Final answer:
A stressful factor such as disease, starvation, or drought can cause a population bottleneck, dramatically reducing the population size. Genetic drift can also be magnified by events that randomly kill a large portion of the population. This can lead to a sudden loss of genetic diversity in the surviving population.
Step-by-step explanation:
On occasion, a stressful factor such as disease, starvation, or drought kills a large proportion of the individuals in a population, producing a population bottleneck that dramatically reduces the population size. This phenomenon is known as a bottleneck effect. Genetic drift can also be magnified by natural or human-caused events, such as a disaster that randomly kills a large portion of the population. This can result in a sudden loss of genetic diversity in the surviving population.