The pesticide DDT was widely used in the 1940s as a method of insect control. In the late 1950s the first DDT-resistant mosquitoes were discovered, and eventually DDT-resistant mosquitoes were found globally. When DDT is used now, the development of DDT resistance in mosquito populations occurs in months rather than years.

Which of the following best explains the observations concerning DDT resistance in mosquitoes?

Natural selection acts on existing variation in a population. Since DDT was used previously, the trait conferring resistance increased in frequency. In subsequent populations exposed to DDT, the trait conferred a huge fitness advantage and therefore further increases in frequency. Development of resistance in a population would therefore occur faster than in previous years.