Biology: Lab 8: Population Genetics and Evolution
Alternative Lab Ideas Tip: "A few months ago there was a discussion in our group about a 'great' genetics lab that used Teddy graham crackers-thanks to some help from NSTA, I found the lab. (Editor's note: Teddy grahams may have changed from hands up/hands down varieties-check current styles and modify names in lab accordingly.) Although the study of biology and life science encompasses many confusing and difficult topics, none seems to be as difficult as natural selection. Students often think that animals consciously adapt to their environment: the peppered moth changing color, the giraffe stretching its neck, and the polar bear turning white so it blends in with the surrounding snow are common misconceptions. Once ingrained in the students' thinking, these mistaken concepts are often difficult to correct. While we agree that there is no 'magic potion' that will guarantee that students understand natural selection, we have developed an activity that models the theory, directly involves the students as the selecting agent, and incorporates a strong motivating force: food. (Remember, when working with food, be sure no chemicals are around and table tops are clean.) Teddy grahams are little crackers that look like bears. Each box contains bears with two traits: one with their hands up and one with their hands down. One trait can be designated as adaptive to the environment, and the other can be considered nonadaptive. The number of boxes you use depends on your class size. The total number of crackers can be considered your population gene pool. How simple or complicated you get depends entirely upon your own goals for the class. Here are two versions of the same activity. The first, 'Natural Selection with Teddy Grahams,' can be used in any basic life science or biology class. The goal is for students to understand the basic concept of natural selection and to have them explain and graph what happens to two populations of bears over time. The second activity, 'Evolution with Teddy Grahams,' incorporates the same ideas from the first activity with the addition of using formulas from Hardy Weinberg Equilibrium for calculating the frequencies of the dominant and recessive genes. The students can calculate the specific frequencies of each genotype over successive generations. These results can lead to a variety of questions regarding the nature of dominance and recessive traits in organisms. You can also discuss why a recessive gene with an unfavorable trait remains in the population. Ultimately, we would like the students to understand that, in order for evolution to occur, there must be a change, over time, of the gene frequency of the organisms.
Natural Selection with Teddy Grahams Divide the students into groups of two and have them read the introductory story that describes the environmental conditions affecting favorable and unfavorable traits. After the story is read, have students follow the prescribed procedure and predict the outcome of each population of bears. You are a bear-eating monster. There are two kinds of bears: happy bears and sad bears. You can tell the difference between them by the way they hold their hands. Happy bears hold their hands high in the air, and sad bears hold their hands down low. Happy bears taste sweet and are easy to catch. Sad bears taste bitter, are sneaky, and are hard to catch. Because of this, you eat only happy bears. New bears are born every 'year' (during hibernation) and the birth rate is one new bear for every old bear left from the last year. First, obtain a population of bears, and have your students record the number of happy bears, sad bears, and the total population for each generation in a data table. Second, eat three happy bears. (If you do not have three happy bears, then eat the difference in sad bears.) Obtain a new generation from the teacher, repeat the steps for two generations, and graph each population result. Have students write a paragraph explaining what happened to each population of bears and why. Have them contrast the conclusions with their original predictions. Evolution with Teddy Grahams Use the same story and procedure as in the natural selection scenario, but add the following information. Since the happy trait is recessive, the happy bears are homozygous recessive. In addition, because the sad trait is dominant, the sad bears are either homozygous or heterozygous dominant. Using the two formulas, calculate the gene frequencies for both the dominant and recessive genes and the genotypes that are represented in the population. Begin with calculating the percentage of homozygous recessive organisms and then use this number for the remaining calculations. We suggest that you ask your students the following questions: 1. Explain which trait is not favorable. 2. Which phenotype is reduced in the population? 3. What specifically happens to the genotypic frequencies from generation one to generation four? 4. What occurs when there is a change over time of the genotypic frequencies? 5. Explain what would happen if the selection pressure changed and the recessive gene was selected for. 6. What would happen if it were better to be heterozygous (Aa)? Will there be homozygous bears? Explain. 7. What happens to the recessive (a) gene over successive generations? 8. Explain why the recessive gene (a) does not disappear from the population.
We have found these two activities to be an effective means for enhancing students' understanding of Darwin's theory. Because they are directly involved with being the actual selective agent, students begin to conceptualize this theory and realize that environmental and genetic factors are influential in determining the survival of organisms. Students eventually form the conceptual link between an organism's appearance and behavior, and the idea that genetic material determines hose traits. Ideally, they replace previous Lamarckian thoughts with new ones that reflect the current understanding of evolution and natural selection. Students come to understand that genetics determine the survival of an organism, but these organisms cannot actively change to fit their surrounding environment. The motivating factor of food appears to enhance the success of the activity. Any edible product that can represent the two traits will work. Other suggestions are for the red and green Christmas M&Ms and Ritz Pieces crackers. (Put the M&Ms in the freezer until needed. Be sure to use a classroom where toxic chemicals are not used if students are to eat their 'prey.') We have been successful in using these activities from middle school life science to AP Biology, and we believe they would be equally effective with upper elementary children."
-- Robert W. Blake Jr., doctoral student, Education Department, Curriculum Instruction and Evaluation, University of Illinois, Chicago, Illinois; -- Albert C. Wartski, biology teacher, Poly High School, Long Beach, California; -- Lynn Marie Wartski, biology teacher, St. Anthony High School, Long Beach, California. Formulas: 1: p + q =1; 2: p2 + 2pq + q2 = 1 -- Sande Ivey, Bangor Area Senior High School, Bangor, Pennsylvania. 9/10/99 Question: "Does anyone have an interesting or unusual way to introduce AP Lab 8-Hardy Weinberg Equilibrium?" Answer: "There is a great lab in the lab manual to accompany Starr and Taggart (a great text, by the way) that involves using colored beads to find gene frequencies. It is always a big favorite with my AP classes. The beads come from Carolina." -- Anne Soos, Stuart Country Day School, Princeton, New Jersey. 2/5/00