DDT resistance of mosquitoes to pesticides: Earlier DDT (dichloro-diphenyl trichloroethane) has been supposed to be the best pesticide for controlling the population of the mosquitoes which spread malaria. Under the National Malaria Eradication Programme, DDT was extensively sprayed to kill the mosquitoes. This made the mosquitoes to adapt and become resistance to the DDT and other pesticides. This can be explained on the basis of natural selection.

Before the discovery of DDT, the mosquito population had more DDT-sensitive and less DDT- resistant mosquitoes. As DDT was not being used, the DDT resistant remained undetected, unidentified and dominated by DDT sensitive mosquitoes. But when the use of DDT as an insecticide started, the DDT resistant mosquitoes had a competitive survival mosquito with advantage over their counter parts. Then only with the resistant genotypes were able to survive and reproduce. The DDT resistant characteristic strain multiplied and spread over more and more members of the population. Gradually their number in the population increased while the DDT sensitive type were finally eliminated, making the mosquito population resistant to DDT.