EvoDots.Tutorial.fm

EvoDots TutorialWhat makes populationsevolve?The flat periwinkle is a small (~ 1 cm long) marine snail that lives in the intertidal zone in New England. Among this snail’s predators is the European green crab. As it’s name suggests, the European green crab is not native to North America. It is an invasive species that reached the East Coast from Europe early in the 19th century. Prior 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 exposed periwinkle populations north of Cape Cod to a new agent of natural selection.Software for Evolutionary Analysis © 2002 Jon C. Herron 1 2 What makes populations evolve?Biologist Robin Seeley investigated whether flat periwinkle popula-tions evolved in response to predation by green crabs. Seeley found, in a museum, samples of pre-1900 periwinkle shells collected at Apple-dore Island, north of Cape Cod. She compared these old shells to shells she collected herself at the same place. Seeley measured the spire height and thickness of each shell. Seeley’s data are depicted in the graphs below. As the graphs–and the photos– show, the crab popu-lation on Appledore island in the early 1980s was, indeed, dramatically different than it was in 1871. The snails had, on average, shells that were both thicker and flatter than those of their ...