(survival). As embodied in current theory, tradeoffs between fitness components drive the evolution of diverse life-history traits in extant organisms (Stearns, 1992; Roff, 2002). In the present chapter we are primarily concerned with the cost of reproduction to viability and how this cost scales with colony size. Fitness tradeoffs gain special significance during the transition from unicellular to multicellular life for several related reasons (Michod, 2006; Michod et al., 2006): (i) fitness tradeoffs often create a covariance effect at the group level so that group fitness is augmented beyond the average fitness of component cells (see Eq. 1); (ii) fitness tradeoffs based on preexisting life-history variation provide a basis for the origin of altruistic interactions within the group (see Origin of Reproductive Altruism); and (iii) fitness tradeoffs between survival and reproduction, if of convex curvature, may select for cells specialized for reproductive and survival-related functions of the group (see Cost of Reproduction and Covariance Effect).
How do groups become individuals? Our hypothesis is that fitness tradeoffs drive the transition of a cell group into a multicellular individual through the evolution of cells specialized at reproductive and vegetative functions of the group. We have modeled this hypothesis (Michod, 2005, 2006; Michod et al., 2006) and have tested our models in two ways. We first ask whether a life-history gene present in the unicellular ancestor was coopted to be an altruistic gene in the multicellular Volvox carteri (Fig. 7.1E) (Nedelcu and Michod, 2006). By answering this question we address how an altruistic gene may originate, that is, by cooption of an existing life-history tradeoff gene. Second, we ask why reproductive altruism arises only in the larger members of the volvocine group. Our answer is that the selective pressures leading to reproductive altruism stem from the increasing cost of reproduction with increasing group size (Solari et al., 2006a,b).
Altruism refers to a behavior or interaction that benefits other individuals at a cost to the individual exhibiting the behavior. Altruism is widely appreciated to be the central problem of social evolution. It is also central to the reorganization of fitness during evolutionary transitions, as already mentioned, because altruism trades fitness from the lower level, the costs of altruism, to the higher level, the benefits of altruism.
In the multicellular green alga V. carteri, reproductive altruism is a property of the small flagellated somatic cells. V. carteri consists of ≈2,000 permanently biflagellated somatic cells and up to 16 nonflagellated reproductive cells. Terminal differentiation of somatic cells in V. carteri involves the expression of regA, a master regulatory gene that encodes