This textbook provides an account of the basic principles governing the central concept of evolutionary theory: selection. It describes how the experimental study of evolution has elucidated the process of selection and how it drives evolutionary change. The author has written a simple text that avoids mathematical arguments or technical details, while giving an introduction to the field. The book is organized as a series of short sections, each designed to make a particular point, and illustrated whenever possible by experimental results. The volume's logical organization should make it readily accessible to all undergraduate and graduate students taking courses in evolution, evolutionary or ecological genetics, or any allied field in biology.