Michod, Richard E., and Denis Roze. “Cooperation and conflict in the evolution of individuality. III. Transitions in the unit of fitness.” Lectures on mathematics in the life sciences (1999): 47-92.
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The evolution of multicellular organisms is the premier example of the integration of lower levels into a single, higher-level individual or unit of fitness. Explaining the transition from single cells to a multicellular organism is a major challenge for evolutionary theory. We provide an explicit genetic framework for understanding this transition in terms of the increase of cooperation among cells withingroups and the regulation of conflict within the cell group—the emerging organism. Cooperation is the fundamental force leading to new levels of organization and selection. While taking fitness away from lower level units (its costs), cooperation increases the fitness of the new higher level unit (its benefits). In this way, cooperation may create new levels of selection and higher levels of fitness. However, the evolution of cooperation sets the stage for conflict, represented here by the increase of deleterious mutants during development. The evolution of a means to regulate this conflict is the first new function at the organism level. The developmental program evolves so as to reduce the opportunity for conflict among cells. An organism is more than a group of cells related by common descent; to exist organisms require adaptations that regulate conflict within. Otherwise, continued improvement of the organism is frustrated by within-organism
variation and change during development. The evolution of modifiers of within-organism
change are a necessary prerequisite to the emergence of individuality and the continued
well being of the organism. Heritability of fitness and individuality at the new level
emerge as a result of the evolution of organismal functions that restrict the opportunity
for conflict within and ensure cooperation among cells. Conflict leads—through the
evolution of developmental adaptations that reduce it—to greater individuality and
harmony for the organism.