Stephen Stearns, teaching Yale's Evolution and Medicine course, lays out three ways biologists have historically framed evolutionary change. He contrasts typological thinking, which sorts organisms into fixed categories, with population thinking, which treats variation within a group as the raw material of natural selection, then adds tree thinking, the practice of reading relationships among species as branching lineages rather than a ladder of progress. Stearns explains why the shift from typological to population thinking marked a real conceptual break in biology, and how tree thinking reshapes the way medical researchers interpret traits shared across species. The lecture is part of a numbered series built around the 2015 Sinauer textbook Evolutionary Medicine, and this short segment sets up vocabulary and habits of mind the rest of the course will use when applying evolutionary reasoning to disease and human biology.