Final answer:
A selective sweep at one locus leads to reduced variation in flanking loci due to the 'hitchhiking' effect, where these loci are carried along with the selected allele, limiting recombination and genetic diversity in those regions.
Step-by-step explanation:
A selective sweep at one locus can indeed affect variation at adjacent loci. The correct answer to the question is A) variation in flanking loci is reduced. This phenomenon occurs because a selective sweep involves a beneficial allele becoming fixed in a population so quickly that there is little time for recombination to break the linkage between the selected allele and neighboring loci. As a result, these adjacent loci 'hitchhike' along with the selected allele, leading to reduced genetic variation in those areas. This process is important to consider in genetic mapping because recombination rates can vary, affecting how we interpret mapping information. Furthermore, understanding the concept of selective sweeps is also helpful in assessing the action of natural selection on allele frequencies in a population.