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 Heredity, heritable traits characteristic of a population over generation ...

, negative selection or purifying selection is the selective removal of

alleles An allele is a variant of the sequence of nucleotides at a particular location, or locus, on a DNA molecule. Alleles can differ at a single position through single nucleotide polymorphisms (SNP), but they can also have insertions and deletions ...

that are deleterious. This can result in stabilising selection through the purging of deleterious genetic polymorphisms that arise through random mutations. Purging of deleterious alleles can be achieved on the

population genetics Population genetics is a subfield of genetics that deals with genetic differences within and among populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as Adaptation (biology), adaptation, s ...

level, with as little as a single

point mutation A point mutation is a genetic mutation where a single nucleotide base is changed, inserted or deleted from a DNA or RNA sequence of an organism's genome. Point mutations have a variety of effects on the downstream protein product—consequences ...

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 of linked 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 called background selection. This effect increases with lower mutation rate but decreases with higher recombination rate. Purifying selection can be split into purging by non-random mating ( assortative mating) and purging by

genetic drift Genetic drift, also known as random genetic drift, allelic drift or the Wright effect, is the change in the Allele frequency, frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene va ...

. Purging by genetic drift can remove primarily deeply recessive alleles, whereas natural selection can remove any type of deleterious alleles.

Negative selection in haploid compared to diploid tissue

The idea that those

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

s of an organism that are expressed in the

haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...

stage are under more efficient

than those genes expressed exclusively in the diploid stage is referred to as the “masking theory”. 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 yeast ''

Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungal microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have be ...

''. 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.

References

Selection Classical genetics Population genetics {{genetics-stub

Negative selection in haploid compared to diploid tissue

See also

References