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This is a probability that declines exponentially toward 1/4 as t increases. In contrast, if the nucleotide at a site is not initially a G then PG(t) = 0. The probability the site remains a G over time is or as a recurrence equation that applies to any two time periods one generation apart. This recurrence equation can also be expressed as the change in the probability that an initial nucleotide site remains unchanged over time. Since the change in a quantity is the difference between what it is now and what it was one time step t in the past, APG(t) = PG(t+1) - PG(t). The change in the probability that a given nucleotide is found at a site over one generation is then

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