Effective size of the population

In describing the dynamics of fixation of mutations, it is necessary to consider not only the probability with which a mutation will become fixed in a population of a certain size, but also the time required on an average for fixation of a mutation. The probability that a newly formed mutation will be fixed is equal to 1/2N. Similarly, the average time required for fixation of one mutation is proportional to the size of the population. However, this is a case of direct proportionality. M. Kimura derived that the average time for fixation of a mutation by genetic drift is equal to 4Ne generations, where Neis the effective size of the population (the effective size of the population is a term that will be explained in Section V.3.2.1) For a population with an effective size of 30,  fixation of a neutral mutation will thus require an average of 120 generation periods.

The graph describing the shape of the time distribution required for fixation of a mutation by genetic drift is highly asymmetric. The asymmetry of the graph reflects the fact that it is highly improbable that a mutation will become fixed sooner than in 0.8Ne generation periods and a great many mutations require substantially more time than the average 4Negeneration periods.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more