Adaptation, interactions between species and their environments, and change over time are fundamental principles in biology. They represent aspects of two of the big ideas in science: evolution and natural selection. To help students understand these ideas, the authors developed the "Guppy Game." In this article, they describe the game and how they used it in an AP biology classroom to model the dynamic processes of population genetics and environmental influence on behaviors. The Guppy Game models the balance between the pressures of natural selection and sexual selection. While playing the game, students learn science content in an interactive, student-driven manner. (Contains 5 figures.)