 Random genetic drift refers to the chance fluctuations in gene frequency that arise in finite populations. In many evolutionary models, including those outlined in section 3 above. the population is assumed to be very large (technically, infinite) precisely in order to abstract away from such fluctuations. But though mathematically convenient, this assumption is often unrealistic. In real life populations, particularly those of small size, stochasticity is an important source of evolutionary change. Thus a given allele may increase or decrease in frequency not because of any effect it has on organismic survival or reproduction, but simply by chance. Understanding such stochastic changes in gene frequency, and their interaction with natural selection, is a major topic in population genetics, past and present.