Darwinian Snails Lab free essay sample

Modified by John Braverman from EvoBeaker Introduction The flat periwinkle is a small snail that lives on seaweeds growing on rocky shores in New England. Among the snail’s enemies is the European green crab. As its name suggests, the European green crab is not native to North America. It traveled from Europe early in the 19th century. Before 1900, the green crab did not occur north of Cape Cod, Massachusetts. After the turn of the century, however, the crab expanded its range northward, and is now found as far north as Nova Scotia. The crab’s range expansion introduced periwinkle populations north of Cape Cod to a new predator. Biologist Robin Seeley suspected that New England’s periwinkle populations have evolved due to predation by green crabs. In a museum, Seeley found an 1871 collection of periwinkles from Appledore Island, north of Cape Cod. She compared these old shells to new shells she had gathered herself at the same place. We will write a custom essay sample on Darwinian Snails Lab or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page Seeley measured the thickness of each shell. As the graphs and photos in the illustration (reprinted from Seeley, 1986) show, the snail population on Appledore Island in the early 1980s was, indeed, dramatically different from the snail population that was there in 1871. The snails had, on average, shells that were thicker than those of their ancestors. The 1980s population also showed a somewhat smaller range of variation in shell thickness. The flat periwinkles living on Appledore in the early 1980s were descendants of the snails that were living there in 1871. Therefore, we can describe the change in the population as descent with modification, or evolution. Figure. Results of Robin Seeley’s research. Note the figure on the right is called a histogram. See text for more information. How did this descent with modification, this evolution, happen? The mechanism of evolution is the subject of this lab. You will do experiments on a model population to explore how evolution works. Then you will return to Seeley’s flat periwinkles to see how the model applies to them. Instructions: Follow along this document and do the experiments and exercises. Write your answers on a print-out, type them in this very document, type them in a blank Word document, or do both hardcopy and computer document. The exercises require some screenshots, so you will need at least one computer file for that. Submit computer files through Blackboard. You may work in pairs, but each student must turn in a document. This counts as the second reflection paper. The last two questions should be done individually, as indicated. If you do not have time to complete this lab in class, you may come and use the software in SC 209 at other times. Contact Fr. Braverman if you need access. Exercise 1: A Model of Evolution by Natural Selection [ 1 ] Launch the SIMBIO VIRTUAL LABS program. Select DARWINIAN SNAILS from the EvoBeaker Labs option. [ 2 ] You will see a population of snails scattered around the Rocky Coastline on the left. Take a closer look at these snails by double-clicking on one of the snails. A window will pop up showing you an enlarged view of that snail and the thickness of its shell. Examine at least 9 other snails in this way. [ 1. 2. 1 ] Which shell thicknesses are most common? Which are least common? 5 units is the most common thickness of the shell with 16 snails, while the least common thickness of the shell is 7 units, with 1 snail. The most common thickness seems to pile up in the middle units around 5 while the thinnest shell seems to tail off at the ends at 1 and 8. [ 3 ] Look at the histogram on the right side of the screen. This sorts the snails on the coastline by shell thickness, and shows the number of snails in each category. [ 1. 3. 1 ] Are the proportions of snails with different shell thicknesses in the histogram similar to the sample of snails you examined in Question 2. 1? Yes there are the same proportion of snails in the picture as the histogram. You will now become a European green crab! You will feel especially crabby if you are not getting enough to eat, and the best snacks available on the coastline are these tasty looking snails. All you have to do is crack their shells by pounding on them with your claw. [ 4 ] Before beginning your feast, copy the histogram of shell thicknesses and save it in a text document. To do this, move your mouse to the center of the histogram, right-click (Windows) or Control-click (OSX) and select â€Å"Copy View to Clipboard†. Finally, open a new document in your word processor and use the paste command to paste the histogram into the document. Label this graph Exercise 1: Starting Population so that you remember what the snails looked like when you first got to the Coastline. Exercise 1: Starting Population [ 1. 4. 1 ] First, make a prediction. What do you think will happen to the shell thickness distribution in this population of snails as you eat some of them? I think that we will see the graph becoming skewed to the right as snails with thinner shells are those that will become easier targets and less work to eat. [ 5 ] Begin the simulation by clicking the GO button (the left-most button in the Controls panel). The snails will start to crawl around. The starting population size is 50, as shown by the Current Snail Population item below the Coastline. [ 6 ] Get your claw ready for action by clicking on the CLAW tool (the crab claw button in the Tools panel). You are now officially a European green crab. [ 7 ] Find a snail you want to eat and start clicking on it. When you claw at it enough times, the shell will crack, you’ll eat what’s inside, and the snail will disappear. The CURRENT SNAIL POPULATION will also show that there is one fewer snail on the Coastline. [ 8 ] Notice the CRAB HAPPINESS SCORE below the Coastline. This score will go up every time you eat a snail, but it will go down every time you click on a snail with your claw, because the more effort you expend to get your meal, the ‘crabbier’ you become. The coastline started with 50 snails, and it takes 25 snails to fill you up. Keep eating snails until you have eaten 25 (with 25 remaining), and try to maximize your CRAB HAPPINESS SCORE while doing this. [1. 8. 1 ] Do you notice any differences in the histogram of shell thicknesses now compared to the starting snail population? If so, explain why this change happened. Yes the distribution of shell thickness drastically changed. There were no snails left with a shell thickness of 1 or 2. The distribution because skewed to the right because those with thicker shells survived more easily than those with a thinner shell because the crab needed to put in much more work to eat those with a thicker shell than those with a thinner shell. [ 9 ] You, the crab, go away for a while after your big meal, and the snails have a chance to reproduce. Stop the model by clicking the STOP button (the square button in the Controls panel). Then allow the snails to reproduce by clicking the REPRODUCE button. Each of the surviving snails generates two new snails by cloning, then dies.