Final answer:
Selection is the evolutionary force that results in adaptive changes in allele frequencies, primarily through the process of natural selection, which favors beneficial alleles and traits for reproductive success.
Step-by-step explanation:
The evolutionary force that results in adaptive changes in allele frequencies is selection. Natural selection works by selecting for alleles that confer beneficial traits or behaviors, while selecting against those with deleterious effects. This process of adaptation leads to a higher reproductive success for those who possess favorable traits, thereby increasing the frequency of those beneficial alleles in the population over time. Mutations introduce new alleles into a population, providing raw material for evolution, while genetic drift and gene flow can also alter allele frequencies, though not necessarily in an adaptive manner. Nonrandom mating changes the gene pool by causing individuals to mate with certain phenotypes more than others, which can affect the distribution of alleles but is not typically a force for adaptive change.