Hopi Hoekstra, Harvard's Alexander Agassiz Professor of Zoology, argues that modern genetics can finally answer a question Darwin could only guess at: which specific pieces of DNA does natural selection actually act on? She builds her case around a single mutation in wild mice, tracing how one change in a pigment gene can shift a mouse's coat color enough to mean the difference between blending into sand-colored soil or standing out to a hawk overhead. The talk, delivered in Harvard's Science Center D, walks through how her lab pinpointed that mutation and connected it directly to survival rates in the field rather than just in the lab. It is a narrow case study used to make a broader point about evolution: that natural selection's work can now be read at the level of individual genetic letters, not just observed in populations over generations. The lecture stays close to her own mouse research rather than surveying the whole field of DNA science.