On unity in organisms and its evolution

We interpret unity in an organism as coordination that promotes economy, flexibility, and robustness, in a combination that aids persistence in a niche. We survey aspects of coordination, at the molecular level and in the physiology and development of cells and multicellular organisms. Combinations of economy, flexibility, and robustness can be used in a top-down strategy for designing networks of modules and regulation that produce appropriate coordination. Selection for these combinations has driven the evolution of modules through competitive optimization.