Stephen Stearns, professor of evolutionary biology at Yale, opens his Evolution and Medicine course with the mechanics of natural selection itself. Paired with the 2015 Sinauer textbook Evolutionary Medicine, this segment lays out the logic Darwin built his theory on: variation, heritability, and differential survival and reproduction, and why these three conditions are enough to produce adaptation without any designer behind it. Stearns frames selection as a statistical process acting on populations rather than a force acting on individuals, setting up the vocabulary the rest of the course will use to explain why bodies get sick, age, and fail in the particular ways they do. Short and dense, it functions as the conceptual foundation for the medical applications that follow in later lectures rather than a standalone survey of evolutionary theory.