In
population genetics
Population genetics is a subfield of genetics that deals with genetic differences within and between populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as Adaptation (biology), adaptation, ...
, the Hill–Robertson effect, or Hill–Robertson interference, is a phenomenon first identified by
Bill Hill and
Alan Robertson in 1966. It provides an explanation as to why there may be an evolutionary advantage to
genetic recombination.
Explanation
In a population of finite but effective size which is subject to
natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...
, varying extents of
linkage disequilibria (LD) will occur. These can be caused by
genetic drift
Genetic drift, also known as allelic drift or the Wright effect, is the change in the frequency of an existing gene variant (allele) in a population due to random chance.
Genetic drift may cause gene variants to disappear completely and there ...
or by
mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
, and they will tend to slow down the process of
evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
by natural selection.
This is most easily seen by considering the case of disequilibria caused by mutation:
Consider a population of individuals whose
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...
has only two genes, ''a'' and ''b''. If an advantageous mutant (''A'') of gene ''a'' arises in a given individual, that individual's genes will through natural selection become more frequent in the population over time. However, if a separate advantageous mutant (''B'') of gene ''b'' arises before ''A'' has gone to fixation, and happens to arise in an individual who does not carry ''A'', then individuals carrying ''B'' and individuals carrying ''A'' will be in competition. If recombination is present, then individuals carrying both A and B (of genotype AB) will eventually arise. Provided there are no negative
epistatic
Epistasis is a phenomenon in genetics in which the effect of a gene mutation is dependent on the presence or absence of mutations in one or more other genes, respectively termed modifier genes. In other words, the effect of the mutation is dep ...
effects of carrying both, individuals of genotype ''AB'' will have a greater selective advantage than ''aB'' or ''Ab'' individuals, and ''AB'' will hence go to fixation. However, if there is no recombination, AB individuals can only occur if the latter mutation (B) happens to occur in an Ab individual. The chance of this happening depends on the frequency of new mutations, and on the size of the population, but is in general unlikely unless A is already fixed, or nearly fixed. Hence one should expect the time between the A mutation arising and the population becoming fixed for AB to be much longer in the absence of recombination. Hence recombination allows evolution to progress faster. [Note: This effect is often erroneously equated with "clonal interference", which happens when ''A'' and ''B'' mutations arise in different wildtype (''ab'') individuals and describes the ensuing competition between ''Ab'' and ''aB'' lineages.]
[ There tends to be a correlation between the rate of recombination and the likelihood of the preferred haplotype (in the above example labeled as ''AB'') goes into fixation in a population.
Joe Felsenstein (1974) showed this effect to be mathematically identical to the Fisher–Muller model proposed by ]R. A. Fisher
Sir Ronald Aylmer Fisher (17 February 1890 – 29 July 1962) was a British polymath who was active as a mathematician, statistician, biologist, geneticist, and academic. For his work in statistics, he has been described as "a genius who ...
(1930) and H. J. Muller (1932), although the verbal arguments were substantially different. Although the Hill-Robertson effect is usually thought of as describing a disproportionate build up of fitness-reducing (relative to fitness increasing) LD over time, these effects also have immediate consequences for mean population fitness.
See also
*Clonal interference
Clonal interference is a phenomenon in evolutionary biology, related to the population genetics of organisms with significant linkage disequilibrium, especially asexually reproducing organisms. The idea of clonal interference was introduced by A ...
*Genetic hitchhiking
Genetic may refer to:
*Genetics, in biology, the science of genes, heredity, and the variation of organisms
**Genetic, used as an adjective, refers to genes
***Genetic disorder, any disorder caused by a genetic mutation, whether inherited or de nov ...
References
{{DEFAULTSORT:Hill-Robertson Effect
Genetics in the United Kingdom
Population genetics
Evolutionary biology