John Avise is one of the most distinguished evolutionary biologists of our time. His groundbreaking work with mitochondrial DNA created the entire discipline of phylogeography and his work on the Pleistocene refugia hypothesis redirected scientific thinking about patterns of distribution. Spanning a remarkable thirty-five-year career, the essays gathered here were rewritten from his previously published articles and represent the first single-volume collection of Avise's work. Moving through various questions in evolutionary biology, these eclectic essays reveal Avise's unique perspectives on major topics in the field. From how to define a species to the folly of faulty applications of cladistics to connections between conservation and evolutionary biology, On Evolution takes the reader on a personal journey into the mind of one of the world's leading evolutionists.