Genetic drift can be defined as a random force leading to allele extinction. Genetic drift involves the chance/random mating which can occur between members of a population, and it is a strong force for gene pool change in small populations. Two extreme cases of genetic drift are the bottleneck and founder effects. Genetic drift is more extreme in random changes in small populations; if a population is small, there is a chance some alleles present in parental groups will be not passed on at all.

The bottleneck effect occurs when a catastrophic event or a period of adverse conditions drastically reduces the size of a population. In this scenario, certain alleles may be lost through chance. If some portion of a population survives the catastrophe, the original population's gene pool cannot be recovered. The expanded population can only carry the alleles that existed in the population that survived the event. As a result, the gene pool will now see an indication of the bottleneck that occurred long after the population has recovered. Smaller variation in surviving individuals will exist. The founder effect is a particular example of genetic drift. A few individuals who move to a new area and become isolated from a larger population may not carry all the alleles which were previously present in the original population. This means that the isolated population has less genetic diversity in comparison to the original population. To summarise, it is an extreme case of genetic drift in a small population that migrates away from a large population, carrying with it an unrepresentative set of alleles. A key example of the founder effect is seen in North America, when 200 people formed the Amish community in the area. One of the individuals carried a recessive allele for Ellis-van Creveld syndrome, which has symptoms including polydactyly, dwarfism and a hole in the heart; this condition has been common in the region ever since.