Dictyostelium is an amoeba which, when starvation is imminent, aggregates to form fruiting bodies consisting of a stalk of dead cells that supports spores. Since different clones may be involved in such aggregations, cheater strategies may emerge that allocate a smaller fraction of cells to stalk formation, thus gaining a reproductive advantage. We model the evolutionary dynamics under the realistic assumption that the number of clones involved in aggregations follows a random distribution, and show that under these assumptions evolutionary branching into cheater and altruist allocation strategies may occur.