Forging an evolutionary individual from separate replicators

Major evolutionary transitions occur when independently replicating particles unite into a higher-level individual. How selection comes to work on particle collectives and generates reliable heredity is unresolved. By packaging red and green plasmids in yeast cells we made plasmid communities units of selection: cell boundaries supplied discreteness, segregation generated variation, and cell division reproduced communities. Communities colouring cells yellow were preferentially propagated. Initially, yellow parents rarely produced yellow offspring, but continued selection aligned replicative fates of the two plasmids favouring recombinants that fused with the endogenous 2-micron element. Chimera stability was secured by mutations in FLP1, which encodes a recombinase that otherwise resolves fused replicons. Reliable collective heredity, here, a derived product of higher-level selection, marked completion of an evolutionary transition in individuality.