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14. Natural Selection A. Natural Selection Simulation: Predator and Prey Objectives At the end of this lab you should be able to: 1. Define the concepts of natural selection and genetic drift. 2. Differentiate between natural selection and genetic drift with respect to how they influence evolutionary change in small populations. Introduction Have you ever wondered how it happened that many In this simulation, a predator will visually “hunt” and “kill” its prey. Two important factors in determining the environmental “fitness” of organism will be studied. The first factor is the genetically determined ability of an organism to see various shapes, colors and movements. The second factor, camouflage, is the structural adaptation that allows potential prey to blend in with their environment, thereby avoiding detection by potential predators. organisms are so well suited for their environment? Consider Materials how night vision in nocturnal animals allows them to find food or how a deer’s quickness allows it to avoid many Each group needs: predators. These variations that help an organism survive • A container of with 40 mixed beads, 10 beads in each of 4 colors • A piece of multicolored fabric with a complex pattern (about 50 cm x 30 cm) • 4 small containers, each with beads of one of the four colors (about 40 beads in each container) • Colored markers in the same colors as the beads • 2 labeled petri dishes: “survivor” dish and “graveyard” dish • Masking tape, sticky side out, wrapped around 3 pencils are called adaptations. Organisms have developed many different kids of adaptations that help them survive in their environments. Charles Darwin observed many species of animals and plants in the Galapagos Islands that were unique to the islands, but still similar to species he had seen elsewhere. Darwin developed a theory called natural selection to explain how species change over time. He described natural selection as the process by which populations change in response to their environment. As individuals better adapted to their environment survive, they are able to reproduce and pass their variations on to the next generation. Evolution can be a difficult subject of study in real time. Evolutionary change generally occurs at extremely slow rates. This simulation is designed to speed up the process, so we can get a glimpse of the process in a shorter time frame. Procedure 1. Spread the fabric out on the lab bench. This will act as your habitat. The beads will act as the prey. 2. Assign roles in your group so that you have a “habitat manager” who manages the fabric and bead counts, a “data recorder” and “chip collectors” that will act as predators. 14. Natural Selection 3. Answer the question below and then finish the If…Then hypothesis by predicting which colors will be left in the largest number after 3 rounds. What colors are found on your material? Data Analysis Table 14.1: Sample Data Hypothesis: If natural selection occurs then….. 4. All group members should turn their backs on the habitat while the habitat manager sprinkles the 40 beads onto the fabric and spreads them evenly. 5. The “predators” pick out 30 beads one at a time in the following manner: look at the fabric and select a “prey” by only using eyesight to locate the bead. Take the first bead that stands out - look away between each “hunt”. Table 14.2: Natural Selection Simulation Results Round One Survivors 6. Put the bead victims in the “graveyard” dish. The habitat manager should count them carefully as they are hunted, until 30 are removed from the fabric and placed in the dish. Offspring (Survivors x 3) Offspring + Survivors Totals Offspring (Survivors x 3) Offspring + Survivors Totals Offspring (Survivors x 3) Offspring + Survivors Totals 7. Stop hunting when 10 survivors are left on the fabric. 8. Remove the remaining 10 bead survivors from the fabric and place them in the “survivor” dish. Count the number of each color and record the data in Table 14.2 (see sample data in Table 14.1). Round Two Survivors 9. Assume that each survivor produced three offspring. Multiply the number of each survivor by 3 and put the results in the data sheet. The four totals should add up to 30. 10. Add the number of survivors of each color to the corresponding total from step 9 and record the results in the Totals column of Table 14.2. Make sure the total number of all four colors is 40. 11. Take the number beads of each color shown in the Totals column from the four single-color bead contains and once again have the habitat manager scatter 40 beads onto the fabric to start round 2. 12. Repeat steps 5-11 two more times for a total of three rounds. 14-2 Round Three Survivors 14. Natural Selection 3. Did any bead colors become extinct? If so, which ones and why do you think this happened? Graphing Display your results in a graph with the vertical (Y) axis being the total number of beads after each round for each of the four colors (include also the starting populations, 10 of each color) and the horizontal axis (X) being the three rounds plus “round zero”, the starting point. Use Graph 14.1 or the graph paper provided. Review Questions 1. Was one bead color represented more than others in the first generation of survivors? 4. Which colors had the greatest surviving populations? Why? 2. Did the population of this bead increase or decrease in the second generation? Graph 14.1: Natural Selection Simulation Results Number of Beads after each Round 40 30 20 10 0 Start Round 1 Round 2 Round 3 Rounds 14-3 14. Natural Selection B. Genetic Drift Simulation In this simulation, the “prey” will be killed by a falling tree that smashes into their habitat. Data Analysis Table 14.3: Genetic Drift Simulation Results Round One Procedure Survivors 1. Identify your roles in the team: The Falling Tree “smashes” a pencil onto the fabric. The Habitat Manager manages the fabric and bead count. The Recorder records the data in the data tables. Offspring (Survivors x 3) Offspring + Survivors Totals Offspring (Survivors x 3) Offspring + Survivors Totals Offspring (Survivors x 3) Offspring + Survivors Totals 2. The habitat manager sprinkles 40 beads onto the fabric and spreads them evenly. 3. With eyes closed, the Falling Tree gently rolls or drops the taped pencil randomly onto the habitat and removes the prey that stick to it. Be sure to watch for escapees that get knocked off the fabric! Put them back on the fabric! Round Two Survivors 4. Put the beads from the pencil in the “graveyard” dish, counting them carefully as they are “killed” until all 30 are removed. It may take a few rolls of the pencil to achieve the precise number of 30 beads. If a roll yields a cumulative number “killed” larger than 30, beads from that roll should be put back on the fabric and the pencil re-rolled, if necessary repeating this procedure of replacing and rerolling until exactly 30 beads have been removed. 5. Check to make sure there are 10 survivors left in the habitat. 6. Remove the remaining 10 bead survivors from the fabric and place them in the “survivor” dish. Count the number of each color and record it in Table 14.3. Round Three Survivors 7. Multiply the number of each survivor by 3 and put the results in Table 14.3. The total of all four colors should add up to 30. 8. Add the number of survivors of each color to the corresponding total from step 8 and record the results in the Totals column of Table 14.3. Make sure the total number of all four colors is 40. 9. Then, take the number of beads of each color shown in the Totals column from the four single-color bead containers and once again scatter 40 beads onto the fabric to start round 2. 10. Obtain a new taped pencil and repeat steps 3-9 two more times for a total of three rounds. 14-4 Graphing Display your results in a graph with the vertical (Y) axis being the total number of beads after each round for each of the four colors (include also the starting populations, 10 of each color) and the horizontal axis (X) being the three rounds plus “round zero”, the starting point. Use Graph 14.2 or the graph paper provided. Use colored markers for the beads. 14. Natural Selection Review Questions 1. What does each of the simulation components represent in nature? 2. What are at least two other variables that affected how the simulation played out? Do these same variables exist in nature? The bead: The pencil: The step of increasing the 10 survivors to 40: 3. What if the two mechanisms, natural selection and genetic drift, were both operating in the simulation? Predict the outcome. The fabric: 4. Compare and contrast the graphs of the Natural Selection Simulation and the Genetic Drift Simulation. The bead colors: The elimination of bead colors: The different rounds: Number of Beads after each Round Graph 14.2: 40 Genetic Drift Simulation Results 30 20 10 0 Start Round 1 Round 2 Round 3 Rounds 14-5