Stephen Stearns, teaching Yale's Evolution and Medicine course, explains how phylogenetic methods illuminate cancer as an evolutionary process within the body. Drawing on the textbook Evolutionary Medicine, this segment treats tumor cells as populations subject to mutation and selection, tracing how techniques borrowed from studying species relationships can reconstruct the history of a cancer's clonal lineages. Stearns discusses what this evolutionary framing reveals about tumor heterogeneity and why cancers that look similar under a microscope can have very different evolutionary trajectories. The lecture is a short module within a longer course sequence, aimed at students who already have some grounding in evolutionary biology, and it connects abstract phylogenetic concepts to a concrete medical problem: understanding why cancers evolve resistance and how that evolution might be tracked or anticipated. It is a compact, focused piece rather than a full standalone treatment of the subject.