Figure 1. Schematic of evolutionary dynamics with fluctuating selection.

Cells within a population can mutate between two states, represented as green (adapted to environment 1) and red (adapted to environment 2). Population bottlenecks occur during the environmental changes, and only a fraction of cells (indicated as dashed squares) are passaged to the next environment. In environment 2, two non-adaptive alleles, differing in origin, coexist: the green circles represent a residual subpopulation which originated from the previous episode, while the green triangles represent novel mutants derived from red individuals. The lineage view shows the phenotypic state of each ancestral cell along a lineage of a single surviving cell at the end of the experiment. Depending on the composition of the bottleneck, the population adapts by the derived allele mechanism (A) or the ancestral allele mechanism (B). Note that in (B), during the expansion of the red phenotype in environment 2, the ancestral cell along the lineage is non-adaptive (green) hence it does not divide, as indicated by a single circle on the lineage in environment 2.