Darwinian Snails Lab

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Darwinian Snails Lab

BIO 360 – God and Evolution – Fall 2012

Modified by John Braverman from EvoBeaker


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. 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.


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...
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