Under what condition would a mutation have the most impact on allele frequency

I think this happens when there is no gene flow or when the population is small. Small population, germ line mutation, beneficial mutation that gets into many progeny and is of  high chance of luck. Genetic drift involves a process in which allele frequencies within a population change by chance only, as a result of sampling error from generation to generation. Genetic drift occurs mostly in small population since the infrequently occurring alleles face a greater chance of being lost.