Why is it that mutation, acting alone, has little effect on allele frequency, and therefore ...

If the mutation rate for a gene is known, we can use the HardyWeinberg formula to calculate the new allele frequency resulting from mutations in that gene. Let's use the dominant trait achondroplasia as an example. For this calculation, we assume that initially only homozygous recessive individuals with the genotype dd (normal stature) were present in the population. We will also assume that mutation has added new mutant (D) alleles to each generation at the rate of 1X10-5 . To change the frequency of the recessive allele (d) from 1.0 (100) to 0.5 (50) at this rate of mutation will require 70,000 generations, or 1.4 million years. Our conclusion in this case is that mutation alone has a minimal impact on the amount of genetic variation present in a population. In other words, mutation generates all new alleles, but except for very small populations, drift, migration, and selection are the forces that change the frequency of alleles in a population.