We're going to investigate the effect of natural selection on a small population of mosquitoes with a genetic disease. Let's imagine that the kdr_dis is a recessive disease allele. This means that if a mosquito has two copies of this allele then it suffers from a disease. Let's also imagine that this disease means that the mosquito will die before reaching adulthood. Let's use the virtual mosquito lab to simulate this situation. We're going to look at a population of 10 mosquitoes with an allele frequency of 0.5 kdr_dis, for each generation the mosquitoes that are homozygous for the kdr_dis allele die before mating and some of the other mosquitoes mate multiple times so that the population size stays constant.

Notice that unlike previous simulations the frequency kdr_dis tends towards 0 and doesn't seem to fluctuate randomly. The reason that the frequency tends towards 0 is that individuals with 2 copies of kdr_dis don't contribute any alleles to the next generation. We can say this more scientifically by saying that individuals with 2 copies of the disease allele have a fitness of 0. Fitness is a measure of the number of alleles that an organism contributes to the next generation. If an organism has a low fitness, then it is unlikely to survive to adulthood and mate.