Hill-Robertson effect
Encyclopedia
The Hill-Robertson effect is a population genetics
phenomenon first identified by Bill Hill
and Alan Robertson
in 1966. It describes an evolutionary advantage to genetic recombination
.
, random linkage disequilbria
will occur. These can be caused by genetic drift
or by mutation
, and they will tend to slow down the process of evolution
This is most easily seen by considering the case of disequilibria caused by mutation:
Consider a population of individuals whose genome
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
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.
Joe Felsenstein
(1974) showed this effect to be mathematically identical to the Fisher-Muller model proposed by R.A. Fisher (1930) and H.J. Muller (1932), although the verbal arguments were substantially different.
Population genetics
Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...
phenomenon first identified by Bill Hill
William G. Hill
William George "Bill" Hill, OBE, FRS, FRSE is a British geneticist and statistician. He is professor emeritus at Edinburgh University since his retirement in 2002. He was elected to the Royal Society of Edinburgh in 1979, Royal Society of London in 1985 and appointed OBE in 2004.- External links...
and Alan Robertson
Alan Robertson
Alan Robertson FRS was an English population geneticist. Originally a chemist, he was recruited after the Second World War to work on animal genetics on behalf of the British government, and continued in this sphere until his retirement in 1985...
in 1966. It describes an evolutionary advantage to genetic 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...
.
Explanation
In a population of finite size which is subject to natural selectionNatural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
, random linkage disequilbria
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...
will occur. These can be caused by 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...
or by mutation
Mutation
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...
, and they will tend to slow down the process of 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...
This is most easily seen by considering the case of disequilibria caused by mutation:
Consider a population of individuals whose 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....
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
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
AB) will eventually arise. Provided there are no negative epistatic
Epistasis
In genetics, epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is called epistatic, while the phenotype altered or suppressed is called hypostatic...
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.
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...
(1974) showed this effect to be mathematically identical to the Fisher-Muller model proposed by R.A. Fisher (1930) and H.J. Muller (1932), although the verbal arguments were substantially different.