Muller's ratchet

The negative effect of accumulating irreversible deleterious mutations may not be prevalent in organisms which, while they reproduce asexually, also undergo other forms of recombination.

[4] Also, laboratory experiments have confirmed the existence of the ratchet and the consequent extinction of populations in many organisms (under intense drift and when recombinations are not allowed) including RNA viruses, bacteria, and eukaryotes.

Also, purifying selection, to some extent, unburdens a loaded population when recombination results in different combinations of mutations.

In contrast, the genomes of mitochondria and chloroplasts do not recombine and would undergo Muller's ratchet were they not as small as they are (see Birdsell and Wills [pp. 93–95]).

However, reductions in genome size, especially in parasites and symbionts, can also be caused by direct selection to get rid of genes that have become unnecessary.

[11] Furthermore, a vertebrate fish, Poecilia formosa, seems to defy the ratchet effect, having existed for 500,000 generations.

This has been explained by maintenance of genomic diversity through parental introgression and a high level of heterozygosity resulting from the hybrid origin of this species.

[12] In 1978, John Haigh used a Wright–Fisher model to analyze the effect of Muller's ratchet in an asexual population.

[13] If the ratchet is operating the fittest class (least loaded individuals) is small and prone to extinction by the effect of genetic drift.

In an asexual population which suffers from ratchet the frequency of fittest individuals would be small, and go extinct after few generations.

Following each click, the rate of accumulation of deleterious mutation would increase, and ultimately results in the extinction of the population.

[14] Early RNA replicators capable of recombination may have been the ancestral sexual source from which asexual lineages could periodically emerge.

[15] Muller's ratchet under such ancient conditions would likely have impeded the evolutionary persistence of the asexual lineages that were unable to undergo recombination.

[14] Since deleterious mutations are harmful by definition, accumulation of them would result in loss of individuals and a smaller population size.

Small populations are more susceptible to the ratchet effect and more deleterious mutations would be fixed as a result of genetic drift.

This creates a positive feedback loop which accelerates extinction of small asexual populations.

[16] It appears that mutational meltdown due to Muller’s ratchet can be avoided by a little bit of sex as in the common apomictic asexual flowering plant Ranunculus auricomus.