Fig. 3

(a) Speed of evolution (increase in fitness) with increasing fitness of the parameter set. For all initial starting fitness, the rate of increase in fitness decreases as time (or the current fitness) increases. However, for small initial fitness (0.0826), the first phase of the evolutionary trajectory includes potentiating mutations which facilitate a more rapid increase in fitness. This phase is followed by the second phase where the rate of increase of fitness decreases with time. (b) Speed of evolution is maximum at intermediate fitness values. The trajectory shown by small initial fitness (blue filled square in Fig. 3(a)) can be visualized as movement on a landscape whose shape is characterized by the peak as shown in the lower panel. In such a scenario, the speed of evolution (fitness gain per mutation) is maximal at intermediate fitness values, and is lower for both cases, when fitness is too high or too low