PRACTICE PROBLEMS 61. Li and collaborators calculated the average sequence divergence for synonymoussubstitutions in a large number of protein-coding genes, and for a large number ofcomparisons of mammals of different orders. The average divergence, corrected formultiple hits, was 0.744. All the orders of mammals are believed to have had a commonancestor about 80 My ago. Calculate the average rate of synonymous substitution formammalian genes. Then estimate the mutation rate for mammals, assuming that theaverage generation time is about 1 year.2. In mammals the rates of synonymous substitution differ between nuclear andmitochondrial genes. The rate is 4.6 X 10-9 bp substitutions per bp per year in thenucleus, and about 50 X 10-9 in the mitochondrion. What, if anything, can you inferfrom these data about the mutation rates in mammalian nuclei and mitochondria?3. Two evolutionary geneticists compared the base sequences of the gag genes of threedifferent viruses that cause AIDS. The results are as follows:differences base pair differencesbetween per 1,500 base pairsLAV-1a and pv.22 6LAV-1a and ARV-2 42pv.22 and ARV-2 45(a) Based on these data, which is the correct phylogenetic tree for these viruses (A, B, orC)? A B C(b) The rate of evolution of these viruses is incredibly high, on the order of 10-3substitutions per site per year. How long ago did the most distantly related of theseAIDS viruses diverge?4. Answer each of the following questions with M for Mutation rate, BS for BalancingSelection, DS for Directional Selection, D for random Drift, or I for Inbreeding.(a) The average resistance of houseflies to the insecticide DDT increased rapidly afteruse of the insecticide began. Resistance is a quantitative trait involving a number ofdifferent genes. The increase in resistance is most likely due to .(b) The difference in evolutionary rates between synonymous and nonsynonymoussubstitutions is due to differences in .(c) The high frequency of the HbS allele for sickle cell anemia in black populations isdue to .5. Many animals and plants are on a list of endangered species, and these have greatlyreduced population sizes. Even if they do not become extinct, there will be significantconsequences for their population genetics. Circle the letters of any of the followingwhich are likely consequences of the reduced population size:(a) reduced nucleotide diversity(b) reduced rate of molecular evolution(d) increased mutation rate(e) establishment of pure lines6. Suppose that during the evolutionary divergence of wheat and maize from theircommon ancestor, both lineages had an average population size of 106 plants and anaverage mutation rate of 5 X 10-10 mutations per base pair per generation. Furtherimagine that in the rbcS gene, which codes for an enzyme called rubisco, the averagefixation probability for nonsynonymous substitutions is 10-7.(a) Write the equation for the rate of molecular evolution as a function of theseparameters.(b) Calculate the rate of nonsynonymous substitutions that you would expect to see inthese rbcS genes.(c) Calculate the rate of evolution in a pseudogene for the two
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