Group Size Optimality and Stability
The group size that maximizes the average fitness of group members; the group size which, if prevalent in a resident population, cannot be evolutionarily invaded by any slightly different group size.
Adaptive explanations for group living have suggested that individuals choose to live in groups of optimal size where the inclusive fitness of each group member is maximized (Caraco and Wolf 1975; Pulliam and Caraco 1984; Rodman 1981). Yet Sibly (1983) pointed out that the optimal group size may not be stable; the stable group size to be observed in nature will frequently exceed the optimum. This happens because a solitary outsider will do better by joining a group of optimal size rather than by remaining alone, even though the average fitness of group members is reduced by the addition of a newcomer (Clark and Mangel 1984; Sibly 1983). The size of a group is expected to increase until group insiders do not fare any better...
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