Schematic illustration of (a) simultaneous and (b) successive range expansion (quarter circles of radial expansions). In panel (a), the green and blue colored areas represent two populations that have the same growth properties. In the inoculation zone (lower left) the two populations are well mixed. During expansion, genetic drift at the expansion front causes the two populations to segregate into sectors, which reduces intermixing and local population diversity. The boundaries are parallel to the direction of expansion because the growth properties of the two populations are identical. Panel (b) represents a successive range expansion of a primary population (blue) and a secondary population (green). The successive expansion could cause a segregation of the primary from the secondary population perpendicular to the direction of expansion, which would lead to two successive expansions and an even further reduction in intermixing and local population diversity.