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Genetic drift

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Genetic drift is the the change in frequency of a neutral allele in a population over time due to statistical effects.

Traits held by a smaller number of individuals within the population tend to be removed from the population, while traits of the majority tend to become fixed among all members of the population. In this way genetic drift counters the increase in genetic diversity caused by mutation. Genetic drift occurs within all populations, although it is more pronounced in small, isolated populations. However, since the number of new mutations increases linearly with population size, the number of total new mutations that are eventually fixed by genetic drift is independent of the number of individuals and is proportional to the mutation rate. This constant fixation of neutral mutations by genetic drift, also known as the molecular clock, can thus be used to reconstruct the time at which two related populations became reproductively isolated from each other.