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INTEGRATION OF EVOLUTIONARY FORCES

INTEGRATION OF EVOLUTIONARY FORCES - The sickle cell story...

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INTEGRATION OF EVOLUTIONARY FORCES Selection leads to an increase in the average fitness of the population. Illustrated by the following example. Recall that the average fitness ("w bar") = w AA p 2 + w Aa 2pq + w aa q 2 . Now consider the case where p=.2, q= .8 so f(AA)=0.04, f(Aa)=0.32 and f(aa)=0.64. If their respective fitnesses are 1.0, 0.2 and 0.1, then w bar = . 04(1)+.32(.2)+.64(.1) = .132. If selection were to continue for a number of generations q would decrease and p would increase. Lets say that we recalculated the average fitness when p=0.8 and q=0.2: thus f(AA)=.64, f(Aa)=.32 and f(aa)=.04 so w bar = .64(1)+.32(.2)+.04(.1)=.708. When will the average fitness reach a maximum? when the deleterious allele (a) is selected out of the population so the only genotype is AA, the most fit genotype. (How would this differ if there was dominance?)
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Unformatted text preview: The sickle cell story with a new twist. There are actually more than just two alleles relevant to the sickle cell story. The normal allele (A) and the sickle allele (S) produce the genotypes traditionally considered: AA is normal but susceptible to malaria, AS individuals are not severely debilitated, but they are resistant to malaria, SS individuals are severely affected and rarely survive to reproduce. A third allele (C) results in the following genotypes and phenotypes: AC is normal but susceptible, SC has mild anemia and CC is normal and resistant to malaria. The following approximate fitnesses have been assigned to these genotypes: Genotype w phenotype AA 0.9 malarial susceptibility AS 1.0 malarial resistance SS 0.2 anemia AC 0.9 malarial susceptibility SC 0.7 anemia CC 1.3 malarial resistance...
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