Visual Displays of Information in Understanding Evolution by Natural Selection

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1 Name: Date: Visual Displays of Information in Understanding Evolution by Natural Selection The alpine skypilot is a purple perennial wildflower that is native to western North America. It grows in dry and rocky sites at high altitudes in mountain ranges like the Rockies and Cascades. It has also been the subject of interesting studies of evolution by natural selection particularly focusing on flower size and pollinators. At the highest altitudes, the skypilot is pollinated only by bumblebees, whereas at lower elevations it is also pollinated by flies. We will use data from studies by Candace Galen (1989, 1996) to learn about the ways biologists study evolution by natural selection and the visual displays of information that are involved. Part I. Do offspring resemble their parents? For flower size to evolve by natural selection, the trait of flower size must be heritable to some degree. That is, offspring must resemble their parents in flower size. If flower size is heritable, offspring of parents with small flowers should have smaller flowers, on average, than offspring of parents with large flowers. 1. How might we establish whether flower size is heritable in a population of alpine skypilots? List your ideas here. Graphs and Natural Selection Page 1

2 One way of measuring heritability is by making a graph called a scatterplot in which offspring s trait values are listed on the y-axis (the dependent variable) and parents trait values are listed on the x-axis (the independent variable). A scatterplot is a common type of graph in all fields of biology and medicine. Using the flower size data on the attached Table 1, make a scatterplot in the space below. Be sure to label the x-axis and the y-axis. The first data point ( Flower 1 ) has been plotted for you Judging from the graph you just made, would you say that flower size is heritable to some degree? That is, do alpine skypilots tend to have flowers that are similar is size to their maternal parent? 3. Would you say that alpine skypilots have the same flower size as their maternal parent? List some reasons why plants might not have exactly the same flower size as their maternal parent. Graphs and Natural Selection Page 2

3 Part 2. Do bigger flowers have more offspring? In order for flower size to evolve by natural selection, flower size must affect the number of offspring a plant has, either directly or through effects on survival. Since flowers are the reproductive organs of the plant, it seems reasonable that flower size could directly affect the number of offspring a plant has. 4. How might flower size affect the number of offspring a plant has? Would you expect bigger flowers to have more or fewer offspring? Why? Write your hypothesis below. 5. If a researcher went out and measured a bunch of alpine skypilot flowers and then later counted the number of seeds from each flower, what results would you predict if your hypothesis is correct? What results would you predict if your hypothesis is incorrect? 6. Draw a scatterplot to simulate how such results might look in the space below at left. The specific numbers don t matter, just the general arrangements of points in the graph. Number of seeds Number of seeds Flower size Flower size 7. What results would you predict if your hypothesis is incorrect? Describe your predicted results below, then draw a scatterplot to simulate how such results might look in the space above at right. Again, only the general arrangement of points in the graph matters. Graphs and Natural Selection Page 3

4 The researcher Candace Galen measured a bunch of alpine skypilot flowers in the wild and later counted the number of seeds from each flower. Some of her data are attached in Table 2. Use these data to make a scatterplot of number of seeds as a function of flower size. When biologists say as a function of, it means that the word before this phrase is the dependent variable (y-axis) and the word after this phrase is the independent variable (x-axis). When you make this scatterplot below, be sure to label both axes (plural of axis) with both titles and numerical scales. 8. Judging from this graph, would you say that flower size influences the number of seeds a flower produces? If so, what is the direction of the trend? How does this relate to your original hypothesis stated in question 4? Graphs and Natural Selection Page 4

5 Part 3. Do pollinators cause natural selection on flower characteristics? As we saw in Part 2, bigger flowers have more seeds than smaller flowers. Why might this be? One hypothesis is that the bumblebees that pollinate alpine skypilots are attracted to bigger flowers. If this were the case, bumblebees could cause natural selection on flower size, which would lead to the evolution of larger flowers. 9. Design an experiment to test the hypothesis that bumblebees prefer large flowers, and that this leads to the evolution of large flowers. Describe your experimental design and the results you predict if the hypothesis is correct. Also describe the results you would predict if the hypothesis is false. In your design, be sure to name your dependent and independent variables, and don t forget to include an experimental treatment and a control treatment. Graphs and Natural Selection Page 5

6 The researcher Candace Galen decided to test this hypothesis in the following experiment. At her field site in the natural habitat of alpine skypilots, she allowed bees to pollinate a large sample of flowers. This was her experimental treatment. For her control treatment, she built a big net around a large sample of skypilots so that bees would not be able to pollinate them, and she then pollinated a random subset of these skypilots by hand. She collected the seeds from these two treatments, germinated them in a greenhouse, and then planted them at random locations in their natural habitat. Six years later she measured the flower sizes of all the plants that grew from these seeds. 10. What results would you predict from this experiment if the hypothesis (stated in question 9) is correct? Remember, there are two treatment groups random pollinated (control), and bee pollinated (experimental). Think about how these groups can be compared, and what similarities or differences you would predict. Graphs and Natural Selection Page 6

7 One way that biologists typically represent information concerning the traits of individuals in a population is in a histogram or frequency distribution, which is a kind of bar graph. In a histogram, all of the numbers in a certain range (in this case, flower size) are counted and displayed together. On the x-axis are the bins, or categories, of flower size, and on the y-axis is the number of individuals counted in each flower size category. The basic layout is shown below. Number of individuals Flower size (mm) 11. Draw on the above layout an prediction of what you would expect if the hypothesis (stated in question 9) is true. Each flower size category will have two bars one for the experimental treatment and one for the control treatment. You should shade these two sets of bars in different ways in order to tell the difference between them, and you should include a key that labels the two treatment groups. 12. Now, on the graph layout below, you can plot the real data from Candace Galen s experiment. These are listed in the attached Table 3. Although the axes are already titled, you will need to add a numerical scale to the y-axis. Remember to shade the experimental treatment differently than the control treatment, and to include a key for the two treatment groups. Number of individuals Flower size (mm) Graphs and Natural Selection Page 7

8 13. Also listed on the attached Table 3 are the average, or mean, flower sizes for the experimental and control treatments. On the graph on the previous page, draw a dotted line representing the experimental mean across the x-axis straight up to the top of the graph. Then draw a dashed line representing the control mean in the same way. How do these two averages compare? 12. How do these data bear on the original hypothesis that bumblebees prefer large flowers and that this causes the evolution of larger flowers? Do you think the hypothesis is correct or incorrect? 13. What additional experiments could be done to further our understanding of how bumblebees influence the evolution of flower size in this population of alpine skypilots? Graphs and Natural Selection Page 8

9 Table 1. Maternal flower Offspring flower size (mm) size (mm) Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower

10 Table 2. Flower Size (mm) Number of Seeds Flower Size (mm) Number of Seeds Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower Flower

11 Table 3. Flower Size (mm) Bumblebee treatment Control Treatmen t Mean: