In 1859 Darwin described a deceptively simple mechanism that he called "natural selection," a combination of variation, inheritance, and reproductive success. He argued that this mechanism was the key to explaining the most puzzling features of the natural world. The exact nature of the Darwinian process has been controversial ever since. Draws on new developments in biology, philosophy of science, and other fields to give a new analysis and extension of Darwin's idea. The central concept used is that of a "Darwinian population," a collection of things with the capacity to undergo change by natural selection. From this starting point, new analyses of the role of genes in evolution, the application of Darwinian ideas to cultural change, and "evolutionary transitions" that produce complex organisms and societies are developed..