(a) Multicellular C. reinhardtii (left) settle faster than a contemporary population undergoing settling selection that remained unicellular (right), forming a pellet after 20 min of settling on the bench (cultures shown are 72 h old). (b) Cells are held in place by a transparent extracellular matrix, indicated by arrows. (c) Motile propagules released from multicellular clusters (phase contrast microscopy); (d) the ancestral unicellular growth form. Note that c and d are phenotypically identical. (e) Cluster formation from a single cell. Cells form clusters by ‘staying together’ after mitotic reproduction, not aggregation of unrelated cells. All scale bars are 25 μm.