Cooperation and conflict in the evolution of individuality. I. Multilevel selection of the organism

This article studies the transition in evolution from cells to multicellular organisms. The issues considered are applicable to all major transitions in the units of evolution that share two themes: the emergence of cooperation and the regulation of conflict among the lower-level units, in this case, cells. Explicit genetic models of mutation and selection both within and between organisms are studied in sexual and asexual haploid and diploid organisms without a germ line. The results may be understood in terms of the differing opportunities for within-and between-organism selection under the different reproductive modes and parameter values. Cooperation among cells increases when the fitness covariance at the level of the organism overcomes within-organism change toward defecting cells. Selection and mutation during development generate significant levels of within-organism variation and lead to significant variation in organism fitness at equilibrium. The levels of cooperativity attained can be low, even with reproduction passing through a single-cell zygote stage and the high kinship that entails. Sex serves to maintain higher levels of cooperation and lower levels of within-organism change. Fixed size may help organisms reduce conflict among cells.