In evolution

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

ary genetics

Genetics

Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

, Muller's ratchet (named after Hermann Joseph Muller

Hermann Joseph Muller

Hermann Joseph Muller was an American geneticist, educator, and Nobel laureate best known for his work on the physiological and genetic effects of radiation as well as his outspoken political beliefs...

, by analogy with a ratchet

Ratchet (device)

A ratchet is a device that allows continuous linear or rotary motion in only one direction while preventing motion in the opposite direction. Because most socket wrenches today use ratcheting handles, the term "ratchet" alone is often used to refer to a ratcheting wrench, and the terms "ratchet"...

 mechanism) is the process by which the genome

Genome

In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

s of an asexual

Asexual reproduction

Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...

 population

Population

A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

 accumulate deleterious mutations

Genetic deletion

In genetics, a deletion is a mutation in which a part of a chromosome or a sequence of DNA is missing. Deletion is the loss of genetic material. Any number of nucleotides can be deleted, from a single base to an entire piece of chromosome...

 in an irreversible manner.

Muller proposed this mechanism as a theory to explain the evolution of sex

Evolution of sex

The evolution of sexual reproduction is currently described by several competing scientific hypotheses. All sexually reproducing organisms derive from a common ancestor which was a single celled eukaryotic species. Many protists reproduce sexually, as do the multicellular plants, animals, and fungi...

. Although Muller's ratchet is proposed to explain the success of sexual reproduction

Sexual reproduction

Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...

 over asexual reproduction

Asexual reproduction

Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...

, 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

Genetic recombination

Genetic recombination is a process by which a molecule of nucleic acid is broken and then joined to a different one. Recombination can occur between similar molecules of DNA, as in homologous recombination, or dissimilar molecules, as in non-homologous end joining. Recombination is a common method...

. This effect has also been observed in those regions of the genomes of sexual organisms which do not undergo 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

Genetic load

In population genetics, genetic load or genetic burden is a measure of the cost of lost alleles due to selection or mutation...

. In theory, the genetic load carried by asexual populations eventually becomes so great that the population goes extinct. In sexual populations, the process of genetic recombination allows the genomes of the progeny to be different from the genomes of the parents. In particular, progeny genomes with fewer mutations can be generated from more highly mutated parental genomes by putting together in progeny genomes mutation-free portions of parental chromosomes.

Among protists and prokaryotes there is a plethora of supposedly asexual organisms. More and more are being shown to exchange genetic information through a variety of mechanisms. In contrast, the genomes of mitochondria and chloroplast

Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s do not recombine and would undergo Muller's ratchet were they not as small as they are. 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 specially 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 non-recombining chromosomes or chromosomal regions such as the mammalian Y chromosome

Y chromosome

The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs...

 (with the exception of multi-copy sequences which do engage intrachromosomal recombination and gene conversion), should also be subject to the effects of Muller's ratchet. Such non-recombining 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. It is not easy to ascribe cases of genome shrinkage or fast evolution to Muller's ratchet alone.

Because Muller's ratchet relies on genetic drift

Genetic drift

Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...

, it turns faster in smaller populations and it is thought to set 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

Bdelloid

Bdelloidea is a class of rotifers found in fresh water and moist soil. Bdelloids typically have a well-developed corona, divided into two parts, on a retractable head. They may move by swimming or crawling...

 rotifers, for example, appear to have been asexual for nearly 40 million years.

Origin of the term

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

Joe Felsenstein

Joseph "Joe" Felsenstein is Professor in the Departments of Genome Sciences and Biology and Adjunct Professor in the Departments of Computer Science and Statistics at the University of Washington in Seattle...

in his 1974 paper, "The Evolutionary Advantage of Recombination".