I bet this is different for Dictyostelium purpureum (a slime mold with kin recognition), because loners that join the stalk would improve the survival of their genes. Whereas this species, Dictyostelium discoideum, will form stalks with unrelated strangers, and thus always faces genetic pressure creating loners and cheaters who don't sacrifice themselves to join the stalk. It just turns out that mutating to not join the stalk mutates something else (they don't know what) that hurts your reproduction more. Source: https://www.discovermagazine.com/planet-earth/us-and-them-am... (Carl Zimmer, 2006)