Fig. 2.

The process of mutation accumulation in clusters. (a) An illustration of the principle of mutation accumulation. Each of the four mutations, A–D, is acquired step-by-step but a large fitness gain is realized only when all of them are present. As the four mutations would become highly prevalent nearly concurrently, the trajectories of these mutations in figure 1 would appear to overlap strongly. (b) The actual process of mutation accumulation in the evolution of the Delta strain in India. Each row represents a particular nucleotide site and these mutations fall into three groups, labeled E, M, and L (early, middle, and late, respectively).