On the reorganization of fitness during evolutionary transitions in individuality

The basic problem in an evolutionary transition is to understandhow a group of individuals becomes a new kind of individual,possessing the property of heritable variation in fitness atthe new level of organization. During an evolutionary transition,for example, from single cells to multicellular organisms, thenew higher-level evolutionary unit (multicellular organism)gains its emergent properties by virtue of the interactionsamong lower-level units (cells). We see the formation of cooperativeinteractions among lower-level units as a necessary step inevolutionary transitions; only cooperation transfers fitnessfrom lower levels (costs to group members) to higher levels(benefits to the group). As cooperation creates new levels offitness, it creates the opportunity for conflict between levelsas deleterious mutants arise and spread within the group. Fundamentalto the emergence of a new higher-level unit is the mediationof conflict among lower-level units in favor of the higher-levelunit. The acquisition of heritable variation in fitness at thenew level, via conflict mediation, requires the reorganizationof the basic components of fitness (survival and reproduction)and life-properties (such as immortality and totipotency) aswell as the co-option of lower-level processes for new functionsat the higher level. The way in which the conflicts associatedwith the transition in individuality have been mediated, andfitness and general life-traits have been re-organized, caninfluence the potential for further evolution (i.e., evolvability)of the newly emerged evolutionary individual. We use the volvocaleangreen algal group as a model-system to understand evolutionarytransitions in individuality and to apply and test the theoreticalprinciples presented above. Lastly, we discuss how the differentnotions of individuality stem from the basic properties of fitnessin a multilevel selection context.