Chapter 16: How Populations Evolve

Transcription

1 Chapter 16: How Populations Evolve AP Curriculum Alignment Evolution is a change in the genetic makeup of a population over time, with natural selection its major driving mechanism. This is a major component of Big Idea 1. In Chapter 16, we learn how the allele frequencies in a population can change, which is termed microevolution. Certain human events can also alter the gene pool of a population. One human activity, the overuse of antibiotics, has resulted in the evolution of resistant bacterial strains, such as MRSA. There are five conditions that if present will allow a population to stay at equilibrium and no evolution will result. These conditions are rarely met and are part of the Hardy-Weinberg equilibrium that is depicted with the equation p 2 + 2pq + q 2 = 1. Students will learn about these conditions and how to use the H-W equation in this chapter. Parts of Chapter 16 also correlate to Big Idea 4. Diversity within a population can be beneficial to the population overall. Species and populations with low diversity are at risk for more deleterious genetic effects as well as extinction. ALIGNMENT OF CONTENT TO THE CURRICULUM FRAMEWORK Big Idea 1: The process of evolution drives the diversity and unity of life. Enduring understanding (EU) 1.A: Change in the genetic makeup of a population over time is evolution. Essential knowledge (EK) 1.A.1: Natural selection is a major mechanism of evolution. a. According to Darwin s theory of natural selection, competition for limited resources results in differential survival. Individuals with more favorable phenotypes are more likely to survive and produce more offspring, thus passing traits to subsequent generations. b. Evolutionary fitness is measured by reproductive success. c. Genetic variation and mutation play roles in natural selection. A diverse gene pool is important for the survival of a species in a changing environment. d. Environments can be more or less stable or fluctuating, and this affects evolutionary rate and direction; different genetic variations can be selected in each generation. e. An adaptation is a genetic variation that is favored by selection and is manifested as a trait that provides an advantage to an organism in a particular environment. f. In addition to natural selection, chance and random events can influence the evolutionary process, especially for small populations. g. Conditions for a population or an allele to be in Hardy-Weinberg equilibrium are: (1) a large population size, (2) absence of migration, (3) no net mutations, (4) Mader, Biology, 12 th Edition Chapter

2 random mating and (5) absence of selection. These conditions are seldom met. h. Mathematical approaches are used to calculate changes in allele frequency, providing evidence for the occurrence of evolution in a population. To foster student understanding of this concept, instructors can an illustrative example such as: Graphical analysis of allele frequencies in a population Application of the Hardy-Weinberg equilibrium equation Essential knowledge (EK) 1.A.2: Natural selection acts on phenotypic variations in populations. a. Environments change and act as selective mechanism on populations. To foster student understanding of this concept, instructors can choose an illustrative example such as: Flowering time in relation to global climate change Peppered moth b. Phenotypic variations are not directed by the environment but occur through random changes in the DNA and through new gene combinations. c. Some phenotypic variations significantly increase or decrease fitness of the organism and the population. To foster student understanding of this concept, instructors can choose an illustrative example such as: Sickle cell anemia Peppered moth DDT resistance in insects d. Humans impact variation in other species. To foster student understanding of this concept, instructors can choose an illustrative example such as: Artificial selection Loss of genetic diversity within a crop species Overuse of antibiotics Essential knowledge (EK) 1.A.3: Evolutionary change is also driven by random processes. a. Genetic drift is a nonselective process occurring in small populations. b. Reduction of genetic variation within a given population can increase the differences between populations of the same species. Big Idea 4: Biological systems interact, and these systems and their interactions possess complex properties. Enduring understanding (EU) 4.C: Naturally occurring diversity among and between components within biological systems affects interactions with the environment. Essential knowledge 4.C.3: The level of variation in a population affects population dynamics. a. Population ability to respond to changes in the environment is affected by genetic diversity. Species and populations with little genetic diversity are at risk for extinction. To foster student understanding of this concept, instructors can choose an 244 Mader, Biology, 12 th Edition, Chapter 16

3 illustrative example such as: California condors Black-footed ferrets Prairie chickens Potato blight causing the potato famine Corn rust affects on agricultural crops Tasmanian devils and infectious cancer b. Genetic diversity allows individuals in a population to respond differently to the same changes in environmental conditions. To foster student understanding of this concept, instructors can choose an illustrative example such as: Not all animals in a population stampede. Not all individuals in a population in a disease outbreak are equally affected; some may not show symptoms, some may have mild symptoms, or some may be naturally immune and resistant to the disease. c. Allelic variation within a population can be modeled by the Hardy- Weinberg equation(s). Concepts covered in Chapter 16 also align to the learning objectives that provide a foundation for the course, an inquiry-based laboratory experience, class activities, and AP exam questions. Each learning objective (LO) merges required content with one or more of the seven science practices (SP), and one activity or lab can encompass several learning objectives. The learning objectives and science practices from the Curriculum Framework that pertain to how populations evolve are shown in the table below. Note that other learning objectives may apply as well. LO 1.1 The student is able to convert a data set from a table of numbers that reflect a change in the genetic makeup of a population over time and to apply mathematical methods and conceptual understandings to investigate the cause(s) and effect(s) of this change. LO 1.2 The student is able to evaluate evidence provided by data to qualitatively and quantitatively investigate the role of natural selection in evolution. LO 1.3 The student is able to apply mathematical methods to data from a real or simulated population to predict what will happen to the population in the future. LO 1.4 The student is able to evaluate data-based evidence that describes evolutionary changes in the genetic makeup of a population over time. LO 1.5 The student is able to connect evolutionary changes in a population over time to a change in the environment. LO 1.6 The student is able to use data from mathematical models based on the Hardy-Weinberg equilibrium to analyze genetic drift and effects of selection in the evolution of specific populations. LO 1.7 The student is able to justify data from mathematical models based on the Hardy-Weinberg equilibrium to analyze genetic drift and the effects of selection in the evolution of specific populations. Mader, Biology, 12 th Edition Chapter

4 LO 1.8 The student is able to make predictions about the effects of genetic drift, migration and artificial selection on the genetic makeup of a population. LO 4.25 The student is able to use evidence to justify a claim that a variety of phenotypic responses to a single environmental factor can result from different genotypes within the population. LO 4.26 The student is able to use theories and models to make scientific claims and/or predictions about the effects of variation within populations on survival and fitness. Key Concepts Summary Microevolution Microevolution is the change is allele frequencies within a population over time. A population is the unit upon which evolution acts: populations evolve and not individuals. Population genetics A population that is in equilibrium and not evolving is said to be in Hardy- Weinberg equilibrium. o The conditions for Hardy-Weinberg Equilibrium are rarely met but include: large population random mating no immigration or emigration (no gene flow) no mutations no natural selection o The Hardy-Weinberg equation, p 2 + 2pq + q 2 is used by population geneticists to determine whether or not evolution is occurring. o A population is assessed at two different time periods to determine if allele frequency has changed. If allele frequency has occurred, microevolution has also occurred. Genetic mutations can cause small changes in the allele frequency of a single allele. Gene flow is the movement of alleles between populations by immigration or emigration, causing the gene pools of the original population to change. Genetic drift occurs when a portion of a large population is removed from the original population. There are two types genetic drift: Founders affect occurs when a portion of a large population immigrates to a new location. This population may interbreed and further reduce its variability. Bottleneck effect is when the population may be reduced due to an environmental disaster. The reduced population loses genetic variability and thus allele frequencies are changed. 246 Mader, Biology, 12 th Edition, Chapter 16

5 Mate selection can also have an effect of allele frequencies over time. Types of selection There are three types of natural selection on populations. Most polygenic traits in a population will form a normal distribution curve. Natural selection favors certain variations in these traits. Stabilizing selection occurs when both extreme variations are selected against. Directional selection occurs when one variation is selected against. Disruptive selection occurs when the middle variation is selected against. Populations maintain genetic diversity even when natural selection is occurring. In sexually reproducing organisms, the heterozygous condition maintains the recessive allele. In some cases, such as sickle cell allele, the heterozygote has an advantage such as resistance to malaria. Key Terms allele frequency bottleneck effect cost-benefit analysis cystic fibrosis directional selection disruptive selection dominance hierarchies fitness founder effect gene flow gene pool genetic drift Hardy-Weinberg principle heterozygote advantage microevolution mutation polygenic population reproductive isolation sexual dimorphism sexual selection sickle-cell disease stabilizing selection territoriality territory Teaching Strategies Class time: Three 45-minute class periods Day 1: Lecture on microevolution, genetic drift, and Hardy-Weinberg equilibrium 30 minutes Activity 1: practice using the Hardy-Weinberg equation 15 minutes Day 2: Activity 2: Introduce the Rock Pocket Mice unit by a short lecture and showing three video clips that are outlined below 15 minutes) Activity 3: have students begin this activity. Review transcription and translation beforehand as necessary - 30 Minutes). Mader, Biology, 12 th Edition Chapter

6 Day 3: Finish the Activity 3 and review the correct answers 25 minutes Discuss the heterozygous advantage as demonstrated in the sickle cell allele 20 minutes Suggested Approaches The HHMI Biointeractive unit of the Rock Pocket Mouse presents students with the opportunity to analyze data in a method that is similar to how population geneticists analyze DNA and determine what changes have occurred to produce changes in the alleles. This provides students with an opportunity to review transcription and translation and to tie the analysis of DNA sequences to evolution. The summative assessment for microevolution should be combined with macroevolution. Student Misconceptions and Pitfalls Generally students believe that an organism changes in order to fit into its environment. The Rock Pocket Mouse activity clearly demonstrates that a change in DNA has caused a change in the allele for fur color. Students generally do not think that an organism can mutate its own DNA on purpose. 248 Mader, Biology, 12 th Edition, Chapter 16

7 Suggested Activities Activity 1: Hardy-Weinberg practice problems Have students complete the subsequent worksheet to review the principles of Hardy- Weinberg equilibrium, and to gain comfort manipulating the equation. Activity 2: Rock Pocket Mouse unit from HHMI This series of videos, animations, and activities from HHMI gives students insight into how mutations interplay with environmental change to effect change on a population through natural selection. Start by showing how the environment changed due to volcanic activity and what effect this change had on natural selection. This short video conveys this information: Next, show an animation that depicts how natural selection occurs in the rock pocket mice due to predation: Finally, you will show a video that depicts scientists conducting field studies on Rock Pocket Mice: Activity 3: Molecular Genetics of Color Mutations in Rock Pocket Mice Download the student worksheet and teachers materials at the following website. Students will analyze the DNA sequences for light and dark colored mice. First they will transcribe the DNQ sequence and then translate it to find the amino acid sequences to compare. A series of analytical questions follow. Mader, Biology, 12 th Edition Chapter

8 Hardy-Weinberg practice You have sampled a population for the frequency of the A and a alleles of a particular gene. You know that the percentage of the homozygous recessive genotype is 36%. Using that 36%, calculate the answers to the following questions. 1. What is the frequency of the aa genotype? 2. What is the frequency of the a allele? 3. What is the frequency of the A allele? 4. What are the frequencies of the genotypes AA and Aa? 5. What are the frequencies of the two possible phenotypes if A is completely dominant over a? 250 Mader, Biology, 12 th Edition, Chapter 16

9 Answers to Activity 1: Hardy-Weinberg practice 1. 36%, as given in the problem itself. 2. The frequency of aa is 36%, which means that q 2 = 0.36, by definition. If q 2 = 0.36, then q = 0.6. Since q equals the frequency of the a allele, then the frequency is 60%. 3. Since q = 0.6, and p + q = 1, then p = 0.4; the frequency of A is by definition equal to p, so the answer is 40%. 4. The frequency of AA is equal to p 2, and the frequency of Aa is equal to 2pq. So, using the information above, the frequency of AA is 16% (p 2 = = 0.16) and Aa is 48% (2pq = 2*(0.4* 0.6) = 0.48). 5. Because A is totally dominate over a, the dominant phenotype will show if either the homozygous AA or heterozygous Aa genotypes occur. The recessive phenotype is controlled by the homozygous aa genotype. Therefore, the frequency of the dominant phenotype equals the sum of the frequencies of AA and Aa, and the recessive phenotype is simply the frequency of aa. Therefore, the dominant frequency is 64% and, in the first part of this question above, you have already shown that the recessive frequency is 36%. Mader, Biology, 12 th Edition Chapter

10 Student Edition Chapter Review Answers Answers to Assess Questions 1. c; 2. c; 3. c; 4. b; 5. d; 6. d; 7. b; 8. c; 9. d; 10. c; 11. c; 12. c; 13. b Answers to Applying the Big Ideas Questions 1. An inherited disease that affects the nervous system results in patients that are homozygous recessive at a certain locus that has two alleles, B and b. In a population of 150 people, 2 people have genotype bb. a) Calculate the allele frequencies of both B and b for this population, and determine the frequencies that would be evident if the population is in Hardy-Weinberg equilibrium. b) Explain how your results in part (a) indicate whether or not this population is evolving. Essential Knowledge Science Practice Learning Objective 1.A.1: Natural selection is a major mechanism of selection. 2.2: The student can apply mathematical routines to quantities that describe natural phenomena. 1.3: The student is able to apply mathematical methods to data from a real or simulated population to predict what will happen to the population in the future. 4 points maximum. Calculations of gene frequency and the appropriate explanation of the results may include: Descriptions of kind of data (1 Explanations (1 point each) point each) In this population of 150, there would be 300 alleles: 194 B and 106 b. (p= 0.65; q= 0.35) Since the actual numbers of alleles in the population are not what would be expected in equilibrium, this population may be evolving. If this population was in Hardy- Weinberg equilibrium, there would be 63 BB individuals, 69 Bb individuals, and 18 bb individuals. (p 2 = 0.42; 2pq= 0.46; q 2 = 0.12) In order to have Hardy-Weinberg equilibrium and not evolve, the population would have to be (1) sufficiently large, (2) without migration, (3) lack mutations, (4) experience random mating, and (5) experience an absence of selection. 252 Mader, Biology, 12 th Edition, Chapter 16

11 The actual population presented is not equal to what would be expected in Hardy-Weinberg equilibrium (46 BB, 102 Bb, and 2 bb). Natural selection or random events can influence the evolutionary process, especially in small populations. 2. Variation in molecular units provides cells with a wider range of functions. Explain how multiple copies of alleles, as seen with a heterozygote advantage, provide evidence for this idea. Essential Knowledge Science Practice Learning Objective 4.C.1: Variation in molecular units provides cells with a wider range of functions. 6.2: The student can construct explanations of phenomena based on evidence produced through scientific practices. 4.22: The student is able to construct explanations based on evidence of how variation in molecular units provides cells with a wider range of functions. 3 points maximum. Explanations may include (1 point each): Multiple copies of alleles or genes may provide new phenotypes. A heterozygote may be a more advantageous genotype than a homozygote under particular conditions, since with two different alleles, the organism has two forms of proteins that may provide functional resilience in response to environmental stresses. Sickle-cell disease is due to an abnormal form of hemoglobin (Hb). The people with sickle-cell disease (Hb s Hb s ) tend to die early and leave few offspring (low fitness). Malaria is a disease caused by a protozoan parasite that lives in and destroys red blood cells of the normal homozygote Hb A Hb A. Individuals with malaria also have fewer offspring (low fitness) due to early death or debilitation caused by the disease. Heterozygous individuals (Hb A Hb S ) have an advantage because they don t die from sickle-cell disease or from malaria, (the parasite isn t able to survive with the sickle-shaped red blood cells that lose potassium) keeping all of the alleles to be maintained in the population. The recessive allele for sickle-cell disease (Hb s ) has a higher frequency in regions where malaria is also prevalent. This is an example of stabilizing selection. In the United States, where malaria is not prevalent, the frequency of the HBs allele is declining among African Americans, because the heterozygote has no particular advantage in this country. Mader, Biology, 12 th Edition Chapter

12 Another example of the heterozygote advantage can been seen in cystic fibrosis where the recessive allele causes cystic fibrosis, but the agent that causes typhoid fever can use the normal version of a protein defective in patients with cystic fibrosis. The recessive allele is maintained in the population. Answers to Applying the Science Practices Questions Think Critically 1. highest 22 percent; lowest 0 percent 2. between 110 and 120 generations 254 Mader, Biology, 12 th Edition, Chapter 16

13 Additional Questions for AP Practice 1. Cystic Fibrosis (CF) is one of the most common life threatening genetically inherited conditions affecting Caucasians. CF is caused by two copies of a recessive allele that causes a non-functional membrane protein. In the UK, the incidence of cystic fibrosis is about 1/2,500 live births. What is the frequency of the CF allele in the UK? 2. Describe the effect of variation within a population on survival and fitness of the population. 3. Snails from the same population can exhibit drastically different shell colors. Provide justification for the presence of such different phenotypes. 4. What environmental change can be connected to the change in allele frequencies of the color of the pepper moth? Mader, Biology, 12 th Edition Chapter

14 Grid-In Questions 1. Susceptibility to poison ivy is a recessive trait. If 64% of a population has the trait for susceptibility to poison ivy, what percent of the population are heterozygous? 2. Albinism (aa) is a rare homozygous recessive trait that is carried in many species. It was found that in a population of squirrels in Upstate New York, that 90% of the population had A alleles and 10% carried a alleles. Assuming that the population was in Hardy-Weinberg equilibrium, what would we expect to find for the genotypic frequencies of the homozygous recessive trait in the F2 generation? 256 Mader, Biology, 12 th Edition, Chapter 16

15 Answers to Additional Questions for AP Practice 1. The incidence of cystic fibrosis is equal to q 2 so 1/2,500 = = q 2. q = = The frequency of the cystic fibrosis is therefore Variation within a population increases that population s chance for survival because if conditions change, there is a good chance that at least some of the variants would survive. If the chances of survival are increased by variation, then the chances of living to have offspring, which is the fitness of the population, would increase as well. A fit population is a population that is producing offspring. 3. Gastropods produce offspring by sexual reproduction. Even though this population lives in the same environment, there is great variety due to the mixing of genetic material during fertilization and during crossing over in meiosis. 4. Prior to the industrial revolution, the light allele had a higher frequency that the dark allele for peppered moths. The light colored moths were harder for predators to see on the light lichen covered tree bark. After the industrial revolution, the dark allele for moth color was in greater frequency. The industrial pollution had darkened the tree bark and now the advantage of being harder to spot by predators was experienced by the darker moths. Mader, Biology, 12 th Edition Chapter

16 Answers to Grid-In Questions 1. Chapter: 16 How Populations Evolve Answer: 32% p2 + 2pq + q2 = 1 homozygous recessive: q2= 64% = 0.64 q = 0.64 = 0.8 homozygous dominant: 4% q + p = p = 1 p = p =0.2 p2 = 0.04 = 4% heterozygous: 32% 2pq = 2 (0.8)(0.2) = % 2. Chapter: 16 How Populations Evolve Answer: 0.01 Solution: A = 0.90 a = 0.10 p2 + 2pq + 22 = 1 AA = (0.90)2 = 0.81 Aa = 2(0.9)(0.1) = 0.18 aa = (0.10)2 = Mader, Biology, 12 th Edition, Chapter 16