159. PDF | ISI | PubMed | DOI | Journal | This is the most recent and most comprehensive review of research around Muller's ratchet currently available.
- Felsenstein, J. (1974). The evolutionary advantage of recombination. Genetics 78, 737-756.
- Lynch, M., Butcher, R. B. D. & Gabriel, W. (1993). The mutational meltdown in asexual populations. J. Hered. 84, 339-344.
- Lynch, M. & Gabriel, W. (1990). Mutation load and the survival of small populations. Evolution 44, 1725-1737.
- Muller, H. J. (1964). The relation of recombination to mutational advance. Mutat. Res. 1, 2-9.
- Muller, H. J. (1963). The need for recombination to prevent genetic deterioration. Genetics 48, 903-903.