Figure 2.

Two examples of empirical biallelic fitness landscapes on four loci. Arrows are directed from lower fitness genotypes to higher and fitness optima are circled. Examples of adaptive dynamics are highlighted with thick black arrows. (a) The E. coli β-lactamase data of Chou et al. (2011) predict a smooth landscape with no sign epistasis. Thus, it contains a single optimum (1111). (b) Prediction based on P. falciparum dihydrofolate reductase growth rate data in the absence of pyrimethamine (Lozovsky et al. 2009). It has two peaks (0011 and 1111) and both single sign (an example in yellow; ) and reciprocal sign epistasis (example in red; ). Based on Figure 1 in Szendro et al. (2013).