Alleles resistant to organophosphate insecticides first appeared in Asia and were introduced into mosquito populations in the United States via ; these alleles quickly became fixed via .

Mutation & genetic drift

Explanation:

A mutation is characterized as a lasting change to the DNA succession in a quality. This change moves the hereditary message conveyed by the quality and can modify the amino corrosive arrangement of the protein the quality encodes. This implies future cells created by the quality will just convey a specific characteristic.  Genetic Drift is the change in the hereditary structure of a populace after some time because of possibility or irregular occasions. In instances of hereditary float, for example, catastrophic events or periods of irregular climate, the age that makes due to repeat won't really be the fittest, yet the most fortunate. Hereditary float doesn't allude to a particular change in hereditary cells, rather to arbitrary events that impact a population's genetic makeup.Hence, the right answer of the fill up the blank is "mutation and genetic drift".