Summary of the dynamics of the simulations of two successive spatial expansions. A first spatial expansion (upper graph) is simulated in a subdivided population of 2,500 demes arranged as a two-dimensional stepping-stone (50 × 50 demes). Each shade of gray denotes the limit of the area of occupied demes at different steps of the progression of the expansion (darker gray shows the first steps while lighter gray shows the more recent steps). At the end of the first spatial expansion a bottleneck is simulated in taking 50 individuals from a peripheral deme from the first grid to start a second spatial expansion (lower graph) in an unoccupied grid. Red crosses show sampling locations at the end of the second spatial expansion.