V23.0058 Evolution SAMPLE EVOLUTION MIDTERM EXAM 100 points total +20 bonus points Part I. Darwin et al. 1. [4 pts.] In the Origin of Species, what are the two main theses that Darwin proposed to explain biological diversity? a. b. 2. [12 pts.] Scientific thought, like all human endeavor, has been strongly influenced by many factors, including cultural environment and its own history. During the history of thought about the origin of biological diversity, three main hypotheses have been formulated that can be tested with empirical data. What are these hypotheses (you may use schematics to aid your explanation)? A. Separate ("Special") Creation B. Transformism (Lamarckian evolution) C. Phylogenesis (Darwinian evolution) Briefly describe how the following evidence supports (i.e., is consistent with or is predicted by) or refutes (i.e., is either contrary to or not predicted by) these hypotheses: a. On three continents (South America, Africa, Australia) at the same latitude, very different faunas and floras occupy very similar environments. On the other hand, these forms are more similar to those at different latitudes on the same continent. Hypothesis A: Hypothesis B: Hypothesis C: b. Endemic species on oceanic islands are most similar to species on the nearest mainland, regardless of environmental differences. Hypothesis A: Hypothesis B: Hypothesis C: c. Among the general rules of the non-arbitrary heirarchical classification system begun by Linnaeus is that the less adaptive is a character, the more important it may be for classification. Hypothesis A: Hypothesis B: Hypothesis C: d. Members of a taxonomic group share "Unity of Type" in terms of "homologous characters" that may nevertheless have very different functions. Hypothesis A: Hypothesis B: Hypothesis C: 3. [7 pts.] Perhaps the most striking aspect of living forms is the way they appear to be designed for specific purposes (creationist arguments are primarily based on this incomplete observation). Darwin's and Wallace's explanation for the existence of adaptations was revolutionary because it allowed exquisitely "designed" morphologies to be formed independently of any supernatural creative force. This explanation was formulated by Darwin in two parts: the Struggle for Existence and Natural Selection. a. What is meant by the metaphorical "Struggle for Existence" in terms of population growth and two important life history parameters? (Equations are not required.) b. What is meant by Natural Selection? c. What is meant by Fitness? 4. [6 pts.] The Logistic Equation is meant to model population growth when there is a limit on how many individuals can be supported (the "carrying capacity"): dN/dt = rN(1- (N/K)), where N is the population size (number of individuals) at a particular time t, K is the maximum population size allowable (carrying capacity), and r is the natural, intrinsic (maximal) rate of increase. a. What is the general shape of the curve described by this equation? Where is K? b. What happens to population growth rate as population size approaches K? How does this affect competition between individuals for resources? c. Darwin proposed that competition is most severe between closely related populations. Why should this be generally true? d. [***Bonus-2 pts.] Darwin suggests that this replacement of one population by another results in "Divergence of Character". How does this divergence relate to the phylogenetic hypothesis of evolution that Darwin proposed? 5. [5 pts.] The in vitro experiments by Orgel employing the replicase of the Qb RNA virus in association with a known RNA molecule demonstrate how a population of entities that can vary, multiply and inherit can evolve and adapt. a. One way to adapt (increase one's fitness) would be to try out, at random, all possible changes until the best set of changes is found. How does natural selection operate if not by this method? b. [***Bonus-3 pts.] Under what conditions might you NOT expect natural selection to result in the appearance of the best of all possible adaptations? c. [***Bonus-2 pts.] If natural selection arrives at the best conceivable solution infrequently, why is natural selection generally thought to be sufficient to explain the adaptations we see in nature? Part II. Evolution at a Single Locus 1. [11 pts.] Assume that there are two alleles, A and a, at a single genetic locus in a diploid population. The frequencies of these alleles in the population are p and q, respectively. Given these allele frequencies, the Hardy-Weinberg equation predicts the frequencies (D, H and R) of the genotypes (AA, Aa and aa, respectively) in the next generation. a. What is the Hardy-Weinberg equation? b. How are the genotype frequencies of the next generation predicted? D = H = R = c. How many generations will it take to reach Hardy-Weinberg equilibrium (where the frequencies of the genotypes do not change from generation to generation)? d. How can allele frequencies be estimated if the genotype frequencies can be measured? p = q = e. What are five assumptions made by this model? (1) (2) (3) (4) (5) f. [***Bonus-2 pts.] It is highly unlikely that all of these assumptions hold for any natural case. What good is a model like this if we know that it doesn't even begin to model the real world? 2. [14 pts.] Heterozygosity (i.e., H, the frequency of heterozygotes) in a population is affected by population size. This relationship is H[t] = H[t-1](1-(1/(2N))), where H[t] is the heterozygosity of generation t, H[t-1] is the heterozygosity of the generation just prior to generation t, and N is the population size. a. Given this relationship, what happens to heterozygosity over time in the absence of mutation or gene flow (migration)? b. How does heterozygosity change in smaller populations relative to larger populations? c. What is the "founder effect" and how does it affect heterozygosity? d. This change in heterozygosity over time (which results in a change in allele frequency over time) is called ______________________. e. Does this equation predict which of two alleles (A or a) will eventually be fixed? (Yes or No) _____ f. If two or more smaller populations become separated from a larger parental population, what is the likely result in terms of (1) allele frequencies within each population? (2) allele frequencies between the populations? g. If mutations are incorporated into the model, such that new alleles arise in individuals at a low rate, what is the likely result in terms of (1) replacement of alleles by new ones within each population? (2) accumulation of changes between populations? 3. [7 pts.] Natural selection often results in changing allele frequencies (evolution) such that one allele becomes fixed. (This result is also predicted by genetic drift). But different types of selection, such as "frequency-dependent" selection do not necessarily lead to this result. a. Aposematic coloration (a warning signal to potential predators for unpalatability) is only effective when it is common. That is, fitness is positively related to frequency. What is the likely equilibrium frequency of an allele that contributes toward aposematic coloration? b. For Batesian mimicry to work in large populations, how must fitness be related to frequency? Why? What is the predicted result in terms of the frequencies and numbers of alleles that contribute to particular mimicry patterns? c. What is at least one other way in which selection might operate to maintain polymorphism at a single locus? 4. [2 pts.] The rate of increase of an allele in a population is determined not only by its effects on fitness, but also on its dominance relationships with other alleles in the population. Industrial melanism in most lepidopteran species (butterflies and moths) is determined by dominant alleles. How would this dominance effect help explain the fact that few such alleles are actually fixed? 5. [***Bonus-2 pts.] Explain how peak shifts are proposed to occur on an Adaptive Landscape. Part III. Evolution at Two Loci 1. [11 pts.] If two loci (e.g., A and B) are linked, changes in frequency of alleles at one locus may affect allele frequencies at the other locus. The key analytical tool for measuring this "linkage disequilibrium" is "haplotype" frequency (i.e., the frequencies of gametes that contain particular combinations alleles at two genetic loci, A and B). For the following questions, assume that the frequency of a AB haplotype is p[AB], that of ab is p[ab], that of Ab is p[Ab], and the frequency of aB is p[aB]. Also assume that the recombination "distance" between the A and B loci is r. a. A new population is just about to be formed by mixing equal numbers of AABB and aabb homozygotes (in the form of AB/AB and ab/ab genotypes). What are the following haplotype frequencies in this parental generation? p[AB] = p[ab] = p[Ab] = p[aB] = b. What is the measure of linkage disequilibrium, D? What will be the amount of linkage disequilibrium in the F1 generation? D = Is this F1 generation in linkage equilibrium or disequilibrium for the A and B loci? ( Circle the appropriate word.) c. According to the model, a Hardy-Weinberg equilibrium is acheived in a single generation. Is linkage equilibrium also acheived in a single generation? Why or why not? d. If the loci were not linked (r = 0.5), would linkage equilibrium be acheived in a single generation? (Yes or No) _________ e. What factor predominantly determines the amount of time (number of generations) that it takes for linkage equilibrium to be reached between two loci? f. Briefly describe how assortative mating might influence the amount of linkage disequilibrium in a population. 2. [6 pts.] Genetic loci rarely act alone to determine the phenotype. Rather, different genes usually act in "epistatic" or interdependent enzymatic, regulatory or developmental pathways. a. What is meant by "epistasis for fitness"? b. How will linkage disequilibrium between two loci be affected by an epistatic interaction between the two loci for fitness? c. How will linkage disequilibrium between these two loci be affected by the rate of recombination (recombination "distance") between the loci? 3. [2 pts.] In Primula, heterostyly is determined by at least two closely linked, epistatically interacting loci. a. Briefly explain how selection is likely to maintain heterostyly. b. [***Bonus-3 pts.] If the loci were not linked, and yet heterostyly continued to be maintained, what might we predict with regard to the selection coefficients associated with particular haplotypes? Part IV. Evolution of Quantitative Traits 1. [13 pts.] Virtually every trait shows some degree of variation that involves many genes. If the effect of each locus is small compared to the total amount of variation, it is very difficult to identify the individual loci, and we must turn to statistical analysis of variation. a. Phenotypic variance results from the sum of what other variances? b. Heritability "in the narrow sense" is what fraction of the phenotypic variance? c. What does this mean in terms of the resemblance between parents and their offspring? d. If this heritability is 1, what kind of "response" will be evoked by a particular selection differential on the potential parents? e. How will a reduction in heritability affect the rate of evolutionary change? f. For each of the following types of selective responses, describe evolution in terms of the effect on the mean phenotype: (1) Directional Selection (2) Stabilizing Selection (3) Diversifying Selection 2. [***Bonus-3 pts.] During an artificial selection experiment, the response to selection sometimes stops. This is generally not due to lack of variation. What are some possible reasons? 3. [***Bonus-3 pts.] As Darwin recognized (his "Correlation of Growth"), a genotype (one or more genes) often affects two or more characters (like legs and arms, survivability and fecundity). a. What are two possible causes for such genetic correlation? (1) (2) b. How might genetic correlation maintain genetic variation (i.e., stabilize polymorphism)?