Performing multiple rounds of directed evolution is useful not only because a new library of mutants is created in each round but also because each new library uses better mutants as templates. The experiment is analogous to climbing a hill on a 'fitness landscape' where elevation represents the desired property. The goal is to reach the summit, which represents the best achievable mutant. Each round of selection samples mutants on all sides of the starting template (1) and selects the mutant with the highest elevation, thereby climbing the hill. This is repeated until a local summit is reached (2).