The biological explanation of the origin of adaptation replaces the idea of a Designer, and is central to post-Darwinian evolutionary biology. ‘Adaptation and natural selection’ describes the modern theory of adaptation and its biological causes and basis. This is based on the theory of natural selection. Adaptations are recognizable in many details of the lives of animals, plants, and microbes, at every level, down to the cellular machinery and its controls. The topics of artificial selection, heritable variation and its varying kinds, natural selection and fitness, adaptations and evolutionary history, detecting natural selection, and sexual selection are considered in turn.