Stephen Stearns, professor of evolutionary biology at Yale, continues his Evolution and Medicine course with a short segment on developmental plasticity and reaction norms. He explains how a single genotype can produce different phenotypes depending on the environment it develops in, and why this matters for defining what counts as a normal or pathological outcome in a patient. Drawing on the companion textbook Evolutionary Medicine, Stearns connects reaction norm curves to clinical reasoning, showing how the same underlying biology can yield healthy or diseased states under different conditions. The segment is part of a numbered lecture series building toward a broader evolutionary framework for medicine, and this installment focuses narrowly on plasticity as a concept doctors need before they can ask what deviates from a norm.