is augmented over the average fitness of member cells. This chapter takes for granted the advantages of larger group size and considers instead the associated costs of groups and how these costs may be ameliorated so as to enhance the benefits of group living. We wish to understand how groups become individuals. The central idea motivating our hypothesis is that by coping with the fitness tradeoffs and the challenges of group living, the group evolves into a new evolutionary individual.
There are several hypotheses for the evolution of cell specialization. The first involves the evolution of cooperation (versus defection). To cooperate, cells presumably must specialize at particular behaviors and functions. The evolution of costly forms of cooperation, altruism, is fundamental to evolutionary transitions, because altruism exports fitness from a lower level (the costs of altruism) to a higher level (the benefits of altruism). The evolution of cooperation sets the stage for defection, and this leads to a second kind of hypothesis for the evolution of specialized cells involving conflict mediation. If the opportunities for defectors can be mediated, enhanced cooperativity of cells will result in more harmonious functioning of the group. A variety of features of multicellular organisms can be understood as “conflict mediators,” that is, adaptations to reduce conflict and increase cooperation among cells (Michod, 2003): high kinship as a result of development from a single cell, lowered mutation rate as a result of a nucleus, self-policing of selfish cells by the immune system, parental control of cell phenotype, programmed cell death of cells depending on signals received by neighboring cells, determinate body size, and early germ soma separation. These different kinds of conflict mediators require different specialized cell types. The third hypothesis for specialization involves the advantages of division of labor and the synergism that may result when cells specialize in complementary behaviors and functions. The most basic division of labor in organisms is between reproductive and vegetative or survival-enhancing functions.
This chapter is primarily concerned with the division of labor and cooperation hypotheses. As a model system, we are considering volvocine algae cell groups that are of high kinship because they are formed clonally from a single cell. Hence, the opportunity for conflict should be low in these groups. Nevertheless, the opportunity for conflict can increase with the number of cell divisions and can depend on the type of development (e.g., rapid cell divisions, as in some volvocine algae, might not allow enough time for DNA repair). For these reasons, the conflict mediation hypothesis may help explain the early sequestration of the germ line in some volvocine lineages (Michod et al., 2003).
Evolutionary individuals must have heritable variation in fitness-related traits. The fitness of any evolutionary unit can be understood in terms of its two basic components: fecundity (reproduction) and viability