Random fixation of alleles. Alleles go to 1.0 frequency at random. In two different populations, alternate alleles may become fixed. Remember, when one allele goes to fixation, the other alleles are lost. Rate of drift depends on population size.
Single events have a greater impact on smaller populations.
Since alleles are lost at random, there is no correlation with fitness. In fact, in small populations, beneficial alleles can be lost by drift. Migration of fertile individuals into or out of a population. Remember, we are interested in the impact on future generations. So, infertile individuals do not count as they do not affect the allele composition of future generations.