This HHMI Biointeractive activity is designed to supplement/support the film, The Origin of Species: Lizards in an Evolutionary Tree. The resources is based on real measurements from a year-long field study on predation, in which Dr. Jonathan Losos and colleagues introduced a large predator lizard to small islands that were inhabited by Anolis sagrei. The activity is written in a case-study format, in which students are asked to formulate a hypothesis and analyze a set of sample research data from actual field experiments designed to test this hypothesis. The activity illustrates the role of predation as an agent of natural selection. It emphasizes that strong selective pressure can have measurable effects on trait variations in a population within a short time. It also shows that natural selection can change direction in just a single generation.