Slugs of D. discoideum and D. purpureum show partial sorting due to development difference. Although cells of both species aggregate together (A), the majority of cells of D. purpureum forms a slug first and migrates away, taking some D. discoideum cells (indicated by the green) with it (B). The majority of the cells of D. discoideum stays in the mound to later form a slug and migrate (C).
Jack et al. BMC Evolutionary Biology 2008 8:293 doi:10.1186/1471-2148-8-293