Abstract
Evolutionary transitions in the units of selection require the promotion of cooperation and the regulation of conflict among the lower level units. For multicellular organisms to emerge as a new unit of selection, the selfish tendencies of their component cells had to be controlled. Theoretical results indicate organisms may regulate this internal conflict and competition in several ways: by reducing the somatic mutation rate, by sequestering cells in a germ line and by directly reducing the benefits to cells of defecting.