The Nature of Selection presents a powerful analysis of the evolutionary concepts of natural selection, fitness, and adaptation. The book clarifies controversial issues concerning altruism, group selection, and the idea that organisms are survival machines built for the good of the genes that inhabit them. As the book unfolds, it provides a straightforward and self-contained introduction to philosophical and biological problems in evolutionary theory. In the first part, "Evolutionary Theory as a Theory of Forces," Sober offers an illuminating characterization of the structure of evolutionary theory. Besides laying to rest the spurious charge that theory is vacuous and unscientific, Sober describes the role of chance in evolution and pinpoints the characteristic structure of evolutionary explanations. The book"s second part, "The Group Above and the Gene Below: The Units of Selection Controversy," thoroughly explores the problem of the units of selection, superseding the author"s earlier essays, which are widely regarded as the best available treatment of this problem.