Talk:Muller's ratchet

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The paragraph on RNA viruses is completely incorrect. The paper cited makes no mention of Muller or the ratchet effect; in fact, the authors of the paper theorize that it is the failure of the high-fidelity viruses to mutate around the host immune system. The paragraph in this page states that the wildtype high mutation rate allows reversion mutations to escape Muller's ratchet, but this is a fallacy, as a higher mutation rate, by Muller's definition, leads to more deleterious mutations than to reversions. A high mutation rate simply accelerates the ratchet effect. I am going to delete this paragraph in a few days, unless someone can come up with evidence that it should stay.

Estewar01 (talk) 18:57, 9 September 2016 (UTC)

article in need of rewriting

This article needs loads of rewriting. The introductory concept is very vague, and it also fails to indicate the importance of random drift in spreading and fixing the deleterious mutants. Defining the Muller's ratchet as happening in asexual populations is perhaps innapropriate, it is simply better to define it as what happens in chromosomes when there is lack of recombination. There is recombination in asexual genomes, so this is not a simple feature of asexual populations, also, some sexual populations show non-recombinant regions, so again the definition is not appropriate. A figure showing the ratchet mechanisms would be very useful.

80.41.59.236 11:57, 19 June 2006 (UTC)

I've worked in this area. I agree that drift (i.e. sample error) is a critical aspect of the Ratchet. My favorite way to explain the process is to consider the limit distribution of a population at equilibrium for mutation and selection. Such a population has a normal distribution for fitness. The tails of a normal distribution are sparsely populated. In particular, the highly fit tail has few occupants. Every generation is produced by randomly sampling the existing population. Since the population is necessarily finite, the probability is reasonably great that the high fitness tail is, by chance, not represented in the next generation. When this occurs, the high fitness tail is not reformed - it is replaced by a slightly lower fitness group and the limit equilibrium distribution reforms around the new lower fitness mean.

One can visualize this process with a schematic of a normal distribution, highlighting the high fitness tail. If the density of high fitness individuals is p and the population size is K, the probability that no individuals from the high fitness cadre makes it into the next population is (1- p)^K. The probability that this loss happens over g generations is 1 - (1 - (1 -p)^K)^g. As g increases, this probability approaches 1. (I believe this analysis can be pulled from Muller's 1964 paper, though it has been, ahem, some time since I read it).

In sexual populations, the process of sampling is the same, except that recombination continuously throws new individuals out to the tails, reforming the high fitness tail and maintaining the equilibrium distribution over much longer time scales. The longer time scale allows back mutations and other beneficial mutations the opportunity to be increased in frequency by selection.

Gubbish (talk) 18:14, 2 May 2010 (UTC)

The statement "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" is 100% BS. In fact, mitochondria have multiple copies (65 each in humans) of their circular chromosomes, and they many recombine within a mitochondria for all we know (or between mitochondria inside a cell, since most cells have dozens of of them). Certainly their smallness in no way leads to their avoiding Muller's ratchet--after all, some bacteria are smaller. 129.252.89.201 06:48, 7 January 2007 (UTC)speciate

Are you guys editing or just making old fogey style comments? Samsara (talk • contribs) 15:28, 13 March 2007 (UTC)

old fogey comment - something like Muller's ratchet is seen in very small populations even with recombination. see "Risk of Population Extinction from Fixation of New Deleterious Mutations" Lande, 1994. The point of emphasizing genomic size is that the rate of mutation for the genome as a whole is much smaller, reducing the effectiveness of the Ratchet. the possibility of recombination among the small number of mitochondria is not that compelling since the absolute number of genomes is small and each generation there is a severe bottleneck from the very small number of, for example, mitochondria that come through the egg.

Gubbish (talk) 18:14, 2 May 2010 (UTC)

The comments are well put I say. No matter whether an effect makes sense in theory - if it cannot be proven in vivo, to hell with the theory! This is the life sciences, not philosophy class.

So... where has it been found? Dysmorodrepanis (talk) 06:56, 16 February 2008 (UTC)

Indirect evidence in parthenogenetic lizard and fish clade die off (lot of references in Lynch, Lynch et.al.). Empirical evidence in fruit fly experiments, probably referenced in Lynch, work by Houle.

Gubbish (talk) 18:14, 2 May 2010 (UTC)

When the Ratchet was and wasn't introduced

I am not going to edit this page as I am already mentioned in it. But in my opinion the Ratchet is not mentioned in Muller's 1932 paper. I think the clearest way to think of this is that in that paper Muller did invoke what we now know as the Hill-Robertson effect, the interaction of selection and drift at closely linked loci that interferes with response to selection. This was the basis of both Fisher's and Muller's theory of the advantage of recombination. But the Ratchet, in which in the absence of recombination, the fittest class of haplotypes gets lost and can only be regenerated by back mutatiion (or immigration from elsewhere), is first mentioned in Muller's 1958 paper. The followup to that paper also uses the word "ratchet" (as mentioned in this page). Muller (1964) says that the population

an asexual population incorporates a kind of ratchet mechanism, such that it can never get to contain in any of its lines, a load of mutation smaller than that already existing in its present least-loaded lines.

Muller's Ratchet was pointed out to me by Jim Crow which led to me working on it after I began to understand the Hill-Robertson effect and saw that it was the overarching phenomenon that explained the Ratchet as well as the Fisher-Miller argument for the evolution of recombination. Felsenst (talk) 14:15, 7 November 2011 (UTC)

Clarifications Needed

It seems that the ratchet only operates when 2 key conditions are met. (1) back-mutation is absent (or at least very rare in some undefined precise sense) and (2) the population is small so stochastic effects are important. No attempt is made in the article to justify these assumptions, except by references to technical books and articles. Maybe they are good assumptions justified by data, but neither are obviously true. A lay reader will gain the impression that asexually reproducing species will rapidly become extinct, which seems nonsense (even if these species do not exhibit extensive non-sexual recombination). It's completely counterintuitive that the highest-fitness genotype will disappear and be replaced by lower-fitness genotypes, since by definition the highest fitness genotype is best at surviving and reproducing. But of course if the population is small and chance fluctuations are important, even the best genotype can be lost. The article needs to include that important "even",which aids understanding. It also needs to explain why back mutation is even less likely than these rare fluctuations that can remove the best genotype, which is not intuitively obvious. Paulhummerman (talk) 15:52, 26 October 2021 (UTC)