Hill–Robertson effect


In population genetics, 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 size which is subject to natural selection, varying extents of linkage disequilibria will occur. These can be caused by genetic drift or by mutation, and they will tend to slow down the process of evolution by natural selection.
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 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 of gene b arises before A has gone to fixation, and happens to arise in an individual who carries A, then individuals carrying B will have reduced fixation probability. 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. The reduced fixation probability of B will tend to produce an ensemble overabundance of Ab individuals -- an imbalance whose disadvantage can be ameliorated by recombination across populations or demes.
Joe Felsenstein showed this effect to be mathematically identical to the Fisher–Muller model proposed by R. A. Fisher and H. J. Muller, 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 LD over time, these effects also have immediate consequences for mean population fitness.