Exploring Evolution in the Periwinkle (Adapted from SimBio Virtual Labs)

Transcription

1 Biology 160 Name: Exploring Evolution in the Periwinkle (Adapted from SimBio Virtual Labs) OBJECTIVES: Apply the principles of natural selection Explore factors that affect selection Generate testable hypotheses about evolution; practice designing experiments with appropriate controls INTRODUCTION The flat periwinkle, shown at right, is a small snail that lives on seaweeds in the rocky intertidal of New England. The European green crab is one of the snail s top predators. As its name implies, the European green crab is not native to New England; it traveled from Europe sometime in the early 19 th century. Before 1900, the green crab was not found north of Cape Cod. Since that time, however, the crab has expanded its range northward, and is now found as far north as Nova Scotia. Thus the periwinkle snails of Cape Cod are now exposed to this predator. Biologist Robin Seeley hypothesized that the New England periwinkle population had evolved due to predation from green crabs. To test this hypothesis, Seeley obtained a museum s 1871 collection of periwinkles from Appledore Island, north of Cape Cod. She compared these old shells to new shells she had gathered at the same place, measuring the thickness of each shell, as shown at right. (In her data, each number represents a single shell without a unit of measure; Seeley plotted her data as a logarithm of thickness, which has been converted here to an integer. Larger numbers indicate a thicker shell.) Seeley went on to continue her studies of the impact of the European green crab on the periwinkle population in both the field and lab. In this lab exercise, you ll explore some of the methods and findings using a computer simulation. PRELAB: Please answer the following questions before your lab. 1. Construct a histogram on the grid provided at right. You can do this by placing an X on the grid at right to represent each shell. Your first histogram should be based on the data Seeley collected between 1982 and The second should be based on the historic data from Was Seeley s prediction correct? In other words, has either the average shell thickness or the range of variation in thickness changed between 1871 and 1982? Use your histograms to explain!

2 DETAILED METHODS Exercise 1: A Model of Evolution by Natural Selection. 1. To begin, launch the SIMBIO VIRTUAL LABS program on the laptop provided. Select DARWINIAN SNAILS from the EvoBeaker labs option. You will see a population of snails scattered around the Rocky Coastline on the left. You can take a closer look at any of these snails by double-clicking on one of the snails. A window will open showing you an enlarged view of the snail and the thickness of its shell. The histogram on the right side of your screen will show you the shell thickness for this population. Before beginning your work, copy the histogram of shell thicknesses by clicking the COPY tool (to the left of the REPRODUCE button). Then click the histogram. You can now paste this histogram in a Word document on your laptop to allow for easy comparison. (If you have trouble with the copy function, you might try using the print screen feature.) 2. You are now ready to begin your experiments. In this first round, you ll be the European green crab. As the crab, you ll crack the periwinkle shells by pounding on them with your claw. Before beginning your hunt, write out a hypothesis and prediction in the space below to explain what you think the impact of this predation will be on the periwinkle population. Hypothesis (your tentative, testable explanation): Prediction (what you ll see or measure in this experiment): 3. Begin the simulation by clicking the GO button (the left-most button in the Controls panel.) Your snails will start to crawl around. Note that your starting population consists of 50 snails. Begin your hunt by clicking on the CLAW tool (the crab claw button in the Tools panel). Find a snail you want to eat and start clicking on it. When you ve clawed it enough times, the shell will crack, you ll consume the snail, and it will disappear from your simulation. Note that your CRAB HAPPINESS SCORE will go up every time you eat a snail, but down every time you click on a snail because of the effort required to crack the shell. Keep eating snails until you have eaten 25, leaving 25 snails remaining in your population. Work to maximize your CRAB HAPPINESS SCORE as you go! 4. Now allow the snail population to reproduce by first clicking the STOP button (the square button in the Controls panel) and then the REPRODUCE button. Each of the surviving snails will now generate two new snails by cloning. Thus each offspring is identical to its parent. 5. We ll now repeat the hunting process, taking a bit of a shortcut this time to relieve you of the tedium of hunting down snails. Add crabs to your rocky coastline animation by clicking on the ADD CRITTER tool in the Tools panel, just to the right of the CLAW button. Hold the mouse down to access the pop-up arrow and select the crab icon. Then click among the snails in the animation. Each click will add a crab. Add 3-5 crabs to your coastline. Now run the simulation (with the GO) button, allowing the crabs to consume about half of the snails. 6. Repeat the reproduction cycle (described in step 4). Do a third, final season of hunting (as described in step 5) and reproduction (as described in step 4). 7. After your three hunting and reproductive seasons, COPY your final histogram and compare it to the original. What changes do you observe in your population? Do these changes support your hypothesis? Explain. 2

3 Exercise 2A. A Model of Evolution by Natural Selection: VARIATION 1. In this exercise, we ll explore the role of variation in a population. Is variance required? What if all the snails started out the same? To run a simulation on a population of snails with no variation in shell thickness, click the Shell thickness is variable checkbox so that it is no longer selected. Then press the RESET button to get a new population of snails. Examine your histogram to get a sense of the variation in your population. 2. Using the simulator tools describe in Exercise 1, repeat your simulation, taking your snail population through three seasons of hunting and reproduction. 3. Compare your final histogram to your original population. Has this population that lacks variation changed? Why or why not? Exercise 2B. A Model of Evolution by Natural Selection: INHERITANCE 1. In exercise 1, the shell thickness of each snail was identical to that of its parent. Thus shell thickness was completely genetically determined. What if shell thickness was not heritable? To explore the impact of heritability of traits, click the Shell thickness is heritable box so that it is no longer selected. (Note that you will also have to restore the variability to your population by reselecting the shell thickness is variable box.) Click the RESET button and then examine your histogram to confirm your settings, making note of the average thickness in your population as well as the variance. 2. Using the simulator tools describe in Exercise 1, repeat your simulation, taking your snail population through three seasons of hunting and reproduction. 3. Compare your final histogram to your original population. Has this population, in which shell thickness is no longer heritable, changed? Why or why not? Exercise 2C. A Model of Evolution by Natural Selection: SELECTION 1. In this scenario, we ll imagine that the predator, our crab, is especially large and can crack shells regardless of their thickness. These crabs thus feed randomly on the population, without a preference for the thinner shelled snails. To explore the impact of this non-selective predator, click the Survival is selective box so that it is no longer selected. (Do, however, make sure that the Shell thickness is variable and Shell thickness is heritable boxes ARE both selected.) As before, examine your histogram to confirm your settings and make note of the average thickness in your population as well as the variance. Be sure to RESET so you begin with a fresh population! 2. Repeat your simulation again, taking your snail population through three seasons of hunting and reproduction. Note that you ll no longer be able to add individual crabs for the hunt. Instead, you ll need to click the Eat Random Snails button and select 25 snails. 3. Compare your final histogram to your original population. Has this population exposed to random predation changed? Why or why not? 3

4 Exercise 3: The Source of Variation Among Individuals In all the experiments you have done so far, your starting population contained individuals of seven different shell thicknesses. In later generations, some of the thicknesses may have disappeared from the population, but no new shell thicknesses appeared. In real populations, where do new variations come from? The answer is mutations. For our present purposes, a mutation is an error during reproduction. That is, while most offspring may resemble their parents, an occasional mutant offspring will not. 1. To see the role of mutation in evolution, select MUTATION S ROLE from the Select an Exercise menu at the top of the screen. 2. The snails in this new stretch of Coastline have shells up to a thickness of only 4, without the 5, 6, and 7- thickness snails on the last area of Coastline. There is now a Reproduce with mutation box in place of the checkboxes in the last screen, and it is currently not checked, so there are no mutations. As before, add three to five hungry crabs to the Coastline. Let your crabs eat three meals of 25 snails each, letting the snails REPRODUCE in between the crabs meals. Is there a limit to how far predatory crabs can drive shell thickness in the snail population? Why or why not? 3. Now click on the checkbox Reproduce with mutation so that it is checked. 4. Let the snails REPRODUCE. Before you start the model again, use the SELECT tool to examine some pairs of snails. Are the children identical to each other (and to the parent)? Are there cases where one of the children is different from the parent? If so, is the change usually towards a thinner shell, a thicker shell, or is it equally likely to be towards either one? (If the answer isn t clear from this reproduction, look at this again the next time your snails reproduce and then come back and answer this question.) 5. Let your crabs eat at least five more meals of 25 snails, letting the snails reproduce in between. (You can speed the process by just clicking the REPRODUCE button when the snail population drops to 25; you don t have to stop the simulation. The snails will reproduce, and the crabs will keep right on eating.) Can you drive the population further towards thicker shells now (with mutations) than you could before (without mutations)? Explain how this can happen, even though there are just as many mutations towards thinner shells as towards thicker shells. 4

5 Exercise 4: Describing natural selection Here is how Charles Darwin thought adaptive evolution happens. Darwin said that: A. if a population contains variation for some character, AND B. if the variation is at least partly heritable (differences among individuals are at least partly due to differences in the genes they have inherited from their parents), AND C. if some variants survive to reproduce at higher rates than others, THEN the distribution of that character in the population will change over time. Condition C, nonrandom survival and reproduction, is called natural selection. The individuals that survive to reproduce are said to be naturally selected. Together, the three conditions and the conclusion are Darwin s Theory of Evolution by Natural Selection. Using the data from your experiments, describe the conditions under which the snail population will evolve toward thicker shells and the conditions under which it won t. Refer back to your notes and the histograms you saved as evidence. After they were born, did the individual snails ever change their shell thickness or color? If the individuals didn t change, how was it possible for the population to change? Did snails grow thicker shells because the snails needed them in order to survive? If not, where did new thicknesses come from? What role did the predators play in causing the population of snails to evolve? Did they create a need for the snails to change a need to which the snails responded? 5

6 Exercise 5. Designing An Experiment to Test Your Hypothesis Your challenge now is to conduct your own, independent experiment on a more realistic model of snail populations. In this scenario you will work with a snail population that has lived in a crab-free environment for several generations, and a snail population that has lived in a crab-infested environment for several generations. Your task is to design an experiment to answer the following questions: Do snails from the crab-infested environment have thicker shells, on average, than the snails from the crab-free environment? If so, do the snails from these two environments differ because one or both have evolved by natural selection, or do they differ simply because snails can smell crabs and grow thicker shells when they need them? With your lab partner, think carefully about how you might structure one or more experiments to address these questions. Be sure your experiments include appropriate controls! The following list introduces a few tools within the software that may help you design your experiment(s). Potentially Useful Tools: A. Snail Tanks- Select MORE SNAILS from the Select an Exercise menu. You will see the screen change to include two stretches of coastline: West, which lacks crabs, and East, which has crabs. You can probably answer your first research question by examining the histograms at the right side of the window. Below the two coastlines, you will also see 4 experimental tanks that don t contain anything at the moment. You should run this model for a few minutes to see that the snails in these populations have many of the features of real snails. They now reproduce sexually and can do so without your help! They vary in shell thickness and experience occasional mutations. You should see juvenile snails, which appear with a blue color to indicate that they are not yet mature. These snails will grow up, mature, and eventually die of old age. Note that you can move snails into any of the experimental tanks. To do this, click on the SELECT tool (the arrow button). Then click and hold down the mouse button on a snail you want to move. Drag the snail into the tank and let go of this mouse. (Note that you can also arrange specific matings by dragging two adults to an experimental tank, making sure they are touching, and running the model!) Note, too, that you can add a crab to the tank by either dragging a crab from one of the coastlines, or addition crabs using the ADD CRITTER tool. B. Gathering Populations of Snails- Depending on your experiment, you may want to have a variety of different shell thicknesses in a tank, or perhaps many snails with the same thickness! You can set up scenarios like this by clicking and dragging snails from the coastline into your tank. Note that you can drag many snails at once by holding down the SHIFT key while you click on one snail after another. All selected snails will get moved as you click on one and drag it to the tank. You can also do this with juvenile snails by selecting on the blue snails. You will have to double-click on these snails to see their snail thickness as you select them. Recording this thickness will allow you to see whether it changes by the time that snail becomes an adult. C. Banded Crabs- As with many predation studies, these snails in these exercises may be sensing and responding to the crabs presence rather than evolving through natural selection. Note that you can also add crabs to your tanks that have had rubber bands placed around their claws. Thus the snails still see and smell these crabs, but the crabs can no longer feed on the snails. These banded crabs are available to you by clicking the pop-up arrow of the ADD CRITTER tool and selecting the crab picture with the blue bands over its claws. Then click once in an enclosure to add the banded crab. Use these tools to design an experiment to answer your question, taking good notes about your methods and results in the space provided! (Note that your Post Lab will ask you to describe and explain this work!) In the space below, generate a hypothesis for your experiment: 6

7 Beginning Your Experiment: Note that you have four tanks available to you on your screen. Two of these tanks should include eastern snails, and two western snails. Why two tanks for each? Remember that every good experiment needs a control! In the space below, record your set-up for each of your tanks: Expose your experimental tanks to three seasons of hunting. In the space below, describe the changes you observe in each of your snail populations (tanks 1-4). Do these results support your hypothesis? Explain. One Final Question: In Exercise 2, you examined the role of variance, inheritance, and selection. Use your results to explain the role of each of these in natural selection. 7