Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

The probability that certain alleles will become fixed is equal to their frequency in the population.If a new mutation is formed in the population, its original frequency in the gene pool of diploid organisms (containing two copies of each allele) is equal to 1/2N, where N is the number of individuals in the population.Thus, in a population with a size of 100, approximately each two-hundredth selectionally neutral mutation will be fixed by drift.There is substantial probability that a new selectionally neutral mutation will become fixed in a small population.This probability is much smaller in a large population.

            The probability that a new mutation will be eliminated from the population through genetic drift immediately after its formation is very high and is basically not connected with its advantageousness from the standpoint of the biological fitness of the organism (Fisher 1958).In a size-stabilized population of sexually reproducing organisms, each parental pair leaves an average of two progeny and each individual passes two alleles of each gene down to the gene pool of the following generation.These two alleles can have both copies of the mutated alleles (probability p = 0.25) or both copies of the original alleles (p = 0.25), or one copy of the original allele and the second is a copy of the mutated allele (p = 0.5).This means that, in one quarter of cases, the mutated allele disappears from the population before it can even become an object of natural selection.If the mutated allele is not eliminated, its frequency is increased somewhat in the population as there is a probability of 1/3 that both progeny will have the mutated allele – the number of mutated alleles is doubled.Thus, the probability that the mutated allele will disappear from the second generation will be somewhat smaller than 0.25 and will equal approximately 0.18.The probability of disappearance of a mutated allele in subsequent generations is additive, so that, after 5-6 generations, any new allele will disappear from the population simply as a consequence of random processes without much reference to its selectional advantage.This is absolutely true for recessive mutations as the selectional advantage or disadvantage of the mutation can apply only to a homozygote with both alleles mutated.If the mutated allele is present in the population with only small frequency, the probability of the formation of these homozygotes in a panmictic population is almost negligible.This phenomenon (disadvantage for recessive mutations) is sometimes termed Haldane’s seive (Noor 1999)and this is discussed in a slightly different context in Section II.4.1.1.If an advantageous dominant mutation is involved, the situation will be somewhat more favourable for the fate of the new mutation; however, even here, random genetic drift will probably play the most important role in the first generations.