Examination Candidate Number: Desk Number: BA, BSc, and MSc Degree Examinations 2017-8 Department : BIOLOGY Title of Exam: Evolutionary ecology Time Allowed: 2 hours Marking Scheme: Total marks available for this paper: 100 Sec on A: Short Answer / Problem / Experimental Design ques ons (50 marks) Sec on B: Essay ques on (marked out of 100, weighted 50 marks) The marks available for each ques on are indicated on the paper Instructions: Sec on A: Answer all ques ons in the spaces provided on the examina on paper Sec on B: Answer either ques on A or ques on B. Write your answer on the separate paper provided and a ach it to the back of the ques on paper using the treasury tag provided Materials supplied: CALCULATOR For marker use only: 1 2 3 4 5 For office use only: 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 12
SECTION A: Short Answer / Problem / Experimental Design questions Answer all questions in the spaces provided Mark total for this section: 50 1. a) Vertical transmission of symbionts might help explain the maintenance of mutualistic interactions. Outline how, why it might be insufficient to explain all mutualisms, and describe one other mechanism that might also help. (10 marks) Answer: The maintenance of mutualistic interactions is a challenge because of the temptation to cheat (1 mark) i.e. gain benefits from a partner without paying costs (1 mark). Vertical transmission of symbionts is when symbionts are transmitted through host reproduction into offspring (1 mark) This resolves the temptation to cheat because the reproduction of the symbiont is dependent on the reproduction of the host (1 mark), hence harming the host harms the symbiont (1 mark). This is insufficient as an explanation because not all stable mutualisms involve vertical transmission (1 mark), such as Mycorrhizal fungi and plants (1 mark for an example). e.g. Sanctions (1 mark). Cheats are detected (1 mark) and the partner takes action to reduce their fitness so that cheating no longer pays (1 mark). Other mechanisms are possible. Learning outcomes addressed. LO1 Understand and explain the major theories underlying the topics covered in the lectures, in terms of underlying assumptions and predictions. LO2 Evaluate the strengths and weaknesses of those theories by reference to the empirical evidence. Feedback: This was one of the best answered questions, with many people getting full marks. It seems that most people knew a lot of relevant material here, but several did not fully answer the question; for example several did not explain what vertical transmission was, or not successfully, or provided incomplete explanations of how it worked, or did not fully or successfully explain why it could not explain the maintenance of all mutualisms (for full marks I wanted some general examples of types of cases where this was so). Several people mentioned sanctions (the most common alternative given) and gave an example of it without really explaining how it works in principle. Many people tried to describe several mechanisms, and I only gave them full marks if the best description of mechanism deserved full marks: i.e. weak descriptions of three mechanisms did not get full marks. page 2 of 12
2. The figure below shows a phylogenetic tree of some nematode species, along with information on their lifestyle and mode of reproduction. Describe the observed association between reproductive mode and lifestyle, and why it might exist. (10 marks) Example Answer: All species parasitizing animals are outcrossing (1 mark), probably representing at least 4 (maybe 6) independent evolutionary associations (1 mark). Plant parasites however and free living organisms contain a mixture of uniparental and outcrossing species (1 mark). This suggests that uniparental animal parasite species go extinct more rapidly (1 mark), although it might be the case that sexual animal parasites adapt and speciate more rapidly, perhaps driven by co-speciation with their hosts (1 page 3 of 12
mark). High extinction rates of clonal animal parasites might best be explained by the concept of the Red Queen (1 mark), or its longer term equivalent, the Fisher-Muller effect (1 mark). The latter both assume that the speed of adaptation is higher in outcrossing organisms, and might be particularly beneficial when attacking animal hosts because of their adaptive immune systems and general outcrossing behaviour (1 mark). In contrast uniparental species might persist for longer in other situations because of the 2-fold advantage in reproductive rate (1 mark), and the lower incidence of outcrossing in plant hosts and more generalized ecology of free living species, which makes inverse frequency dependence less strong (1 mark). Explanations need to include and explain major concepts, and compare the animal and other ecologies, linking them to these concepts. Learning outcomes addressed: LO1: Understand and explain the major theories underlying the topics covered in the lectures, in terms of underlying assumptions and predictions. LO2: Evaluate the strengths and weaknesses of those theories by reference to the empirical evidence. LO3: Understand a range of research techniques and approaches in evolutionary ecology, along with their limitations, and describe how they can be applied to particular problems in rigorous investigations. LO5: Read, understand and criticise the primary research literature from a range of topics in evolutionary ecology. Feedback: There were some very good answers here, but plenty of people only got about half marks. Some people clearly did not understand what the association between reproductive mode and lifestyle meant, and instead started talking about frequencies of each trait state instead of associations between the two traits. But most people understood what association means. I reserved one mark for some discussion of the number of evolutionary replicates of the association e.g. how often sexuality had arisen in animal parasites, which is rather fewer than the number of species of animal parasites. A couple of very impressive answers included this, or broached this topic. Most of the marks available were for inferring why animal parasites would benefit from outcrossing whilst free living and plant parasites might sometimes benefit more from asexuality. Here, most people managed to get to the red queen and explain that in context (some people gave incomplete explanations which didn t get maximal marks); rather fewer people talked about the two-fold cost of sex, or about why being a plant parasite might lead to less of a red queen situation. If you did all that well, though, you got ten marks. 3. In a parasitic wasp species, researchers determine that the relationship between female offspring body size, S (mm), and clutch size, c, is: page 4 of 12
S = -0.067c+1.54 Whilst the relationship between female body size, S, and adult lifetime fecundity (eggs laid), F, in the laboratory is: F = 112.3S-85.4 a) Using these relationships, calculate the Lack clutch size (show all your working). (3 marks) Answer: Example for clutch size of 2: size = -0.067*2+1.54=1.406; F=72.5, Brood fitness = 72.5*2= 145.0. (1 mark for method applied correctly) Total brood fitnesses up to clutch size of 7: 80.0; 145.0; 194.9; 227.4; 249.6; 254.3; 244.1 (1 mark for these correct), Therefore the Lack clutch size is 6 (where total brood fitness is maximal) (1 mark). 1 mark removed for each mistake, errors though carried forward. Learning outcomes addressed: LO1: Understand and explain the major theories underlying the topics covered in the lectures, in terms of underlying assumptions and predictions. LO3: Understand a range of research techniques and approaches in evolutionary ecology, along with their limitations, and describe how they can be applied to particular problems in rigorous investigations. Feedback: Lots of people got full marks on this part. Some areas where some people fell down: some people forgot that the Lack model is about optimizing total offspring fitness form a brood, and instead tried to optimize individual offspring fitness: they generally only got one mark for that. This and other errors were carried forward into the next part as far as possible. Some people forgot that they were supposed to solve for the maximum total brood fitness over all possible clutch sizes, and instead simply tried to solve for c. No marks for that. Most people attempted a numerical solution, which is fine, and some people didn t bother spending time on repeating the calculations over 7 clutch sizes, but showed that they understood the method well enough, and they got two marks. One impressive individual provided an analytical solution via calculus, which was correct. b) The researchers also calculated the Lack clutch size from a field estimate of fitness as 4. Given that the observed clutch size in the field was 3.9, explain what we can infer about the biology of the species from this and the lab estimate of the Lack clutch size from part a. (7 marks) page 5 of 12
Example answer: The Lack clutch size from the field study is very close to that observed, whilst that from the lab study is larger than that observed (1 mark). The latter is a common observation in tests of the Lack model (1 mark), and usually results from the fact that organisms can rear several broods in their lifetime, which is not assumed under the Lack model (1 mark). In this case two size fitness relationships have been estimated, from the lab and field, and it is common in lab studies of insects for the fitness effects of body size to be underestimated (1 mark). In this case, the smaller estimate of the Lack clutch size conforms to this idea, since a smaller clutch size implies higher fitness for large individuals (1 mark). If we accept the smaller estimate of the Lack clutch size to be the more accurate, that would imply that the wasp generally only rears a single brood (1 mark), since the observed clutch fits the Lack estimate well (1 mark). Learning outcomes addressed: LO2: Evaluate the strengths and weaknesses of those theories by reference to the empirical evidence. LO3: Understand a range of research techniques and approaches in evolutionary ecology, along with their limitations, and describe how they can be applied to particular problems in rigorous investigations. Feedback: this part was one of the worst answered in the exam, though some people did well at it. The vital comparisons to make were between the lab and field predictions of the Lack clutch size, and between the observed clutch size and the field prediction. The former comparison should lead you to try to explain why the field prediction was smaller, and there you can talk about a lot of biology, particularly better estimates of the benefits of large offspring size. I gave credit for talking about fights between wasps here, but in most parasitoid wasps that probably doesn t happen, and the observed benefit of size in the field is probably due to large female wasps being better at finding hosts than small wasps. In the latter comparison you should have concluded that the observed clutch size and field predictions were the same, hence this suggests that the Lack assumption (of laying only a single brood per lifetime) is correct. Unfortunately a large number of people took the tiny difference between these two values to be meaningful about biology, and went on to explain how this must mean that wasps lay more than one brood per lifetime, and/or that hosts might be superparasitized. I gave some credit for this, but never full marks. page 6 of 12
4. The Table below is from a study of genetic differentiation in the Halichoeres genus of coral reef fish living in the tropical western Atlantic. Table 1 Population genetic subdivision among selected geographical locations in species of Halichoeres. [Asterisk (*) represents comparisons of localities in different environments (tropical versus subtropical in H. bivittatus ; inshore versus offshore in H. H. maculipinna )]. Symbols used: distance, straight line distance in km; Φ ST, genetic separation between populations (negative values indicate very high gene flow); **significant population structure at α =0.05.) distance pairwise comparisons (km) Φ ST Halichoeres bivittatus Bermuda inshore Bermuda offshore* 2 0.57** Gulf of Mexico Belize* 1200 0.94** Panama Bahamas 1860 0.004 Gulf of Mexico Panama* 2000 0.93** Florida offshore NE Brazil 4800 0.25** Belize NE Brazil 5100 0.50** Halichoeres maculipinna Florida Keys Bahamas 100 0.10 Florida Keys Venezuela 1780 0.10 St Croix NE Brazil* 3100 0.97** Venezuela NE Brazil* 3200 0.97** With reference to these data, explain what speciation mechanisms are likely to be active in the genus. (10 marks) page 7 of 12
Example answer: The table investigates how genetic differentiation relates to distance and to ecological niches. Theories of speciation differ in the roles played by adaptation to different ecologies, and in the role of isolation by distance (1 mark). For example, allopatric speciation requires isolation by distance (1 mark), so we would expect genetic differentiation only between the most distant populations if isolation occurred through allopatry (1 mark). On the other hand, sympatric speciation and parapatric speciation require less isolation by distance (1 mark) but are likely to involve adaptation to different ecologies (ecological speciation) (1 mark). In Halichoeres bivittatus we see significant genetic differentiation between individuals from different habitats in close proximity, as well as between individuals from the same habitat which are very distant (1 mark). This suggests that ecological speciation can occur without geographic isolation, although allopatric speciation is also possible (2 marks). In the other species, significant genetic differentiation only occurs between individuals from different habitats, although these are always distant comparisons, so the evidence for mechamism is arguably less strong here (2 marks). Up to five marks for pointing out the salient features of the data and up to five for linking these well to different speciation mechanisms. Learning outcomes addressed: LO1 Understand and explain the major theories underlying the topics covered in the lectures, in terms of underlying assumptions and predictions.lo2 Evaluate the strengths and weaknesses of those theories by reference to the empirical evidence. LO3 Understand a range of research techniques and approaches in evolutionary ecology, along with their limitations, and describe how they can be applied to particular problems in rigorous investigations.lo4 Establish and articulate the interactions between ecological and evolutionary processes or investigations.lo5 Read, understand and criticise the primary research literature from a range of topics in evolutionary ecology. Feedback: This question threw many people, but several people did very well at it. A mistake made by several people was to assume that when populations were not significantly differentiated, this implied that speciation was nonetheless happening. In fact it means there is no evidence for speciation occurring. Several people wrote about hybridization. In fact the question is explicitly about how populations of a species differentiate into two, not about how two species hybridize to form a third. Some people appeared to not understand that the left hand column defined a comparison between two populations, appearing to think instead that it defined the range of one population. Several people didn t seem to notice the important information contained in the asterisks in the left hand column, which shows when the populations inhabit different environments, implying the potential for adaptive page 8 of 12
differentiation; in fact this was explicitly discussed in only a minority of cases. Quite a few people did not notice that the first row shows differentiation between two populations in very close proximity. However, the majority of people correctly identified likely cases of allopatry and sympatry and parapatry, and explained why those mechanisms matched the data. 5. a) Briefly outline how you could test if the lack of large bodied rodent species might be due to higher extinction rates in larger species. (4 marks) Example answer: in principle you could estimate how body size affects extinction rates from fossils, phylogenies of living taxa, and from current extinction risks of living taxa. One mark for one of these. The most common approach is the latter (1 mark). You could look up the body sizes and extinction risk from the IUCN website (1 mark) and plot that across the phylogeny, and then use a phylogenetically controlled method to estimate the body-size extinction risk association (e.g. independent contrasts was mentioned in the lecture) (1 marks). Up to 2 marks available for a good description of other approaches (e.g. BiSSE on phylogenies, compare species lifetimes for fossil species). Learning outcomes addressed: LO3 Understand a range of research techniques and approaches in evolutionary ecology, along with their limitations, and describe how they can be applied to particular problems in rigorous investigations. Feedback: Most people got 2 or more marks here; maximal marks were rarer as people were often quite vague about, for example, how you could test the hypothesis. Drawing a graph of some data does not explicitly test an hypothesis: instead you need to refer to a statistical approach. Some people were rather vague about how a phylogeny could be used, or seemed to misunderstand that. b) Suggest two other macroevolutionary explanations for the lack of large bodied rodent species, and outline how you could test one of them (6 marks) Answer: Could be higher speciation rates at low body sizes (1 mark), or small ancestral body size with little body size evolution (1 mark). e.g. for small ancestral body size, one could either reconstruct body size on an evolutionary tree from extant species body sizes (2 marks), and see where the ancestor lies in relation to the modern distribution (1 mark), and quantify the degree of page 9 of 12
change since then using some kind of metric which e.g describes whether closely related species tend to share similar body sizes more commonly than expected by chance (1 mark). Other answers are possible. Learning outcomes addressed: LO1 Understand and explain the major theories underlying the topics covered in the lectures, in terms of underlying assumptions and predictions. LO3 Understand a range of research techniques and approaches in evolutionary ecology, along with their limitations, and describe how they can be applied to particular problems in rigorous investigations. Feedback: Several people didn t provide two mechanisms, and some people s mechanisms were not explicitly macroevolutionary. Again, if you want to test an hypothesis, you need to be explicit about what data you will get, how you will get it, and what statistical approach you will use. Lots of people mentioned (in an oblique way normally) that optimal body sizes might tend to be small. I gave credit for that, but often the proposed tests for this were not very convincing or detailed. 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) Discuss, with examples, how phylogenies can help us to answer questions in evolutionary ecology, and what their limitations might be. Answer: A good answer will explain what a phylogeny is and what information it contains; for example timing of diversification events, evolutionary distances between clades, branching patterns from which speciation and extinction rates can be calculated. It will outline the use of the comparative method for testing hypotheses about trait evolution, and the benefit of phylogeny in identifying evolutionary independent datapoints. Some examples might include: identifying trade-offs in life histories; comparative tests of sex ratio theory; identifying page 10 of 12
macroevolutionary trajectories in specialization. Macroevolutionary applications come from the ability to identify speciation and extinction rates from branching pattern, or to find correlates of extinction risk in extant species. Limitations include that they can often only provide correlative evidence, so causation is hard to pin down, that they might be hard to construct accurately or contain errors (e.g. for diverse taxa, for extinct data, where taxa are on long branches etc), that some phylogenetic patterns are consistent with multiple processes, and are easiest to construct for extant species where molecular data are available, so rarely give detailed information on processes like extinction. Very good answers will give detailed explanations of the comparative method and how it works, and how to extract macroevolutionary information; show a good spread of examples and applications to a broad range of topics in evolutionary ecology, referring those back to the overall properties of phylogenies themselves; show in depth understanding of limitations with examples of those, as well as showing a good understanding of the range of secondary data that can be gathered about species and understood in phylogenetic contexts, linking those back well to fundamental questions. Learning outcomes addressed: LO1 Understand and explain the major theories underlying the topics covered in the lectures, in terms of underlying assumptions and predictions. Lo3 Understand a range of research techniques and approaches in evolutionary ecology, along with their limitations, and describe how they can be applied to particular problems in rigorous investigations.lo4 Establish and articulate the interactions between ecological and evolutionary processes or investigations. Feedback: Only four people attempted this question, so it was obviously not an area where people were very comfortable compared to the other essay, and this was reflected in the answers, which were often very short and weak on breadth and depth, and error-filled, despite there being numerous examples to draw on from throughout the module. There was one quite competent answer though. OR B) The sex ratio is often thought of as one of the best understood life history traits. Discuss to what extent this might be true. Answer: A good answer will explain how we judge an area of evolutionary ecology to be well understood in general; i.e. by reference to mathematical models that make well tested assumptions and predictions, and many strong empirical studies that support the models. It will then outline how sex ratio can page 11 of 12
vary, and the major theoretical models that predict variation in sex ratio such as Fisher/Dusing s theory, Hamilton s theory of LMC etc. It will then refer to the match between theory and data by reference to empirical studies. The very best answers will show a broad range of theory and data, well linked back to the question, perhaps refer to the long history of sex ratio studies, refer to the ease with which theories can be tested both experimentally and by comparative data.the best answers might compare the state of sex ratio studies with a range of other life history traits and compare and contrast the state of the theories and evidence, perhaps relating that back to the history of study band the ease with which traits can themselves be studied empirically. Learning outcomes addressed. LO1 Understand and explain the major theories underlying the topics covered in the lectures, in terms of underlying assumptions and predictions. LO2 Evaluate the strengths and weaknesses of those theories by reference to the empirical evidence. Feedback: The vast majority of people answered this question. There was a large range of marks obtained, with some very good answers indeed. It was pleasing to see most people explicitly try to answer the question, and many people tried to compare the state of sex ratio research to that in other fields, which was a good thing to try. Few people tried to identify what we do not know about sex ratio evolution, (e.g. to what extent sex determining mechanisms constrain sex ratios) but this made a good impression when it was done. Even fewer actually tried to define a set of criteria for what a well-understood area of science might look like in the introduction: that would have been an effective way to start your essay. page 12 of 12