Figure 5

Two hypotheses about how ruggedness of the adaptive landscape might yield the observed results. Schematic shows two hypotheses that could explain the counterintuitive result in which populations that evolved in the +EQU/-AND environment reached higher fitness in the +EQU environment than did those that evolved in the +EQU environment. (A) Populations evolved to the lower fitness peak in the +EQU environment (upper left) because an adaptive valley prevented them from finding the higher peak. However, that valley did not exist in the +EQU/-AND environment (lower left), allowing selection to drive the populations up the single peak. When organisms that evolved in the +EQU/-AND environment were assayed for their fitness in the +EQU environment, they were on the higher peak. (B) The ancestor was between two adaptive peaks, either of which could be reached from that point. However, evolution in the +EQU environment (upper right) predisposed populations toward the lower peak because its initial ascent was steeper or mutations were more likely to move the populations into its domain of attraction. By contrast, evolution in the +EQU/-AND environment (lower right) predisposed populations to evolve toward the higher peak.