Figure 5. Sequence conservation and epistasis affect fitness landscapes.

(a) Fitness landscapes of IGPS orthologues are significantly correlated (R>0.4) for IGPS positions ('open circle' odd numbered libraries, 'filled circle' even numbered libraries) displaying 40 to 100% conservation in the multiple sequence alignment (red box): the fitness landscapes of highly conserved positions are strongly correlated. At the same time, the wide range of correlations before the ‘cliff' of 40% conservation suggests that sequence conservation is not the only determinant of fitness. (b) Mutations that ‘transform' one IGPS orthologue into another primarily have neutral or beneficial effects (pink histogram) compared to all mutations (grey). Epistatic interactions are responsible for the existence of transformative detrimental mutations (minor peak on the pink histogram around s=−1). The distribution of selection coefficients for the transformative mutations is significantly different that than of the null distribution (**P<0.02).