THE purpose here is to discuss the limits of theoretical population genetics. This 100-year-old field now sits close to the heart of modern biology. Theoretical population genetics is the framework for studies of human history (REICH et al. 2002) and the foundation for association studies, which aim to map the genes that cause human disease (JORDE 1995). Arguably of more importance, theoretical population genetics underlies our knowledge of within-species variation across the globe and for all kinds of life. In light of its many incarnations and befitting its ties to evolutionary biology, the limits of theoretical population genetics are recognized to be changing over time, with a number …