This is a graphing activity that demonstrates evolution in action. The case roughly follows the work of Robin Seeley - a biologist who collected flat periwinkle snails at a beach in New England and compared their shell thicknesses to museum specimens collected at the same beach about 100 years prior. The difference is that a snail predator was inadvertently introduced shortly after the initial collection. So Seeley had a snapshot of shell thicknesses before and after the predator was introduced.
Students graph data of snail shell thicknesses and height before and after the predator was introduced. The results indicate that directional selection occurred. Analysis questions get students to consider the forces that may be driving snail thicknesses, and to predict how these changes may also be affecting the evolution of the crab predators.