Which process reduces the amount of allelic variation in a population over time?

Genetic drift is a process that can significantly reduce allelic variation in a population, particularly in small populations. It refers to random changes in allele frequencies from one generation to the next, due to chance events rather than natural selection. Over time, because certain alleles may become fixed (reach a frequency of 100%) while others may be lost (reach a frequency of 0%), the overall genetic diversity of the population can diminish.

In small populations, the effects of genetic drift are magnified since random fluctuations can lead to more pronounced changes in allele frequencies. This can lead to a reduction in genetic variation not only by fixing some alleles but also by erasing others entirely, resulting in a less diverse gene pool.

Other processes such as migration, selection, and mutation contribute to allelic variation in different ways. Migration tends to introduce new alleles into a population, increasing genetic diversity. Natural selection acts on existing variation by favoring certain alleles, which can either maintain or enhance diversity depending on the selective pressures. Mutation is the original source of new genetic variation, adding new alleles to the population. Therefore, while those processes play vital roles in shaping genetic variation, genetic drift specifically reduces it over time.