Biology 152/153 Hardy-Weinberg Mating Game EXERCISE 1 Testing Hardy-Weinberg Equilibrium Hypothesis: The Hardy-Weinberg Theorem says that allele frequencies will not change over generations under the following conditions 1. The population is very large (no genetic drift) 2. There is no differential survival and reproduction among individuals in the population (no natural selection) 3. No mutations 4. No immigration or emigration of individuals into or out of the population (no gene flow) 5. Mating is random (no sexual selection) Prediction: If no evolutionary mechanisms are working on a population (all the assumptions of Hardy-Weinberg are met), then no evolution will occur (there will be no change in allele frequencies over time). 1a. Fill in Table 1. Calculate the initial genotype and allele frequencies. Table 1. INITIAL genotype and allele frequencies for Exercise 1 - NO MECHANISMS population size (# of individuals) = # of individuals in class total # of alleles within the population = 2 x population size = 2 X # of individuals in class Initial genotype frequencies for Exercise 1 - NO MECHANISMS frequency AA = # individuals AA / pop. size = 0.25 frequency Aa = # individuals Aa / pop. size = 0.50 frequency aa = # individuals aa / pop. size = 0.25 * check: frequency AA _0.25_ + frequency Aa _0.50_ + frequency aa _0.25_ = 1.0 Initial allele frequencies for Exercise 1 - NO MECHANISMS frequency A = # A / total # alleles in the gene pool = 0.50 frequency a = # a / total # alleles in the gene pool = 0.50 * check: frequency A 0.50 + frequency a 0.50 = 1.0 1
1b. How could you apply the more general terminology of p s and q s to the above? In other words, in the equation p 2 + 2pq + q 2 = 1.0, which terms refer to the frequencies of AA, Aa, and aa? term p2 refers to frequency of AA term 2pq refers to frequency of Aa term q2 refers to frequency of aa Thus, p refers to what? How about q? term p refers to term q refers to the allele frequency of A the allele frequency of a 1c. Answers will vary. 1d. Answers will vary. The should be a relatively small change between initial and final allele frequencies. 1e. Were your results consistent with the hypothesis and prediction? Hopefully yes -- the allele frequencies will have changed little. 1f. Is this population evolving? Briefly explain why or why not. No - the allele frequencies did not change much, thus no evolution is occurring according to a the definition used by popultion genetics. EXERCISE 2 Simulation of Natural Selection Hypothesis: Natural selection alone can cause evolutionary change in a population. Prediction: If natural selection is acting on a gene pool, then the allele frequencies will change over generations (i.e., then evolution will occur). 2a. Fill in Table 4. Record the initial genotype frequencies and allele frequencies for this simulation (i.e. copy the calculations from Table 1 on page 145). Table 4. INITIAL genotype and allele frequencies for Exercise 2 - NATURAL SELECTION 2
initial genotype frequencies: AA 0.25, Aa 0.50, aa 0.25 initial allele frequencies: A 0.50, a 0.50 2b. Answers will vary. 2c. Answers will vary. The should be a relatively small change between initial and final allele frequencies. 2d. Were your results consistent with the hypothesis and predictions? Hopefully yes -- the allele frequencies will have changed. 2e. Is this population evolving? Briefly explain why or why not. Yes - the allele frequencies did change over generations. 2f. During this simulation, the parental generation included living individuals with the genotype aa. In the F1 generation, all aa individuals died. Speculate how this scenario might occur in nature. The environment may have changed after the P generation had offspring, such that the genotype aa was being selected against under the new environment, whereas aa did fine before the environmental change. EXERCISE 3 Simulation of Mutation Hypothesis: Mutation alone can cause evolutionary change in a population. Prediction: If mutation is acting on a gene pool, then the allele frequencies will change over generations (i.e., then evolution will occur). 3a. Fill in Table 7. Record the initial genotype frequencies and allele frequencies for this simulation (i.e., copy the calculations from Table 1 on page 145). Table 7. INITIAL genotype and allele frequencies for Exercise 3 - MUTATION 3
initial genotype frequencies: AA 0.25, Aa 0.50, aa 0.25 initial allele frequencies: A 0.50, a 0.50 3b. Answers will vary. 3c. Answers will vary. The should be a some change between initial and final allele frequencies. 3d. Were your results consistent with the hypothesis and predictions? Hopefully yes -- the allele frequencies will have changed some. 3e. Is this population evolving? Briefly explain why or why not. Yes - the allele frequencies did change over generations. EXERCISE 4 Simulation of Genetic Drift 4a. Propose a hypothesis that addresses genetic drift. Depending on your hypothesis, predict future allele frequencies of the gene pool (population) based on the effects of gene flow (if/then). Your Hypothesis: Your Prediction(s): Answers will vary. Answers will vary. 4b. Answers will vary. 4c. Answers will vary. 4d. Answers will vary. There may be a change between initial and final allele frequencies. In addition this change will vary among groups. Some may go to fixation at one or the other allele, others may show a smaller change. 4
4e. Answers will vary. There may be a change between initial and final allele frequencies. In addition this change will vary among groups. Some may go to fixation at one or the other allele, others may show a smaller change. 4f. Were your results consistent with the hypothesis and prediction? Maybe -- the allele frequencies will have changed a lot in some groups and little in others. 4g. Is your small population evolving? How about other groups? Briefly explain why or why not. Yes - if the allele frequencies changed over generations. No - if they did not. 4h. Genetic fixation occurs when one allele or the other goes to a frequency of 1.0 (100%). Genetic drift tends towards fixation. Were any of the populations in your class fixed? If so, for which allele? Why might some populations fix at one allele, other populations fix at the other allele, and some not go to fixation at all? Explain your reasoning. Genetic drift tends too cause fixation in small populations. Which allele the population fixes at is random. However, if a population starts fixing in one direction (p =.7 & q =.3), the population will probably fix in that direction given enough time because it has a head start in one direction. In this simulation, fixation is possible at either allele, and some populations may not yet have fixated. It is all due to chance. 4i. Did the final allele frequencies show more or less variation (mix of A and a ) than the initial allele frequencies? Based on the class results, would you say genetic drift increases or decreases variation within a population? Would you say genetic drift increases or decreases variation among populations? Explain your reasoning. Genetic drift tends to decrease variation within a population (extreme case - fixation). Genetic drift tends to increase variation among populations (the populations are independent of one another -- which allele goes to fixation in one population is independent of which allele goes to fixation in another population). Also, the amount of time it takes a population to fix will vary further increasing the amount of variation among populations. 5
Example Outcome: pop 1 pop 2 pop 3 pop 4 pop 5 initial A.5.5.5.5.5 initial a.5.5.5.5.5 final A 1.0.7 0.4.2 final a 0.3 1.0.6.8 variation within a population has decreased variation among populations has increased EXERCISE 5 Gene Flow Gene flow is the migration of individuals into or out of a population. In other words, a population may gain new alleles into its gene pool due to immigration and may lose old alleles due to emigration. Natural selection and genetic drift tend to decrease variation within a population and increase differences among populations because they work toward the predominance of one allele in a population. Gene flow tends to increase variation within a population and decrease differences among populations as alleles mix due to migration of individuals. Imagine if the class ran another mating simulation similar to Exercise 4 (genetic drift). Instead gene flow would be simulated, and the initial allele frequencies were as follows: pop 1 pop 2 pop 3 pop 4 pop 5 pop 6 initial allele freq A 0.60 0.25 0 0.80 1.0 0.90 initial allele freq a 0.40 0.75 1.0 0.20 0 0.10 5a. How would the final allele frequencies change compared to the initial if gene flow was at work? Explain your reasoning. Final frequencies would have tended towards 50/50. 5b. How would the results of gene flow compare to the results from the genetic drift exercise? In other words, explain how the mechanisms differ. Gene flow tends to increase variation within a population -- in this exercise the populations started with varying frequencies depending on the different outcomes in small groups from Exercise 3. For example, if 6
a small population started with frequencies of p =.2 & q =.8, gene flow would change these frequencies towards.5 both. Gene flow tends to decrease variation among populations -- gene flow has a homogenizing effect among populations -- all the populations start to look similar over time. Example Outcome: pop 1 pop 2 pop 3 pop 4 pop 5 initial A 1.0.7 0.4.2 initial a 0.3.1.0.6.8 final A.45.4.5.65.5 final a.55.6.5.35.5 variation within a population has increased variation among populations has decreased (all the pops are closer to p=.5 and q=.5 than initially) EXERCISE 6 Small Group Inquiry 6a. Explain the difference between evolution and natural selection. Can evolution occur without natural selection? Explain your reasoning. Evolution is the result of natural selection (i.e., natural selection can cause evolutionary change). Evolution can occur without natural selection via one or more of the other mechanisms. 6b. Natural selection requires that variation exists within a population. Why? If only one homozygous genotype exists in a population (the population is fixed at one allele), then natural selection cannot occur. The survival and reproduction will be the same among all the genotypes -- either they all live and reproduce equally or they all die. For example, imagine three different final population that resulted in Exercise 3: 7
pop 1. all AA (p=1.0) pop 2. all aa (q=1.0) pop 3. mix (p =.4 and q =.6) In population 1, there are no aa s to select against and never will be unless mutation or gene flow occurs. In population 2, all the individuals would die (no differential survival... none survive). In population 3, variation exists, thus natural selection can occur -- some individuals will survive and reproduce better than others EXERCISE 7 Review Questions 7a. Match the terms on the left with the descriptions on the right. Use the descriptions on the right ONLY once. Hardy-Weinberg equilibrium C evolution D genetic drift E gene flow B mutation F natural selection A A. differential survival and reproduction B. migration into or out of a population C. describes a population NOT evolving D. change in allele frequencies in a gene pool over many generations E. chance events F. change in the DNA of an individual 7b. Which evolutionary mechanism or mechanisms tend to decrease variation within a population and increase differences among populations? genetic drift and natural selection 7c. Which evolutionary mechanism or mechanisms tend to increase variation within a population and decrease differences among populations? gene flow and mutation 7d. What is the ultimate source of variation in a population? mutation 8