This important new volume in the Oxford Series in Ecology and Evolution examines the mechanism and action of natural selection in evolution. Williams offers his own synthesis of modern evolutionary theory - including discussions of the gene as the unit of selection, clade selection and macroevolution, diversity within and among populations, stasis, and other timely and provocative issues central to the study of evolution. Williams' preeminent position in the field ensures immediate and widespread interest in the book among evolutionary biologists, genericists, and their graduate students.