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components of fitness regardless of the curvature of the tradeoff curve. However, stability of the single-cell life habit to groups requires a concave tradeoff in unicells. In cell groups, if the tradeoff remains concave, cells will not specialize, and there will be no variance to speak of and no covariance effect. However, if the tradeoff becomes convex, as a result of, for example, an increasing cost of reproduction, then cells should start specializing in viability and fecundity leading to an increased group fitness according to the covariance effect.

HOW DOES A GROUP BECOME AN INDIVIDUAL?

Let us return to the basic question asked at the beginning of the chapter: How is it that a group becomes an individual? In answering this question we assume that there is a selective benefit for forming groups and for increasing group size. We also assume there is a means of forming groups, such as by cells sticking together after cell division. According to our hypothesis, as colonies increase in size, the costs of reproduction increase and the curvature of the tradeoff between reproduction and viability goes from concave to convex. This convexity of the tradeoff curve selects for specialization in reproductive and vegetative viability-enhancing functions (germ soma specialization). As cells specialize in these essential fitness components, the fitness of the cells declines while the fitness of the group increases. The covariance effect further enhances the fitness of the group. As a result of the specialization of the cells, fitness is transferred from the cell to group level and the group becomes indivisible and an individual.

Underlying this process is high kinship among the cells, which is fundamental to, but not sufficient for, the emergence of individuality (as the volvocine algae teach us). The evolution of altruism within groups trades fitness from the lower level to the higher level, and the evolution of conflict mediation further enhances cooperation while restricting the opportunity for defecting mutants. How does a gene become altruistic? The hypothesis we have tested in the volvocine algae is that life-history genes in unicells may be coopted for reproductive altruism in the group. What are the selective factors involved, and, in particular, why doesn’t altruism originate in the smaller-sized groups? The hypothesis we have tested is that tradeoffs between reproduction and survival become increasingly convex with increasing size selecting for reproductive altruism, that is, soma. In the case of the volvocine algae, soma benefits the group both by enhancing motility and by mixing the surrounding medium allowing for more effective transport of nutrients and waste than would be possible by diffusion alone (Solari et al., 2006a; Short et al., 2006).



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