Final answer:
Natural selection alters allele frequencies by favoring the propagation of beneficial alleles and reducing the prevalence of detrimental alleles at a given locus, directly impacting the genetic diversity within a population.
Step-by-step explanation:
The most accurate generalization regarding the impact of natural selection on allelic variation at a single locus is that it can increase the frequency of beneficial alleles and decrease the frequency of harmful ones.
Natural selection acts on phenotypic variations that affect reproductive success. If an allele provides a phenotype that improves an organism's chances to survive and reproduce, this allele will become more common in subsequent generations. Conversely, alleles that result in a disadvantageous phenotype may be eliminated quickly from the gene pool. Since allele frequencies must total 100%, a rise in one allele's frequency necessitates a fall in another. This dynamic adjustment in allele frequencies is a central aspect of population genetics.
For instance, the case of the peppered moth illustrates the effects of natural selection on allele frequencies in a natural population. The frequency of dark or light wing coloration in moths changed with the level of industrial soot present on trees. In such ways, natural selection guides the evolution of populations by favoring certain alleles over others based on environmental conditions.