This article in NSTA’s September 2013 edition of The Science Teacher describes a unit on natural selection that encompasses eleven 55 minute class periods. A series of activities asks students to explain and make sense of a variety of phenomena, developing their understanding of natural selection over the course of the unit. As a result of engaging in carefully sequenced classroom activities, students make a number of observations and inferences, adding pieces to the natural selection model they are constructing individually and as a class. This unit uses the constructing natural selection model as an anchor for further instruction. Examples of the activities used in the unit include a fish simulation, a sunflower seed activity, and a “worm-eater” game which simulates natural selection, all of which have sense-making discussion components. The unit assessment task asks students to apply what they have learned to explain how the carapaces of different populations of tortoises, found on various islands of the Galapagos islands, changed over time.