The book’s traditional emphasis on scientific reasoning has been maintained and enhanced. For example, every chapter begins with a one-page case study, typically including both a photo and dataset, that presents a research question and a study that answers it. These cases serve to introduce the kinds of problems and research programs that will be covered in the chapter. Streamlined and updated content in every chapter reflects key changes in the field and incorporates new research and examples. A few key highlights are listed below. Heavily revised Chapter 1 (Understanding HIV) includes updated statistics on the status of the pandemic, newer thinking on how HIV causes AIDS, new data on the origin of HIV, and new ideas and evidence on why HIV is lethal. Reorganized Chapter 2 (Evidence for Evolution) features sections on evidence for microevolution, speciation, macroevolution, and common ancestry. Completely revised Chapter 4 (Estimating Evolutionary Trees) provides an improved introduction to tree thinking and includes more detailed explanations of parsimony, maximum likelihood, and Bayesian phylogeny inference. Completely revised Chapter 15 (Phylogenomics and Molecular Adaptation) includes new sections on the evolution of genome architecture, the evolution of mutation rates, and gene families. Coverage of mobile genetic elements and the molecular basis of adaptation have been updated. Heavily revised Chapter 16 (Speciation) includes new sections on mechanisms of divergence, hybridization and gene flow, and what drives diversification. The chapter includes new examples illustrating the application of species concepts as well as new examples of mechanisms of isolation. Completely revised Chapter 19 (Evolution and Development) includes revised coverage of Hox genes, deep homology, developmental constraints and trade-offs, and the evolution of novel traits. The art and design program has been updated to reflect an increasing emphasis on evolutionary genetics and on macroevolution. Computing Consequences boxes offer mathematical elaborations of quantitative topics. An updated media program features chapter quizzes, simulations, activities, and case studies.