University of London

(University College London)

BSc Degree 1997

GENETICS B7: POPULATION AND EVOLUTIONARY GENETICS

Answer ONE question from section A, TWO questions from section B, and ONE question from section C (overleaf). Each section carries 1/3 of the marks on the paper.

Section A. Answer ONE of questions 1-3.

1. Outline the essential features of cladistic analysis. Explain why it sometimes leads to classifications that conflict with traditional views.

2. How does the genetic architecture of adaptations to man-made environmental change differ from that of adaptation to the natural environment?

3. Explain how genetic drift interacts with selection in (a) chromosomal evolution OR (b) warning colour OR (c) survival of endangered populations?

Section B. Answer TWO of questions 4-8.

4. Explain the difference between positive and negative frequency-dependent selection and give brief details of one example of each.

5. "Hybrid zones are natural laboratories for the study of speciation". Discuss.

6. Do females choose their mates mainly on the basis of the overall fitness of males? Give reasons for your answer.

7. What is meant by heritability and how can it be measured?

8. Summarise, using examples, the effects of mutation on fitness.

Remember to answer Section C overleaf!

Section C. Answer ONE of questions 9-10.

9. In Mississippi, larvae of the cotton bollworm is rapidly evolving resistance to pyrethroid insecticides. A sample of 1000 adult cotton bollworm Helicoverpa zea were characterized at the sodium channel gene Nac known to confer resistance:

     Susceptible     Heterozygotes     Resistant

     Homozygotes                       Homozygotes

      Nac₁/Nac₁        Nac₁/Nac₂         Nac₂/Nac₂      Total

         77              154               769         1000

a) Estimate the allele frequencies at the Nac locus.

b) Find the expected numbers given random mating.

c) Use a chi-square test to test whether there is any evidence for significant deviation from Hardy-Weinberg. Apart from selection, suggest three reasons why the genotypes might not be in Hardy-Weinberg equilibrium.

d) Laboratory tests using pyrethroid dosages applied to larvae in the field showed that the relative larval fitnesses of the three genotypes were Nac₁/Nac₁: 0.1, Nac₁/Nac₂: 0.1, Nac₂/Nac₂: 1. Are these fitnesses consistent with your answer in (c)? Why or why not?

e) Assuming the laboratory fitnesses above, what genotypic frequencies and what gene frequency would you expect among adults in the next generation? (Hint: random mating and reproduction occurs before selection on larvae).

10. a) Explain what is meant by the evolutionary term "homology".

A homologous protein has been sequenced from six species, A, B, C, D, E and F, and the sequences have been used to construct a matrix of differences between the species (i.e. a distance matrix):

     A    B    C    D    E

B    22

C    32   26

D    37   42   40

E    34   37   34   40

F    30   34   32   38   28

b) Indicate which two of these six species are the most closely related to one another, and explain your answer.

c) Use the distance matrix to construct an unweighted pair-group mean (UPGM) phenogram for these six species. Show the distances assigned to each branch of the tree, and explain how you have calculated them.

d) Describe the three main assumptions made in this type of tree construction.

e) Discuss, briefly, whether numerical phenetic classifications are objective or subjective.

End of paper