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Plot the data for the shells collected in the 1980s on the topgrid at the right. If you’re working with a partner, one of youshould read the number while the other places an X on the gridfor each shell. We have put an X on the grid for the firstshell (with a thickness of 14); you add the rest. When you havemore than one shell with the same thickness, stack the X’s ontop of each other.

Next, plot the data for the shells collected in 1871 on the bottom grid at the right.

[ 1.1 ] Which snail population has the larger average shell thickness, the populationfrom 1871, or the population from the 1980s?

[ 1.2 ] Which snail population has the larger range of variation in shell thickness?

[ 1.3 ] Was Seeley’s prediction correct? In other words, is the average shell thickness and/or the range ofvariation in thickness different for the 1980s snail population versus the 1871 population?

[ 1 ]

[ 2.1 ] Which shell thicknesses are most common? Which are least common?

[ 3.1 ] Are the proportions of snails with different shell thicknesses in the histogram similar to the sample of snails youexamined in Question 2.1?

[ 4.1 ] First, make a prediction. What do you think will happen to the shell thickness distribution in this population ofsnails as you eat some of them?

[ 8.1 ] Do you notice any differences in the histogram of shell thicknesses now compared to the starting snail population? Ifso, explain why this change happened.

[ 12.1 ] Compare the starting histogram you saved earlier to this fourth-generation histogram. Has the distribution of shellthicknesses changed as you predicted in Step 4? Explain.

[ 16.1 ] Which snails tended to get eaten? Which snails tended to survive?

[ 17.1 ] How does the evolution of the snail population when predatory crabs are present compare to the evolution of the snailpopulation when you acted as predator?

[ 3.1 ] Is there any variation in shell thickness among the snails now?

[ 4.1 ] Make a prediction: Do you think this population of snails will evolve as predators start eating them? Explain why or whynot.

[ 6.1 ] Was there a change in the distribution of shell thickness among the snails? Explain why this is different from when you or thecrabs were eating snails before. Why does the distribution of shell thickness stay the same now, even though predators were eatingjust as many snails?

[ 9.1 ] Do the children have the same shell thicknesses as the parent (are the snails identical within each pair as before)?

[ 9.2 ] Do you think that the distribution of shell thickness in the population will change over several snail generations aspredators start eating snails? Why or why not?

[ 13.1 ] Why are the starting and ending shell thickness distributions similar to each other in shape, even after your crabs had threemeals of the thinnest-shelled snails?

[ 17.1 ] Do you think the distribution of shell thickness in the population will change over time as your super-crab startshaving meals? Why or why not?

[ 20.1 ] Was there a change in the distribution of shell thickness among the snail population? Is this what you expected? Explain.

[ 23.1 ] Are there changes in the shell thickness distribution from the initial pattern? Are these changes consistent among allthree of your trials?

[ 23.2 ] Try to explain any changes you observed in the populations. Consider whether the changes happen for the samereason as the changes in distribution in Exercise 1 of this lab. Could random chance result in any of the types ofchanges you’ve seen?

[ 1.1 ] Using the data from your experiments, describe the conditions under which the snail population will evolve towardthicker shells and the conditions under which it won’t. Refer back to your notes and the histograms you saved as evidence.

[ 2.1 ] Is there a limit to how far predatory crabs can drive shell thickness in the snail population? Why or why not?

[ 4.1 ] Are the children identical to each other (and to the parent)? Are there cases where one of the children is differentfrom the parent? If so, is the change usually towards a thinner shell, a thicker shell, or is it equally likely to be towardseither one?

[ 5.1 ] Can you drive the population further towards thicker shells now (with mutations) than you could before (withoutmutations)? Explain how this can happen, even though there are just as many mutations towards thinnershells as towards thicker shells.

[ 1.1 ] After they were born, did the individual snails ever change their shell thickness or color? If the individualsdidn’t change, how was it possible for the population to change?

[ 1.2 ] Did snails grow thicker shells because the snails needed them in order to survive? If not, where did new thicknessescome from?

[ 1.3 ] What role did the predators play in causing the population of snails to evolve? Did they create a need for the snailsto change – a need to which the snails responded? Or did the predators simply determine which snails survived toreproduce and which didn’t?

[ 1.1 ] What evidence, if any, does Seeley have that the flat periwinkles of Appledore island vary in the thickness of theirshells?

[ 1.2 ] What evidence, if any, does Seeley have that snails with thick shells are more likely to survive than snails with thinshells? (Think carefully about this.)

[ 1.3 ] What evidence, if any, does Seeley have that shell thickness is heritable? (Again, think carefully about this.)

[ 2.1 ] For which of the three conditions is the evidence you have seen the weakest? Explain.