Stephen Stearns, teaching Yale's Evolution and Medicine course built around the Sinauer textbook Evolutionary Medicine, explains why chemotherapy often fails in the long run and what evolutionary theory suggests instead. He frames tumors as populations under selection, where aggressive dosing kills susceptible cells but leaves resistant ones to repopulate the tumor unopposed. The lecture lays out the logic of adaptive chemotherapy, which deliberately holds back on maximum dosing to preserve some drug-sensitive cells that compete with and suppress resistant ones, slowing relapse rather than chasing a cure. Stearns ties the approach back to basic principles of natural selection and resistance evolution covered earlier in the course, treating cancer treatment as an ecological and evolutionary management problem rather than a simple kill-the-tumor exercise. It is a short, dense segment aimed at students already following the course's evolutionary framework.