Final answer:
Genetic drift leads to a decrease in heterozygosity by causing random changes in allele frequencies, especially in small populations, through processes like the bottleneck effect and the founder effect.
Step-by-step explanation:
Genetic drift causes a decrease in heterozygosity over time due to random changes in allele frequencies that occur in small populations. This reduction in heterozygosity occurs because some alleles are randomly passed to the next generation in greater numbers, while others might not be passed on at all. Two prominent examples that illustrate this concept are the bottleneck effect and the founder effect.
For instance, a natural disaster could non-selectively kill a large portion of the population, causing certain alleles to disappear completely. This is the bottleneck effect. Similarly, if a small group leaves to establish a new population elsewhere, the new population's genetic structure may significantly differ from the original population due to the limited genetic variation brought by these founders—this is known as the founder effect. These chance events can lead to the loss of genetic variation, which is essential for natural selection.
Genetic drift can have a considerable impact in small populations where chance occurrences can result in significant shifts in allele and genotype frequencies from one generation to the next. For example, if only two individuals who are homozygote for a certain allele reproduce, the alternate allele might be lost, thereby decreasing heterozygosity within the population.