Understanding Natural Selection: Effects on Rabbit Populations

Natural Selection and Growth of Populations (20 points) Written by Dr. Kimberly Hill-Edwards [Part 2 adapted from HHMI Biointeractive] Part 1: Natural Selection Changes in the characteristics of a population occur through natural selection. Natural Selection occurs when an accidental mutation allows some members of a population to better survive, reproduce, and pass on their traits to their offspring. Using a computer simulation, you will investigate the impact of accidental mutations on a population of rabbits with and without a predator (wolves). 1. Open PHET Natural Selection simulation a. Select ‘Intro’ . You should now see one white rabbit on the screen. 2. Click on ‘Proportions’ in the bottom right corner.

Simulation 1 3. Click “Add a Mate” and let the simulation run for 2 generations. Then click ‘pause’ on the bottom right. a. You can tell how many generations have been born at the top of the screen (the area circled in orange). There is a legend on the left side that shows the distribution of color of the bunnies You should have 18 all white bunnies.

4. Click ‘Play’ and let the simulation run until you have 5 generations. Then click the ‘pause’ button. 5. (0.5 pts): Once the screen reads generation 5, how many bunnies are there, and what is the distribution of color? There are 480 bunnies and they are all white fur. Simulation 2 – Accidental Mutation 6. Click on the orange ‘reset’ button in the bottom right corner (See the orange arrow below).

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7. Click on ‘Proportions’ in the bottom right corner. 8. Click “Add a Mate” and let the simulation run for 2 generations. Then click ‘pause’ on the bottom right. 9. Now we will add an accidental mutation. Click on “Dominant Fur” in the upper right- hand corner. 10. Click ‘play’ and wait 3 more generations. Then click the ‘pause’ button.

11. (0.5 pts) Once the screen reads generation 5, how many bunnies are there, and what is the distribution of color? There are 480 bunnies. The distribution of color is 91% white and 9% brown. 12. (1 pt) How did the accidental mutation of brown fur impact the size of the population? (Hint: Refer to questions 1 and 2 to help answer this question.) In generation 5, the total bunny population did not change. However, there was a change in color range. Bunnies are no longer all white. Simulation 3 – Accidental Mutation and a Predator 13. Click on the orange ‘reset’ button in the bottom right corner. 14. Click on ‘Proportions’ in the bottom right corner. 15. Add Dominant fur mutation and then click ‘Add a Mate’ . Wait until you have generation 3. Click ‘pause’ . 16. (0.5 pts) At generation 3, how many bunnies are there, and what is the distribution of color? There are 54 bunnies total. The distribution of color is 83% white and 17% brown. 17. Add a predator. Click the ‘ Wolves ’ button. 18. Click ‘play’ and let the simulation run until generation 4. Then click ‘pause.’ 19. (0.5 pts) At generation 4, how many bunnies are there, and what is the distribution of color? In generation 4, the total bunnies were 25. The distribution of color is 48% white and 52% brown. 20. Click ‘play’ again and let the simulation run until generation 6 . 21. (0.5 pts) At generation 6, how many bunnies are there, and what is the distribution of color? There are 48 bunnies in generation 6. The distribution of color is 19% white and 81% brown

22. (1 pt) What happened to the color of the rabbit population over time? What role did the wolves play? The dominant color of the bunny population has become brown. Wolves killed most white bunnies. Because white is easy to see in the wild, the wolves took most of the white bunnies, which is why brown is the predominant color. There is no consideration given to the white fur of a recessive white bunny in genetics 23. (1 pt) Summarize how natural selection impacted the color of the bunny populations. In natural selection, dominant brown fur survives, while recessive white fur is easily removed. White bunnies attract wolves the most, while most remaining bunnies remain brown. Part 2: Population Growth Population dynamics are how a population changes over time, including how fast it gains or loses individuals. How a population grows is described by Exponential Growth (J- Curve) or Logistic Growth (‘S - Curve’). Through computer models, you will investigate what determines how quickly a population grows and how large it gets. PROCEDURE 1. Open the Population Dynamics Model 2. C lick on “Launch Interactive” (See the yellow arrow in the image below) and read through the section “Below are some Examples of what understanding population dynamics can help us do”.

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Part A: Exponential Growth Model (J-curve) 3. Scroll down and c lick on “Exponential Growth Model”. Read the introduction information. 4. Now click on “Go to the Simulator to Explore Exponential Growth” located at the bottom to access the exponential model simulator. The population size and how quickly it changes is determined by the size of the initial population at the start of the model (N o ) and the growth rate of the population ( r ). You will investigate the impact of both of these factors on the size of a rabbit population.

5. (1.5 pts) The r value determines quickly a population grows. Set the initial rabbit population size (N o ) to 100. Set the maximum value of the y-axis to 2000. Then test the growth rates listed in the table below. Use the graph to determine when the population size reaches 2000 rabbits. You may need to alter the x-axis scale to determine the time. 6. (1 pt) Summarize the trend (i.e. How does the amount of time to reach 2000 rabbits change as you increase the growth rate?) Increasing population growth decreases the time it takes to reach 2000 rabbits . 7. (1.5 pts) Now let’s see the impact of initial population size (N o ) on how quickly a rabbit population grows. Set the population growth rate ( r ) to 0.5. Set maximum value of the y-axis to 2000. Then test the initial population sizes listed in the table below. Use the graph to determine when the population size reaches 2000 rabbits. You may need to alter the x-axis scale to determine the time. N o (Initial Population Size) Time to reach 2000 rabbits 10 10 100 6 1000 1 r -value (population growth rate) Time to reach 2000 rabbits 0.1 30 0.5 6 0.8 3 This sets the maximum x-axis value This sets the maximum y-axis value

8. (1 pt) Summarize the trend (i.e. How does the amount of time to reach 2000 rabbits change as you increase the size of the initial population?) With increasing population size, the time necessary to reach 2000 rabbits decreases 9. (1 pt) Does a exponential model predict that population growth will ever slow down or decline? Does this model accurately reflect reality? Why or why not? Population growth is not slowed down by conditions, as it is influenced by real-life factors like diseases and weather, making this model unrepresentative of reality. 10. (1 pt) What happens to a population that follows exponential growth that overshoots the ecosystem’s carrying capacity? Overpopulation can lead to species extinction in an ecosystem, as resources may run out when a specific population is overpopulated, causing some populations to leave or die off. Part B: Logistic Growth Model (S-Curve) Click on the “Logistic Growth Model” button at the top and read through the description of the logistic model, then click on “Go to the Simulator to Explore the Logistic Growth Model”. Logistic growth is still determined by the size of the initial population and the population growth rate, but logistic growth is limited by carrying capacity, K . Carrying capacity is the maximum number of individuals that the community can support without exhausting resources. 11. (1.5 pts ) Let’s examine the impact of carrying capacity, K on a wolf population. Set the growth rate to 0.6 and the initial population size to 100. Set the maximum value of the y-axis values to 200. Try the carrying capacities in the table below. Carrying Capacity ( K ) Final Wolf Population Size ( N ) 50 50 100 100 150 150

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12. (1 pt) Summarize what happens to the final population size depending on carrying capacity. Populations increase or decrease based on carrying capacity, regardless of where they start. To meet the carrying capacity of the population, gaps must be filled, or the population removed. Now, let’s examine a real world population. In the 1980s, wildebeest and other ungulates in the Serengeti were decimated after they became infected with rinderpest, a virus related to measles. Wildebeest populations began to recover when farmers started vaccinating domestic cattle, which were the source of the virus. Figure 1 shows the population of zebra and wildebeest in the Serengeti from 1955 to 2010. Figure 1 Actual Wildebeest and zebra populations in the Serengeti from the 1950s to 2010. Data from Mduma, Simon AR, A. R. E. Sinclair, and Ray Hilborn. "Food regulates the Serengeti wildebeest: A 40‐year record." Journal of Animal Ecology 68.6 (19 99): 1101-1122 and Grange, Sophie, et al. "What limits the Serengeti zebra population?" Oecologia 140.3 (2004): 523-532. 13. (0.5 pt) What kind of population growth model (exponential or logistic growth model) would you use to represent wildebeest populations? Why?

A logistic growth model will best represent the wildebeest population since it considers the environment's limited resources. 14. (0.5 pt) Were wildebeest populations at their carrying capacity in 1965? Why or why not? Wildebeest populations were not at their carrying capacity in 1965 due to continued growth until 1980 and leveling off. The Zebra population, however, remained at capacity in 1965 due to a consistent population until 2010. 15. (1 pt) If the size of the protected grazing land were doubled, which parameter of logistic growth would most likely change – growth rate, initial population, or carrying capacity? Would it increase or decrease? Why? By doubling the size of protected grazing lands, the logistic model of growth predicts that the carrying capacity of the population would be altered, which refers to the maximum population that can be supported without harming the environment. This increase in protected grazing land would enable a larger population. 16. (1 pt) If predators were removed from the protected grazing area where the wildebeest lived, which parameter of logistic growth would most likely change – growth rate, initial population, or carrying capacity? Would it increase or decrease? Why? Carrying capacity would increase if predators were removed from protected grazing areas. The growth of a species will increase crazily without predators controlling it. 17. (1 pt) If there was a wildfire which parameter of logistic growth would most likely change – growth rate, initial population, or carrying capacity? Would it increase or decrease? In a logistic model of population growth, the carrying capacity of the environment remains constant, but natural events such as wildfires can alter it, causing a decrease in carrying capacity as the fire destroys resources and habits. 18. (1 pt) Summarize how the population growth of the rabbits (Part A) was the same as and different from the population growth of wolves (Part B).

Whether it was due to resources or predators, both populations changed based on the growth rate. Rabbits grew at a different rate from wolves. Since rabbit populations grew exponentially without any limiting factors, their size increased rapidly.

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Natural Selection and Population Growth Lab-KellyBoulis

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