If the initial frequency of an allele is 0.5, you can see that increasing numbers of populations deviate from 0.5 with increases in generation numbers. When comparing the effects of drift in populations of size 10 versus 25, it can also be seen that although changes occur in both, they are more pronounced in the population with the smaller size. Thus N determines the rate of change caused by drift, but even very large populations can be affected by drift if given enough time. Genetic drift, like any other evolutionary force, can only operate when there is genetic variability. As long as p is not equal to 0 or 1, drift will cause changes in allele frequency.
Drift is thus like a genetic fly paper.
These properties of genetic drift have been demonstrated empirically many times but they also are easy to see in computer simulations. Some Properties of Genetic Drift Suppose we run a simulation of genetic drift with an initial allele frequency of 0.5. When allele frequencies are averaged over a number of generations, there is almost no change from the initial allele frequencies of 0.5. The chances of any subpopulation deviating 0.5 and the magnitude of that deviation increase each generation. With increasing time, more and more populations become fixed for one allele.