BSc and MSc Degree Examinations

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1 Examination Candidate Number: Desk Number: Department : BIOLOGY BSc and MSc Degree Examinations Title of Exam: Evolutionary Ecology Time Allowed: 2 hours Marking Scheme: Total marks available for this paper: 100 Section A: Short Answer / Problem / Experimental Design questions (50 marks) Section B: Essay question (marked out of 100, weighted 50 marks) The marks available for each question are indicated on the paper Instructions: Section A : Answer all questions in the spaces provided on the examination paper Section B : Answer either question A or question B. Write your answer on the separate paper provided and attach it to the back of the question paper using the treasury tag provided For marker use only: Office use only: A B Total as % DO NOT WRITE ON THIS BOOKLET BEFORE THE EXAM BEGINS DO NOT TURN OVER THIS PAGE UNTIL INSTRUCTED TO DO SO BY AN INVIGILATOR Page 1 of 11

2 SECTION A: Short Answer / Problem / Experimental Design questions Answer all questions in the spaces provided Mark total for this section: The above figure shows data from a laboratory clutch size manipulation experiment on a parasitic wasp in which clutches of varying numbers of eggs (x-axis) were added to host caterpillars of constant size. The survival of eggs to adulthood is shown by open circles and the solid line, whilst the total number of adult offspring produced is shown by the triangles and the dashed line. a) Explain how you could use these data, and additional data you might gather, to calculate the Lack clutch size for this wasp (6 marks) Answer: The Lack clutch size is the clutch size that maximizes total fitness from a single brood, which is individual offspring fitness multiplied by the number of offspring (1 mark). The graph shows the number of offspring surviving to adulthood rising with clutch size (dotted line) (1 mark). Additional data we would need would therefore be some way of assessing the fitness of these individuals (1 mark), which might mean assessing their size and the fitness consequences of size (e.g. their lifetime reproductive success) (1 mark). Once this has been gathered one could simply multiple the average LRS for offspring of average size for that clutch size by the number surviving to adulthood for each clutch size (1 mark) and find at which clutch size this number peaks, which is the Lack clutch size (1 mark). Answers could also use the survival data incorporated into the fitness measure but would then need to multiply this by the actual clutch size when calculating the total brood fitness. Page 2 of 11

3 Feedback: This was one of the more poorly answered questions. Only a minority of people seemed to understand how to interpret the graph; the graph only shows information about the number of offspring surviving to adulthood (the egg to adult survival multiplied by the clutch size gives you the number of adult offspring produced), so you can t really use either of the curves as a fully developed individual or total brood fitness curve. Information is lacking here on the body size of individual offspring and that probably affects individual offspring fitness, so the extra info would involve measuring the size of surviving offspring from different clutch sizes and the size/fitness relationship. Only a minority of people recognized this. Several people got the gist of how to do things but did not refer to the graph at all, and they were not awarded full marks. b) In this species, the estimated Lack clutch size from lab experiments was 15, but the observed clutch size averaged 30. Outline to what degree this is expected, give one possible explanation, and briefly outline how you might test that explanation (4 marks). Answer: It is unusual for the Lack clutch size to underestimate observed clutch size; normally the theory overpredicts the data (1 mark). The most likely explanation is that the chances of laying more than one clutch are small and that the fitness costs of small size have been overestimated in some way (1 mark). One way in which that might happen is if there is some hidden advantage to being small that is concealed in laboratory studies, such as small wasps finding it easier to find mates or hosts, perhaps if hosts are concealed in places where small size is advantageous. To test these ideas we would ideally need to find out how many hosts adults tend to find in the field, perhaps by some kind of mark-recapture study (1 mark), and one could also compare the size distribution of wasps at birth to those actually finding and attacking hosts in the field, expecting that small ones might be better than expected from lab experiments (1 mark). Other possibilities can be given credit if correct, and showing understanding. Feedback: Most people correctly observed that this is unexpected and that in most studies it is the other way around. However, only a minority of people then suggested that the size-fitness relationship would have been inaccurately estimated in the lab (it would have been overestimated, not underestimated as is usual). A few people also suggested that mortality might have been different in the field, which is correct and could affect the estimated Lack clutch size. It is however, not clear what pattern of mortality should predict a lower Lack clutch size, because it will also depend on the precise size-fitness relationship, which we haven t specified here, but I gave credit to the suggestion. Evaluate the strengths and weaknesses of those theories by reference to the empirical evidence. Page 3 of 11

4 Read, understand and criticise the primary research literature from a range of topics in evolutionary ecology. 2. Breaking some of the assumptions of Fisher/Düsing s theory of equal sex allocation can cause biased sex allocation. Which assumptions and why? (10 marks) Answer: The four assumptions are: costs of producing males and females equal, panmictic mating structure, males and females differ only in mating role, and parental autosomes are under control of the sex ratio (4 marks). Unequal costs alters the cost-benefit ratio of sex allocation, leading to relative overproduction of the cheaper sex (1 mark); Local mating leads to female biased sex ratios because male fitness is improved but female fitness is unaffected (1 mark); if the sexes differ in areas other than mating role then a variety of outcomes are possible; for example, conditional sex expression where environments favour on sex over the other (1 mark), or local resource competition where the dispersing sex can be favoured if resources are limited for residents (1 mark). If other selfish genetic elements are in control, the outcome would be expected to favour the transmission of the genes involved (1 mark) such as female bias which enhances transmission of cytoplasmic elements (1 mark). Feedback: this was generally well answered and explained, and most people got decent marks. Typical reasons for sub-maximal marks included not explaining how the assumption affects the sex allocation, and also giving examples which were inappropriate to the assumption being explained. 3. Page 4 of 11

5 The figure above shows results from experiments on Medick plants grown with different mycorrhizal fungi: Glomus intraradices is a cooperative species that greatly improves Medick growth, and G. aggregatus and G. custos are less-cooperative species that do not improve Medick growth so much. The graph shows how much carbon flows to the fungi from the plant, as measured by the presence of labelled C 13 in fungal RNA. Positive preference values indicate more carbon flowing to the species at the top of each graph. Why might mutualistic interactions be unstable, how do these data help explain the stability of the plant-mycorrhiza mutualism, and to what extent is this information sufficient (10 marks)? Answer: Mutualisms may be unstable because partners are susceptible to cheating (1 mark), whereby each partner is selected in the short term to receive benefits but not pay them back, hence reducing costs of the interaction (1 mark). Mechanisms whereby partners can sense the decisions made by each other (1 mark) and be selective about whether and how they reciprocate (1 mark) offer the opportunity to alter the fitness consequences of cheating in the long run, selecting against it (1 mark). In the above graph carbon flows preferentially to the more cooperative fungal partner (1 mark), which is evidence of discrimination in the rewards given based on Page 5 of 11

6 partner behaviour (1 mark). Whilst this shows some potential to stabilize the interaction, it may not be sufficient (1 mark), because a mechanism is also needed to prevent plants from cheating cooperative fungi (1 mark), which might take the form of fungi only giving nutrients (e.g. Phosphorus) to plants that give them carbon (1 mark). Feedback: Generally there were pretty good answers to this one. Most people got the idea of cheating right, and understood what that was, and most people said that the selective allocation of resource to potential partners would act as a kind of selection agent on cooperation and could explain where the data showed that. People were a bit less good at explaining the sufficiency of the data, but other people correctly suggested that some other mechanisms might also be effective and we might investigate those too to understand their contribution to cooperation, and also we don t know how much carbon is going to the less-cooperative species and whether it s little enough to be effective, and also, as in the model answer, how defection by plants is controlled, if it s a problem. I gave credit to the suggestion that only looking at three fungal species does not indicate that it s a result of the cooperative behaviour of those species. Evaluate the strengths and weaknesses of those theories by reference to the empirical evidence. Read, understand and criticise the primary research literature from a range of topics in evolutionary ecology. 4. What is adaptive dynamics, what assumptions does it make, and what questions can it be used to solve? (10 marks) Answer: Adaptive dynamics is a modelling approach (1 mark) used to solve questions where the fitness of phenotypes that evolve are density or frequency-dependent (1 mark), and have the potential to influence population dynamics (1 mark). The models work by assuming a monomorphic population and then calculating the fitness of a mutation of small effect (1 mark) at population equilibrium in the background of the resident phenotype (1 mark). If the mutant has a higher fitness than the resident at population equilibrium (invasion fitness) (1 mark), then it is assumed to spread to fixation and becomes the resident (1 mark). This is repeated until mutants can no longer replace residents (1 mark). Some questions it has been used to solve include under what circumstances we can expect Page 6 of 11

7 evolutionary branching (1 mark), and under what conditions do polymorphisms between interacting species coexist (1 mark). Feedback: generally a straightforward and well-answered question. Quite a few people gave no indication of the types of question the approach is applied to. Establish and articulate the interactions between ecological and evolutionary processes or investigations. 5. The figure above shows a) how the wingspan of pterosaur species changed over their evolutionary history (as well, for comparison, as that of birds) and b) how the pterosaur species are related to each other. The diameters of the symbols at the tips Page 7 of 11

8 of the branches of the tree in b) are proportional to log(wingspan). The thick grey line in a) is the maximum wingspan of birds at that time. Using these data, explain what macroevolutionary processes have likely contributed to the frequency distribution of pterosaur wingspans (10 marks) Answer: Plotting all the wingspans together on a log scale, they appear to be right skewed (1 mark). However, this appears to have changed significantly over time, with early species/clades starting off small, and there is a trend towards larger average sizes over time (1 mark). This appears to be a largely anagenetic increase in size over time (i.e. sizes appear to increase with time within several clades) (1 mark). Starting off small with a trend towards larger size means that larger species have had less time to diversify, explaining their relative lack of richness (1 mark). Extinction rates might have been high in small species because many appear to exist on short branches (1 mark) and this might partly explain the trend to larger size distributions over time (1 mark), although because there are also clusters of small species, they might also have high speciation rates (1 mark). Several large species also exist on long branches, suggesting that large species do not speciate very rapidly (1 mark). There is extinction of several small species when birds first arise and the trend of increase in body size tends to track that of maximal size in birds (1 mark), suggesting that pterosaurs compete with similarly sized birds, selecting for size increases in pterosaurs (character displacement) (1 mark). Feedback: This was one of the less well-answered questions. Very few people actually referred to the frequency distribution of wingspans and how it might look, and most of those didn t notice that the axis scale is logged. I was pleased that many people identified that the small species suffer high extinction and the large species suffered low extinction rates. It s less obvious how speciation rates vary with size from the figure, but many people made suggestions, for which I gave credit. Very few people wrote explicitly about anagenetic changes within lineages. Only a minority of people mentioned that bird body sizes could have influenced pterosaur body sizes, through character displacement, even though the data are quite strongly suggestive of that. There were a number of other ultimate influences on body size suggested, seemingly rather plucked out of the general literature but there was nothing on the figure to suggest these. There were a small number of excellent answers which were very close to the model answer though. Evaluate the strengths and weaknesses of those theories by reference to the empirical evidence. Page 8 of 11

9 Read, understand and criticise the primary research literature from a range of topics in evolutionary ecology. SECTION B: Essay question Answer one question on the separate paper provided Remember to write your candidate number at the top of the page and indicate whether you have answered question A or B Mark total for this section: 50 EITHER A. What approaches can be used to explain why some clades contain more species than others? Answer: A good essay will focus on defining different approaches, which should include sources of data, and should also include potential types of process that can explain differences in species richness. A good essay will cover the value of fossil and phylogenetic evidence as well as taxonomic richness of extant groups, and conservation status. It will also cover proximate processes of some kind, e.g. macroevolutionary (speciation and extinction rates), and/or ultimate (e.g. ecological/behavioural). An excellent answer would be comprehensive in its coverage of processes and data and provide lucid descriptions of each and be Page 9 of 11

10 well populated with examples illustrating how these approaches can be applied, as well as giving some potential answers. Evaluate the strengths and weaknesses of those theories by reference to the empirical evidence. Establish and articulate the interactions between ecological and evolutionary processes or investigations. Read, understand and criticise the primary research literature from a range of topics in evolutionary ecology. Feedback: Only 2 people chose this essay, so there is not a big sample to comment on. One of these contained a good range of material and paid attention to the approaches aspect of the question and got a first class mark. OR B. To what extent do we understand why there are both sexual and asexual species on Earth? Answer: A good answer will set out approaches to answering the question that provide the necessary understanding, for example defining the primitive and derived state, why one evolved from the other, why one sometimes reverts to the other, and possibly the macroevolutionary success of both strategies. Since the lecture focussed on selective forces governing origins and maintenance of sex from asexual reproduction a good essay will probably focus on outlining what those forces are, and explaining them and some of the evidence supporting them. A good essay must contain an evaluative element that attempts to define how close our understanding might be to complete. An excellent answer would include elements not necessarily present in the lecture (e.g. macroevolution) and make more explicit reference to extinction processes as well as speciation. It could also be exceptional in breadth of processes covered or in the lucidity of explanations given or the quality and quantity of evidence explained, as well as providing a very well justified evaluation of the extent to which our understanding is complete. Page 10 of 11

11 Evaluate the strengths and weaknesses of those theories by reference to the empirical evidence. Establish and articulate the interactions between ecological and evolutionary processes or investigations. Read, understand and criticise the primary research literature from a range of topics in evolutionary ecology. Feedback: the vast majority of people chose this essay, and there were some very good answers with a good number of people making an attempt at addressing the to what extent bit of the question. A fair percentage of people were not very critical of the studies that had been done, but several were, and had sensible ideas about where research was lacking, or weaknesses of some of the studies we had covered in class. If people did this and covered a good range of the ideas and evidence about how sex might have originated and be maintained, whilst also explaining why it might not be, they got a very good mark. Weaker essays tended not to address the to what extent part of the question, or did so uncritically, or omitted for example why asexual reproduction might still exist, or omitted some of the key ideas and evidence about the origins and maintenance of sex. Page 11 of 11