BI 310

Evolution

Problem Set II

Answer each of the following questions (10 points each; 100 points total).

1. Using your own words, describe the difference between an experimental study, an observational study, and a comparative study. What sorts of questions are they each suited for? Give an example of each type of study from the Adaptation lecture or the textbook.

2. What is an evolutionary trade-off? Why do they occur? Give two examples. How does the occurrence of trade-offs illuminate the general question of whether all traits are adaptive?

3. Males in many species often attempt to mate with strikingly inappropriate partners. Ryan (1985) describes the túngara frogs clasping other males. Some orchids mimic female wasps and are pollinated by amorous male wasps – who have to be fooled twice for the strategy to work. Would a female túngara or a female wasp make the same mistake? Why or why not? (Think of general explanations that are applicable to a wide range of species.)

4. The graphs in the figure below show the variation in lifetime reproductive success of male versus female elephant seals (Le Boeuf and Reiter, 1988). Note that the scales on the horizontal axes are different. Why is the variation in reproductive success so much more extreme in males than females? Draw a graph showing your hypothesis for the relationship between number of mates and reproductive success for male and female elephant seals. Why do you think male elephant seals are four times larger than females? Why aren’t males even bigger?

5. When a Thomson’s gazelle detects a stalking cheetah, the gazelle often begins bouncing up and down with a stiff-legged gait called stotting (see figure below). Stotting was originally assumed to be an altruistic behavior that distracts the cheetah from the gazelle’s kin and also alerts the gazelle’s kin to the presence of the predator, at considerable risk to the stotting gazelle. However, T.M. Caro (1986, 1994) reports that stotting does not seem to increase the gazelle’s risk of being attacked. In fact, once a gazelle begins to stott, the cheetah often gives up the hunt. a) If Caro is right, how does

C
(the cost of stotting) for a gazelle compare to

C
(the cost of trilling, a.k.a. alarm calls) for a Belding’s ground squirrel? b) Do you think stotting is altruistic, selfish, spiteful, or cooperative (mutualistic)? If you are not sure, what further studies could you do to answer this question?

6. With respect to mutualisms, what is “the cheater problem”?

7. What are Batesian mimicry and Müllerian mimicry?

8. After a gene is duplicated, what eventual evolutionary outcomes are possible? What is the most likely outcome?

9. Codon bias is a phenomenon whereby certain codons are substituted more often in phylogenetic lineages than other (synonymous) codons that encode the same amino acid. What might account for codon bias?