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.
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics