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Michod et al. (2006). Additivity of fitness effects is the simplest assumption possible, and it corresponds to group selection of type 1 in the terminology of Damuth and Heisler (1988) and likely applies early in evolution as groups first start forming. For example, in the volvocine green algae, flagellar action is a main adaptive capacity underlying viability, and the forces contributed by cells to group motility are nearly additive as cells start forming groups (Roff, 2002; Michod et al., 2006). Nevertheless, the assumption of additivity of the contributions of cells to the viability of the group may be relaxed, and the general point underlying the covariance effect still holds (Michod et al., 2006).

As illustrated in Fig. 7.4, if one cell has a high reproductive effort (and hence a low viability and a low cell fitness), this may be compensated for by another cell with high viability (and hence a low fecundity and a low cell fitness) (Michod et al., 2006). Consequently, even though each of these cells by itself would have a low fitness, together they can bring a high fitness to the group, especially under conditions of convexity of the tradeoff. This kind of joint effect, whereby multiple cells may contribute more to the group than could each alone, does not require additivity (Michod et al., 2006). Also, this kind of joint effect would not be possible if group fitness were simply assumed to be the average of the cell fitnesses.

Concerning the transition from single cells to cell groups, the model predicts the following. Single cells must be generalists as far as their

FIGURE 7.4 Two cells jointly specializing in reproduction and viability. Cell i specializes in reproductive effort, bi, with less effort put into vegetative functions, vi. Cell j does the reverse. Alone they would each have low fitness, but together in a group they may have high fitness if the tradeoff between reproduction and viability is convex.

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