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Molecular evolution and neutral theory - What limits the rate of evolution by selection?


Genetic load

Haldane's argument for a limit on the rate of natural selection is better understood when the theoretical concept known as genetic load is explained.

Genetic load is a number between 0 and 1 and it measures the extent to which the average individual in a population is inferior to the best possible kind of individual. The genetic load equals the relative chance that an average individual will die before reproducing because of the deleterious genes that it possesses. Ignoring frequency-dependent selection, it is calculated as follows:

Suppose there are a variety of genotypes in the population, each with its characteristic fitness; one genotype has a higher fitness than the rest and we call its fitness wopt. We can also measure the average fitness of the whole population; it is just the fitness of each genotype multiplied by its frequency: it is called mean fitness and is symbolized by v .

The general formula for genetic load (L ) is as follows:

L = (wopt - v ) / wopt

If all the individuals in the population have the optimal genotype, then

v = wopt and the load is zero.

If all but one have a genotype of zero fitness then v = 0 and L = 1.

The cost of selection can be expressed as a genetic load: while selection is fixing the favored allele, there will be some inferior and some superior alleles in the population. We can therefore define a substitutional load with the same formula as above: the substitution of one gene for another by selection requires mortality in the population. This mortality places an upper limit on the rate of natural selection because, as we have seen, too much selection will drive the population extinct.

The geneticist Linda Partridge explains the importance of genetic load.

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