The intrinsic conformational flexibility of proteins creates structural heterogeneity, giving rise to conformational ensembles within the energy landscape. When conformational ensembles harbor distinct functional sub-states, mutations can reshape the conformational landscape, thereby altering the distribution of functional sub-states and driving the evolution of novel functions. In this review, we provide a conceptual framework that elucidates the importance of functional sub-states and how evolution can select them. We highlight key studies that have uncovered functional sub-states and discuss recent insights into the transitions of functional sub-states during evolutionary trajectories. Finally, we outline critical techniques for studying functional sub-states, address the challenges faced in analyzing these sub-states, and explore future advancements in the field of protein evolution.