image: Populations adapt to their environment by accumulating new mutations that increase their fitness. This is often thought of as an ascent up a fitness landscape, analogous to climbing a mountain. But very little is known about the topography of such landscapes in nature, which has made it hard to predict how a population would move in this landscape and how fast it would adapt. Our work provides a glimpse into the shape of a fitness landscape for populations of lab yeast and shows that adaptation can in some cases be predictable. The picture of adaptation that we find is surprisingly simple. Each of our yeast populations adapts by taking one of an enormous multitude of diverse mutational trajectories towards higher fitness. Predicting which trajectory a population will take is impossible. Yet, despite a great deal of randomness in which mutations populations get, they adapt at highly predictable rates and eventually all converge to the same fitness. This is illustrated in the figure. We generated several "founder" yeast strains (three balls on the sides of the well) from a common ancestral strain represented by a ball at the bottom of the well. We then allowed these founders to adapt further by acquiring additional mutations (arrows). As further mutations occurred, populations diverged in the genetic level moving farther and farther away from the common ancestor and from each other in "genotype space." At the same time, initially less fit populations caught up in fitness to more fit ones, and we observed that fitness trajectories of all populations converged, despite increasing genetic divergence. We found that this striking "catching up" phenomenon was caused by a peculiar pattern of genetic interactions between individual mutations called "diminishing returns epistasis." Consider the "orange" mutation shown here. Whenever this mutation occurs in a low-fitness strain near the bottom of the well, it confers a big fitness increase, but when the same mutation occurs in a high-fitness strain near the top of the well it confers only a small increase. Surprisingly, the fitness increment provided by each mutation depends only on how far up the well it happens (i.e. on the fitness of the strain it occurs in), but not on the specific location of that strain in the genotype space, leading to a well-like shape of the fitness landscape. This image relates to a paper that appeared in the 27 June, 2014, issue of Science, published by AAAS. The paper, Sergey Kryazhimskiy at the Harvard University in Cambridge, Mass., and colleagues was titled, "Global epistasis makes adaptation predictable despite sequence-level stochasticity." view more
Credit: [Image courtesy of Sergey Kryazhimskiy]