Measuring ruggedness in fitness landscapes

Abstract

How important are interactions among mutations for adaptation? Obviously, no gene functions in isolation, but it is possible that assuming that mutations have independent effects could still give a good prediction for how adaptation proceeds. In PNAS, Nahum et al. (1) use an elegant combination of simulations and experiments with Escherichia coli to show that even in adaptation over the course of a few weeks involving only a handful of mutations, interactions among those mutations can have very large effects. The authors detect these effects using a clever indirect method based on the effect of spatial mixing on the evolution of their experimental populations. Interactions among mutations are typically visualized using the “fitness landscape” metaphor introduced by Sewall Wright (2), which analogizes contours of organismal fitness with the contours of physical topography. Most commonly, the horizontal axes of the landscape represent the space of possible genotypes with the height of the landscape representing the fitness of the corresponding genotype (3, 4) (Fig. 1). The ruggedness of the fitness landscape is a measure of the prevalence of fitness interactions among genes: in smooth landscapes with a single peak, each mutation has a fixed effect on fitness (Fig. 1 A ), whereas in rugged landscapes with multiple peaks (Fig. 1 B ), the effect of each mutation can depend on the other mutations an individual has. Fig. 1. Fitness landscapes. The horizontal axes represent the space of different combinations of genotypes, and the vertical axis is individual fitness as a function of genotype. ( A ) Smooth fitness landscape with a single peak and no fitness interactions among genes. Different evolutionary trajectories lead to the same peak. ( B ) Rugged fitness landscape with multiple peaks and pervasive interactions among genes. Different evolutionary trajectories can reach different peaks even from the … [↵][1]1To whom correspondence should be addressed. Email: jvancleve{at}uky.edu. [1]: #xref-corresp-1-1