EVOLUTION OF DDT RESISTANCE IN MOSQUITOES

    The figure portrays the evolution of resistance to the pesticide DDT in mosquitoes (Aedes aegypti) in a suburb of Bankock, Thailand.  Resistance is governed by a single Mendelian locus, R .  The two alleles are designated R and +, for resistant and wild-type, respectively.  Individuals carrying at least one copy of the R allele are resistant to DDT, while +/+ individuals are susceptible.  The figure shows that the frequency of the R/R genotype (open circles, solid line) increased during the period of DDT treatment, indicating that the frequency of R rose rapidly in response to selection imposed by the pesticide. This response is expected, since resistant genotypes have much higher fitness in the presence of DDT than do susceptible genotypes.  After cessation of pesticide treatment in 1968, the frequency of the R allele fell quickly, suggesting that in the absence of pesticides, the resistant allele has deleterious effects, similar to alleles for warfarin resistance.
 

Figure from D. J. Futuyma.  1998.  Evolutionary Biology.  Sinauer, Sunderland, MA.

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