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There are a lot of sci-fi plots where mutagens, especially radiation, cause some organism to evolve at an accelerated rate. Obviously evolution does not occur within a single organism and most of the authors behind these works are more concerned with finding a justification for a desired plot rather than adhering to known science. Nonetheless, I'm curious to know if there is any well-known case where such a scenario is true. (To get one obvious reply out of the way, all evolution that we've observed on Earth occurs in the presence of the background radiation experienced on Earth. Also, although mutagens by definition increase the rate of mutation, those mutations can of course be harmful, so the question is whether the effective "rate of evolution" can be increased rather than the rate of mutation.)

Is there any well-known case where introducing mutagens can increase the "rate of evolution" in an experimental evolution study?

By "rate of evolution", I'm informally referring to either the amount of time or the number of generations that pass before some beneficial mutation occurs and is selected for. For example, in the E. Coli strains from Lenski's E. coli evolution experiments, are there any known cases where a mutagen reduces the amount of time or the number of generations required for a non-citrate-metabolizing E. coli strain to acquire the ability to metabolize citrate?

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  • $\begingroup$ The Ames test comes to mind, though it’s specifically designed to detect beneficial mutations in response to a mutagen. $\endgroup$ – canadianer May 3 '18 at 16:08
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Yes, although what you mean by increasing the rate of evolution is a little vague. Plant breeders regularly use mutagens of various types to increase the mutation rate. The mutants are then screened for useful traits, and useful mutants are cross-bred with existing lines.

Chemical mutagens like EMS and DMS, radiation and transposons are used to generate mutants with desirable traits to be bred with other cultivars – a process known as Mutation Breeding. Classical plant breeders also generate genetic diversity within a species by exploiting a process called somaclonal variation, which occurs in plants produced from tissue culture, particularly plants derived from callus. Induced polyploidy, and the addition or removal of chromosomes using a technique called chromosome engineering may also be used.

Wikipedia: Plant breeding

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Do mutagens ever increase the “rate of evolution”? [..] By "rate of evolution", I'm informally referring to either the amount of time or the number of generations that pass before some beneficial mutation occurs and is selected for

This is a very unusual definition of "rate of evolution" but I'm happy to consider it if you want.

If more mutations happen per generation, then it takes less time for a beneficial mutation to happen. So yes!

By rate of evolution, some people may like to refer to the rate at which neutral mutations fix (fixation=reaching a frequency of 1 in the population). If $\mu$ is the mutation rate, then $2N\mu$ mutations are produced each generation (assuming diploidy), and a fraction $\frac{1}{2N}$ of them fix (as they are neutral), this means that the rate of fixation is $\frac{2N\mu}{2N} = \mu$. So, again increasing the mutation rate increases the rate of fixation of neutral mutations. It is actually true also for deleterious and beneficial mutations but the computation is a little more complicated.

Note however, that there is a point after which the mutation rate is so high, that your population might be drawn to exctinction. This is called a mutational meltdown. Mutational meltdown is unlikely to happen unless the population size is already small to start with and/or the mutation rate is really really high.

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  • $\begingroup$ But OP seems to be looking for a documented case where the rate of fixation of advantageous mutations (a combination of mutation rate, strength of selection, and population size) has been shown to increase in an environment with a higher overall mutation rate ... $\endgroup$ – Ben Bolker May 3 '18 at 17:03
  • $\begingroup$ Perhaps you can also comment about mutation-selection balance. I guess that would be a relevant factor here. $\endgroup$ – WYSIWYG May 4 '18 at 9:32

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