Innatural selection,negative selection[1] orpurifying selection is the selective removal ofalleles that aredeleterious. This can result instabilising selection through the purging of deleteriousgenetic polymorphisms that arise through random mutations.[2][3]
Purging of deleterious alleles can be achieved on thepopulation genetics level, with as little as a singlepoint mutation being the unit of selection. In such a case, carriers of the harmful point mutation have fewer offspring each generation, reducing the frequency of the mutation in the gene pool.
In the case of strong negative selection on a locus, the purging of deleterious variants will result in the occasional removal oflinked variation, producing a decrease in the level of variation surrounding the locus under selection. The incidental purging of non-deleterious alleles due to such spatial proximity to deleterious alleles is calledbackground selection.[4] This effect increases with lower mutation rate but decreases with higher recombination rate.[5]
Purifying selection can be split into purging by non-random mating (assortative mating) and purging bygenetic drift. Purging by genetic drift can remove primarily deeply recessive alleles, whereas natural selection can remove any type of deleterious alleles.[6]
The idea that thosegenes of an organism that are expressed in thehaploid stage are under more efficientnatural selection than those genes expressed exclusively in the diploid stage is referred to as the “masking theory”.[7] This theory implies that purifying selection is more efficient in the haploid stage of the life cycle where fitness effects are more fully expressed than in the diploid stage of the life cycle. Evidence supporting the masking theory has been reported in the single-celled yeastSaccharomyces cerevisiae.[8] Further evidence of strong purifying selection in haploid tissue-specific genes, in support of the masking theory, has been reported for the plant,Scots Pine.[7]
