Competition between genetically identical organisms is considered insignificant in evolutionary theory because it is presumed to have little selective consequence. We argue that competition between genetically identical cells could improve the fitness of a multicellular organism by directing fitter cells to the germ line or by eliminating unfit cells, and that cell-competition mechanisms have been conserved in multicellular organisms. We propose that competition between genetically identical or highly similar units could have similar selective advantages at higher organizational levels, such as societies.