Fig. 7. Some evolutionary trajectories of a peptide sequence undergoing mutation. Mutations in the peptide sequence can cause an increase in fitness (e.g. enzyme activity, green), loss of fitness (salmon pink) or no change in fitness (grey). Typically, improved fitness mutations are selected for and subjected to further modification and selection. Neutral mutations keep sequences ‘alive’ in the series, and these can often be required for further improvements in fitness, as shown in steps 2 and 3 of this trajectory.