Question

Mosquitoes in tropical regions can carry and transmit the malaria parasite to humans. The mosquitoes used to be killed in these regions by spraying them with the insecticide DDT. The mosquitoes that were earlier sensitive to DDT are now not killed by DDT and grow and multiply even in the presence of DDT. How can you explain this phenomenon

Answer 1

Simply put, evolution. A mutation occurred within the mosquito population and those who had this mutation were not harmed by the DDT and thus had greater fitness, which allowed them to survive and continue to breed (and pass on their DDT resistive mutation).