Mutation–selection balance

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Mutation–selection balance occurs when the rate of elimination of deleterious allele copies equals the rate of creation of new deleterious copies. This equilibrium is an important concept in the study of population genetics and evolution.[1]

Mutations are a weak force in shaping gene pools, but they are also an important source of genetic variation. The majority of genetic mutations tends to be neutral or deleterious; beneficial mutations are much more rare. Selection acts against the deleterious mutations, but these gene changes ensure their place in the gene pool by continually being added. When these rates of change are equal, a population is said to be in equilibrium in terms of mutations.[2]

If the deleterious allele is recessive, the frequency of this mutation at equilibrium is given by the equation q=√(μ/s), where μ is the mutation rate and s is the selection coefficient. The selection coefficient is a number between 0 and 1, signifying the strength of selection against the allele. When the mutation is only slightly selected against (low s) but the mutation rate is high, the equilibrium frequency of the allele, or its frequency in the population, will be high. The opposite is true as well; high s and low μ will yield low q.

The mutation-selection balance can account for why some deleterious alleles continue to persist in a population despite being harmful. For example, spinal muscular atrophy (SMA) is caused by deletions in the telomeric survival motor neuron gene (telSMN). Researchers estimated that these deleterious mutations in particular occur at frequency of about 0.01 in Caucasians. Therefore, s would be around 0.9, indicating that the deleterious mutation be selected out of the population. However, the mutations continue to persist at the same frequency. Researchers found that the mutation rate of telSMN is high enough to counteract the high selection, leading to a consistent frequency in the population.[3][4]

Mutation–selection balance, however, cannot always explain why mutations persist in populations. In particular, cases of heterozygote superiority – like selection for the loss-of-function mutations in the CFTR protein – can go against this equilibrium. (Mutations in the CFTR protein can cause cystic fibrosis but also can lead to greater protection against typhoid fever.)[5]

Beneficial mutation-selection is also a concept that has been discussed by researchers. This equilibrium works in a similar way, balancing loss of variation caused by selection and creation of variation caused by beneficial mutations.[6]

References[edit]

  1. ^ Herron, JC and S Freeman. 2014. Evolutionary Analysis, 5th Edition. Pearson.
  2. ^ Herron, JC and S Freeman. 2014. Evolutionary Analysis, 5th Edition. Pearson.
  3. ^ http://www.sciencedirect.com/science/article/pii/S0002929707602006
  4. ^ Herron, JC and S Freeman. 2014. Evolutionary Analysis, 5th Edition. Pearson.
  5. ^ http://www.nature.com/nature/journal/v393/n6680/full/393079a0.html
  6. ^ http://www.genetics.org/content/176/3/1759.full