USA Biology Olympiad (USABO) Practice Exam

Which factor in a population prevents changes in allele frequencies according to the Hardy-Weinberg principle?

1. Genetic drift

2. Migration

3. Natural selection

4. Random mating

The correct answer is: Random mating

The Hardy-Weinberg principle outlines the conditions necessary for a population to remain in equilibrium, meaning that allele frequencies will remain consistent across generations in an idealized scenario. One crucial factor that allows for maintaining constant allele frequencies is random mating. Random mating ensures that every individual has an equal opportunity to mate with any member of the opposite sex within the population, leading to a stable distribution of alleles. When mating is random, it minimizes the influence of sexual selection or any preference for specific traits, which can otherwise skew allele frequencies over generations. This means that the alleles contributed from the mating pairs are mixed freely, maintaining the original proportions of the different alleles in the gene pool of the population. In contrast, genetic drift, migration, and natural selection all introduce forces that can alter allele frequencies. Genetic drift may cause random fluctuations in allele frequencies, especially in small populations. Migration can lead to changes as new individuals introduce or remove alleles from the population. Natural selection, on the other hand, favors certain alleles over others based on their contribution to an organism's fitness, which can significantly shift allele frequencies over time. Thus, random mating is a critical factor that upholds the conditions of the Hardy-Weinberg principle, ensuring that allele frequencies remain