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
References: Seeley, Robin Hadlock. 1986. Intense natural selection caused a rapid morphological transition in a living marine snail. Proceedings of the National Academy of Sciences, USA 83: 6897-6901. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC386617/ Trussell, Geoffrey C. 1996. Phenotypic plasticity in an intertidal snail: The role of a common crab predator. Evolution 50: 448-45. 2012, SimBiotic Software for Teaching and Research, Inc.