EVOLUTION OF THE INDIVIDUAL

This article studies the transition in evolution from single cells to multicellular organisms as a case study in the origin of individuality . The 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 select ion both within and between organisms are studied. Cooperation among c ells increases when the fitness covariance at the level of the organis m overcomes within-organism change toward defection. Selection and mut ation during development generate significant levels of within-organis m 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 th e organism level. The evolution of these modifiers is the first new fu nction at the emerging organism level and a necessary component of the evolution of individuality.