Final answer:
A ratio of nonsynonymous to synonymous differences that is much greater than 1 indicates the presence of purifying selection.
Step-by-step explanation:
A ratio of nonsynonymous to synonymous differences is used to evaluate the presence of purifying selection. Purifying selection refers to the removal of deleterious genetic variants from a population. When the ratio is much greater than 1, it indicates a higher number of nonsynonymous differences, suggesting that purifying selection has been at work.