Genetic hitchhiking
Encyclopedia
Genetic hitchhiking is the process by which an allele
may increase in frequency by virtue of being linked
to a gene that is positively selected. Proximity on a chromosome
may allow genes to be dragged along with a selective sweep
experienced by an advantageous gene nearby. More generally, genetic hitchhiking can refer to changes in an allele's frequency due to any form of selection operating upon linked genes, including background selection against deleterious mutations.
Whether a neutral allele becomes fixed
is a matter of chance. The traditional view of this stochastic process is that it is dominated by sampling error, that is genetic drift
. But it may instead be dominated by whether the allele is linked
to a good genetic background: this is known as genetic draft.
in the area around it.
--M------A--
On this chromosome the gene M is a mutator allele, increasing the rate of mutation in the surrounding area. A is an allele which is fixed
in the population. Due to the increased mutation rate, the A allele may be mutated into a new, advantageous allele, A*.
--M------A*--
The individual in which this chromosome lies will now have a selective advantage over other individuals of this species, so the allele A* will spread through the population by the normal processes of natural selection
. M, due to its proximity to A*, will also increase in frequency. This hitchhiking only works when M is very close to the beneficial allele that it has created by mutation. A greater distance would increase the chance of recombination
separating M from A*, leaving M alone with any deleterious mutations it may have caused. For this reason, hitchhiking on beneficial mutations is most likely to affect the evolution of mutation rate
s in asexual
species where recombination cannot disrupt linkage.
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
may increase in frequency by virtue of being linked
Genetic linkage
Genetic linkage is the tendency of certain loci or alleles to be inherited together. Genetic loci that are physically close to one another on the same chromosome tend to stay together during meiosis, and are thus genetically linked.-Background:...
to a gene that is positively selected. Proximity on a chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
may allow genes to be dragged along with a selective sweep
Selective sweep
A selective sweep is the reduction or elimination of variation among the nucleotides in neighboring DNA of a mutation as the result of recent and strong positive natural selection....
experienced by an advantageous gene nearby. More generally, genetic hitchhiking can refer to changes in an allele's frequency due to any form of selection operating upon linked genes, including background selection against deleterious mutations.
Whether a neutral allele becomes fixed
Fixation (population genetics)
In population genetics, fixation is the change in a gene pool from a situation where there exist at least two variants of a particular gene to a situation where only one of the alleles remains...
is a matter of chance. The traditional view of this stochastic process is that it is dominated by sampling error, that is 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...
. But it may instead be dominated by whether the allele is linked
Linkage disequilibrium
In population genetics, linkage disequilibrium is the non-random association of alleles at two or more loci, not necessarily on the same chromosome. It is also referred to as to as gametic phase disequilibrium , or simply gametic disequilibrium...
to a good genetic background: this is known as genetic draft.
Advantage to mutators
Consider a hypothetical mutator allele that increases the mutation rateMutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
in the area around it.
--M------A--
On this chromosome the gene M is a mutator allele, increasing the rate of mutation in the surrounding area. A is an allele which is fixed
Fixation (population genetics)
In population genetics, fixation is the change in a gene pool from a situation where there exist at least two variants of a particular gene to a situation where only one of the alleles remains...
in the population. Due to the increased mutation rate, the A allele may be mutated into a new, advantageous allele, A*.
--M------A*--
The individual in which this chromosome lies will now have a selective advantage over other individuals of this species, so the allele A* will spread through the population by the normal processes of natural selection
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...
. M, due to its proximity to A*, will also increase in frequency. This hitchhiking only works when M is very close to the beneficial allele that it has created by mutation. A greater distance would increase the chance 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...
separating M from A*, leaving M alone with any deleterious mutations it may have caused. For this reason, hitchhiking on beneficial mutations is most likely to affect the evolution of mutation rate
Mutation rate
In genetics, the mutation rate is the chance of a mutation occurring in an organism or gene in each generation...
s in 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...
species where recombination cannot disrupt linkage.
Further reading
- Maynard Smith, J.John Maynard SmithJohn Maynard Smith,His surname was Maynard Smith, not Smith, nor was it hyphenated. F.R.S. was a British theoretical evolutionary biologist and geneticist. Originally an aeronautical engineer during the Second World War, he took a second degree in genetics under the well-known biologist J.B.S....
and Haigh, J. 1974. The hitch-hiking effect of a favourable gene. Genetical Research 23, 23-35.