Evolution of the individual

This article studies the transitions in evolution from single cells to multicellular organisms as a case study in the origin of individuality. TIle issues considered are applicable to all major transitions in the units of selection that involve the emergence of cooperation and the regulation of conflict. Explicit genetic models of mutation and selection both within and between organisms are studied. Cooperation among cells increases when the fitness covariance at the level el the organism overcomes within-organism change toward defection. Selection and mutation during development generate significant levels of within-organism variation and lead to variation in organism fitness at equilibrium. This variation selects for germ-line modifiers and other mediators of within-organism conflict, increasing the heritability of fitness at the organism level. The evolution of those modifiers is the first new function at the emerging organism level a necessary component of the evolution of individuality.