Two closely related species of bacteria are found to be fixed for two different electrophoretically detected alleles at a locus encoding an enzyme involved in breaking down a nutrient. How could you test experimentally whether the divergence in enzyme sequences may have caused differences in function and fitness? In Figure 20-5, note that the difference in survival rates be- tween AS and AA genotypes declines as children get older. Offer one possible explanation for this observation. Examining Figure 20-8, explain why the rate of evolution at nonsynonymous sites is lower. Do you expect this to be true only of globin genes or of most genes?