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Unformatted text preview: Evolutionary Mechanisms Spring 2008 Exam #2 Please PRINT your name_________ANSWER KEY_______________ Please SIGN your name, indicating your agreement to abide by the honor code: ______________________________________________ Instructions: Make sure you have a complete exam: your exam should contain 15 questions. You have 75 minutes to work on the exam. The exam is closed book and closed notes. Please write legibly. Short answer questions (# 1-12). These questions can each be answered in a few sentences. 1. The biologist J.B.S. Haldane was once explaining kin selection to some friends in a pub. As the story goes, he scribbled some calculations on an envelope and announced that he would be willing to die for two brothers or eight cousins. Explain his reasoning (4 points). Haldane realized that he was related to each of his brothers by an r of 0.5 and to each of his cousins by an r of 0.125. Thus, he could replace himself (in an evolutionary sense) by ensuring that his two brothers survived (because 2 x 0.5 = 1) or by ensuring that his eight cousins survived (because 8 x 0.125 = 1). Page 2 2. Why are evolutionary rates measured from extinct populations in the fossil record often much lower than those measured in contemporary (extant) populations (4 points)? Rate of evolution in the fossil record are often much lower than those in contemporary populations because of the problem of "time averaging." Fossil data represent the average rate of change over a very long time period. Thus, while evolutionary rates can be very rapid over short time periods, they are not sustained over long periods of time. As a result, the longer the time period over which an evolutionary rate is calculated, the lower the rate of evolution that is measured. 3. Using labeled diagrams, illustrate the essential differences between phyletic gradualism and punctuated equilibrium (5 points). Page 3 4. What are the three critical predictions of the reinforcement hypothesis (4 points)? (1) Hybrids should be less fit than the parental types (2) Character displacement in reproductive traits should occur (3) Hybrids should decline over time 5. Give an example of an adaptation that benefits: (a) a small genetic system, at a cost to its host; (b) a local group of organisms, at a cost to its individual members (4 points). (a) a transposable element; a b-chromosome; an allele that causes gene conversion; an allele that causes segregation distortion; an allele that causes sex-biased inheritance. (b) an allele that codes for kin-selected altruism Page 4 6. (a) What is a ring species? (2 points) (b) Why are ring species important (4 points)? Ring species occur when a chain of interbreeding populations loops around such that the terminal populations coexist without interbreeding. Ring species are important for two reasons: (1) They show that species can form despite ongoing gene flow. (2) They demonstrate that the differences between species are the same in kind (although not in degree) as the differences between populations within a species (i.e., microevolution explains macroevolution). 7. During the early evolution of life, what three factors were thought to be important in enforcing cooperation among different self-replicating entities (4 points)? central control, synergy, genetic similarity Page 5 8. Although organisms occupy almost every habitat in the biosphere, most individuals inhabit only a limited set of ecological conditions. Briefly describe (i.e., in a couple of sentences or less) two general factors that may promote "phylogenetic niche conservatism" (6 points). Any two of the following would do: 1. Lack of variability: there may be a lack of variability in the traits that would allow dispersal across unsuitable habitat. 2. Natural selection: When ecological factors (e.g., extreme temperatures, competition for food) reduce fitness outside of the ancestral niche, NS should favor traits that keep individuals in the niche (e.g., behavioral habitat selection or habitat tracking). Such behavioral habitat selection will reduce opportunities for adaptation to conditions outside the niche, because populations cannot adapt to conditions to which they are not exposed. 3. Pleiotropy: Organismal traits that would allow range expansion may be pleiotropically linked to traits that reduce fitness. 4. Gene flow: Gene flow from the center to the periphery of the range of a species can impede local adaptation to conditions at the edge of the species range. 9. (a) What is the C-value paradox (3 points)? (b) Why is the best interpretation of the C-value paradox (3 points)? The C-value paradox refers to the fact that more complex taxa do not tend to have larger genomes (where the C-value refers to a taxonomic group's haploid genome size. The C-value paradox suggests that genomes consist largely of "selfish" and "junk" DNA. Page 6 10. Speciation is often viewed as a three-stage process. List, and briefly describe (i.e., in one sentence), each of the three stages of speciation (6 points). 1. Isolation is created by reductions in gene flow. 2. Divergence is created when mutation, genetic drift, and selection act on populations separately. 3. Reproductive isolation arises when members of different populations no longer are able to interbreed. 11. (a) What is an epigenetic phenomenon (3 points)? (b) Give an example (3 points). An epigenetic phenomenon is one that occurs "above the genome". It is a change caused not by a change in genes per se (e.g., in DNA sequence), but in the way the genes are expressed. DNA methylation is an example of an epigenetic phenomenon. Methyl tags become added to genes after years of replication (as in stem cells), or due to environmental exposure (to chemicals, inflammatory responses, stress). This may turn off important genes and cause cells to act abnormally, possibly even resulting in cancer. 12. In the table below, fill in the (a) criterion for recognizing species, (b) advantages, and (c) disadvantages associated with each of the three species concepts listed (9 points). Species concept Criterion for recognizing species Advantages Disadvantages Biological Reproductive isolation between populations (they don't breed and produce viable offspring) Reproductive isolation = evolutionary independence Not applicable to asexual or fossil species; difficult to assess if populations do not overlap geographically Morphospecies Populations are morphologically distinct Widely applicable Subjective (researchers often disagree on how much morphological divergence = species) Phylogenetic Smallest monophyletic group on evolutionary tree Widely applicable; based on testable criteria Few phylogenies available Short essay questions (# 13-15). These questions can each be answered in one page (or less). 13. (a) Explain why altruism is a central paradox of Darwinism (2 points). (b) Discuss Hamilton's concept of kin selection and explain how this concept resolves the apparent paradox of altruism (10 points). Your answer should include a discussion of Hamilton's rule and a description of its terms. Altruism represents a central paradox for Darwinism. After all, how can a trait that all, if natural selection favors those individuals that produce the most surviving young, how can a behavior evolve that entails helping others to survive and reproduce, especially if it reduces the helper's own survival and reproduction? Hamilton realized that individuals can propagate their alleles not only by reproducing themselves but also by helping relatives to reproduce. Hamilton (1964) formalized this idea and used it to develop a far-reaching evolutionary principle, Hamilton's rule, which describes the spread of an altruism-promoting allele. This rule incorporates three terms: the coefficient of relatedness, r, between an altruist (the "actor") and the individual receiving help (the "recipient"), where r is a measure of the genetic similarity between individuals by virtue of descent from a common ancestor; the cost of the act, c, in terms of future offspring production that the actor loses by behaving altruistically; the benefit of the act, b, in terms of the extra offspring that the recipient gains. Hamilton's rule states that an allele for altruism will increase in frequency in a population if the following inequality is satisfied: rb - c > 0 In other words, an altruism-promoting allele will increase in frequency in a population if the genetic profit from the altruistic behavior, which equals the recipient's extra offspring production multiplied by the relatedness of the altruist and the recipient, exceeds the genetic loss, which is the decrease in the altruist's offspring production resulting from its efforts to help other individuals. This theory, termed kin selection theory, helps explain the evolution of apparent altruism, for, by helping relatives reproduce, propagates copies of its own alleles. In essence, altruismpromoting alleles spread because they promote care for copies of themselves. Page 9 14. (a) What is character displacement, and how does it arise (6 points)? (b) What role might character displacement play in the origin of species (6 points)? (a) Character displacement refers to trait evolution stemming from selection to lessen competition between species. When two species with similar resource requirements cooccur, those individuals most dissimilar from the average resource use phenotype of the other species will face the least competition and thereby acquire more resources than other members of their population. Consequently, the most divergent individuals will experience the highest fitness, and each population may evolve to be less like the other. (b) An indirect consequence of character displacement (Fig. a below) is that individuals in sympatry with the heterospecific competitor will experience a different competitive environment than conspecifics in allopatry. Consequently, conspecific populations in these two types of environments may diverge in resource-use traits. Between-competitiveenvironment offspring (i.e., offspring produced by matings between individuals from different competitive environments) may therefore produce an intermediate phenotype that is less well adapted to either competitive environment than that produced by withincompetitive-environment offspring (i.e., offspring produced by matings between individuals from the same competitive environment). If such maladaptation limits gene flow between populations in different competitive environments, it may constitute "ecologically dependent postmating isolation," which occurs when "hybrids" are at a selective disadvantage because they are intermediate in phenotype and an intermediate niche is lacking. Moreover, because of this reduced gene flow, populations in divergent selective environments may begin to evolve independently of each other and thereby accumulate alleles that are incompatible with genomes from the alternative environment. Ultimately, postmating isolation between conspecific populations, and possibly even speciation, may emerge as a by-product of interactions between species (Fig. b below). Page 10 15. Discuss one major insight that the field of evolution and development has contributed to our understanding of the evolutionary process (12 points). I accepted anything reasonable here. Some important insights that the field of evo-devo has contributed to evolutionary biology come from an understanding of: (1) (2) (3) (4) Deep homology The origins of novelty Developmental plasticity and polyphenism The role of epigenetic inheritance in development and evolution Page 11 EXTRA CREDIT Define "heterochrony" (2 points). Heterochrony refers to all cases in which the timing or rate of development of one developmental process in the body changes during evolution relative to the rate of another developmental process. ...
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