(A) When a cell (represented by a hiker) is atop or very close to a fitness peak, point mutations with small effects on fitness normally allow the cell to stay in the vicinity of the peak, while large-effect mutations such as aneuploidy typically push the cells towards regions of lower fitness. (B) When a cell is situated in a fitness valley far away from a peak (because of either a physiological growth control mechanism or a strong genetic or environmental perturbation), small-effect mutations rarely bring significant fitness gain in a single step, whereas the large phenotypic leaps brought about by aneuploidy can in some cases bring the cell much closer toward a nearby fitness peak, enabling immediate fitness advantage.