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In
evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...
ary
genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinians, Augustinian ...
, Muller's ratchet (named after
Hermann Joseph Muller Hermann Joseph Muller (December 21, 1890 – April 5, 1967) was an American geneticist who was awarded the 1946 Nobel Prize in Physiology or Medicine, "for the discovery that mutations can be induced by X-rays". Muller warned of long-term dang ...
, by analogy with a
ratchet effect The ratchet effect is a concept in sociology and economics illustrating the difficulty with reversing a course of action once a specific thing has occurred, analogous with the mechanical ratchet (device), ratchet that allows movement in one dire ...
) is a process which, in the absence of recombination (especially in an asexual
population Population is a set of humans or other organisms in a given region or area. Governments conduct a census to quantify the resident population size within a given jurisdiction. The term is also applied to non-human animals, microorganisms, and pl ...
), results in an accumulation of irreversible deleterious mutations. (original paper as cited by, e.g.: ; ) This happens because in the absence of recombination, and assuming reverse mutations are rare, offspring bear at least as much mutational load as their parents. Muller proposed this mechanism as one reason why
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...
may be favored over
asexual reproduction Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the f ...
, as sexual organisms benefit from recombination and consequent elimination of deleterious mutations. 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. This effect has also been observed in those regions of the genomes of sexual organisms that do not undergo recombination.


Etymology

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".


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 deleterious mutation, no genomes with fewer such mutations can be expected to be found in future generations (except as a result of back mutation). This results in an eventual accumulation of mutations known as genetic load. In theory, the genetic load carried by asexual populations eventually becomes so great that the population goes extinct. 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. In sexual populations, the process of genetic recombination allows the genomes of the
offspring In biology, offspring are the young creation of living organisms, produced either by sexual reproduction, sexual or asexual reproduction. Collective offspring may be known as a brood or progeny. This can refer to a set of simultaneous offspring ...
to be different from the genomes of the parents. In particular, progeny (offspring) genomes with fewer mutations can be generated from more highly mutated parental genomes by putting together mutation-free portions of parental chromosomes. Also,
purifying selection In natural selection, negative selection or purifying selection is the selective removal of alleles that are deleterious. This can result in stabilising selection through the purging of deleterious genetic polymorphisms that arise through random ...
, to some extent, unburdens a loaded population when recombination results in different combinations of mutations. Among
protists A protist ( ) or protoctist is any Eukaryote, eukaryotic organism that is not an animal, Embryophyte, land plant, or fungus. Protists do not form a Clade, natural group, or clade, but are a Paraphyly, paraphyletic grouping of all descendants o ...
and
prokaryotes A prokaryote (; less commonly spelled procaryote) is a single-celled organism whose cell lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Greek (), meaning 'before', and (), meaning 'nut' ...
, a plethora of supposedly asexual organisms exists. More and more are being shown to exchange genetic information through a variety of mechanisms. In contrast, the genomes of
mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...
and
chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
s do not recombine and would undergo Muller's ratchet were they not as small as they are (see Birdsell and Wills p. 93–95. 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, especially 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. In sexually reproducing organisms, nonrecombining chromosomes or chromosomal regions such as the mammalian
Y chromosome The Y chromosome is one of two sex chromosomes in therian mammals and other organisms. Along with the X chromosome, it is part of the XY sex-determination system, in which the Y is the sex-determining chromosome because the presence of the ...
(with the exception of multicopy sequences which do engage intrachromosomal recombination and gene conversion) should also be subject to the effects of Muller's ratchet. Such nonrecombining sequences tend to shrink and evolve quickly. However, this fast evolution might also be due to these sequences' inability to repair DNA damage via template-assisted repair, which is equivalent to an increase in the mutation rate for these sequences. Ascribing cases of genome shrinkage or fast evolution to Muller's ratchet alone is not easy. Muller's ratchet relies on
genetic drift Genetic drift, also known as random genetic drift, allelic drift or the Wright effect, is the change in the Allele frequency, frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene va ...
, and turns faster in smaller populations because in such populations deleterious mutations have a better chance of fixation. Therefore, it sets the limits to the maximum size of asexual genomes and to the long-term evolutionary continuity of asexual lineages. However, some asexual lineages are thought to be quite ancient; Bdelloid rotifers, for example, appear to have been asexual for nearly 40 million years. However, rotifers were found to possess a substantial number of foreign genes from possible
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...
events. 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 Introgression, also known as introgressive hybridization, in genetics is the transfer of genetic material from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Introg ...
and a high level of heterozygosity resulting from the hybrid origin of this species.


Calculation of the fittest class

In 1978, John Haigh used a Wright–Fisher model to analyze the effect of Muller's ratchet in an asexual population. If the ratchet is operating the fittest class (least loaded individuals) is small and prone to extinction by the effect of genetic drift. In his paper Haigh derives the equation that calculates the frequency of individuals carrying k mutations for the population with stationary distribution: n_k\ =\ \frac \theta=\ \lambda / s where, n_k\ is the number of individual carrying k mutations, N is the population size, \lambda is the mutation rate and (1-s)^k is the selection coefficient for a genome with k mutations. Thus, the frequency of the individuals of the fittest class (k =0 ) is: \ = N \ \ e^ In an asexual population which suffers from ratchet the frequency of fittest individuals would be small, and go extinct after few generations. This is called a click of the ratchet. Following each click, the rate of accumulation of deleterious mutation would increase, and ultimately results in the extinction of the population.


The antiquity of recombination and Muller's ratchet

It has been argued that recombination was an evolutionary development as ancient as
life Life, also known as biota, refers to matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes. It is defined descriptively by the capacity for homeostasis, Structure#Biological, organisation, met ...
on Earth. Early RNA replicators capable of recombination may have been the ancestral sexual source from which asexual lineages could periodically emerge. Recombination in the early sexual lineages may have provided a means for coping with genome damage. Muller's ratchet under such ancient conditions would likely have impeded the evolutionary persistence of the asexual lineages that were unable to undergo recombination.


Muller's ratchet and mutational meltdown

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. This phenomenon has been called
mutational meltdown In evolutionary genetics, mutational meltdown is a sub class of extinction vortex in which the environment and genetic predisposition mutually reinforce each other. Mutational meltdown (not to be confused with the concept of an error catastroph ...
. It appears that mutational meltdown due to Muller’s ratchet can be avoided by a little bit of sex as in the common
apomictic In botany, apomixis is asexual reproduction, asexual development of seed or embryo without Fertilisation, fertilization. However, other definitions include replacement of the seed by a plantlet or replacement of the flower by bulbils. Apomictic ...
asexual flowering plant '' Ranunculus auricomus''.


See also

*
Evolution of sexual reproduction Sexually reproducing animals, plants, fungi and protists are thought to have evolved from a common ancestor that was a single-celled eukaryotic species. Sexual reproduction is widespread in eukaryotes, though a few eukaryotic species have ...
*
Genetic hitchhiking Genetic hitchhiking, also called genetic draft or the hitchhiking effect, is when an allele changes frequency not because it itself is under natural selection, but because it is near another gene that is undergoing a selective sweep and that is ...
* Hill–Robertson effect


References


External links

*''
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'
webcomic
explaining Muller's ratchet and recombination through the evolution of
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s {{DEFAULTSORT:Muller's Ratchet Evolutionary biology concepts Genetics concepts Population genetics