Fluctuation in the physical environment as a mechanism for reinforcing evolutionary transitions.

We hypothesize a mechanism for reinforcing transitions between levels of selection, involving physiological homeostasis and amplification of variation in the physical environment. Groups experience a stronger selection pressure than individuals for homeostasis with respect to reproductively limiting variables, because their greater longevity exposes them more often to suboptimal physical conditions, and greater physical size means they encompass a larger fraction of any resource/nutrient gradient. Groups achieve homeostasis by differentiation into microcosms with specialist functions, e.g. cell types. Such differentiation is more limited in individuals due to their smaller size and shorter lifespan. Hence tolerance of fluctuation in certain physical variables is proposed to be weaker in individuals than in groups. We show that a trait providing increased tolerance (alpha) to fluctuation (V-V(opt)) in a limiting abiotic variable (V), at relative fitness cost (C), can increase from rarity if the condition alpha.mid R:V-V(opt)|>C is met. Groups also sequester larger absolute quantities of resource than individuals, and group death is less frequent, hence the population dynamics of groups cause resource/nutrient availability to fluctuate with greater amplitude than that of individuals. Increasing the amplitude of fluctuation in a reproductively limiting environmental variable is proposed as a mechanism by which a group can limit reproduction of parasitic "cheat" individuals. Enhancing physical fluctuation is frequency dependent, hence only an increase in tolerance to fluctuation can explain the group's increase from rarity. However, once groups reach intermediate frequencies, a positive feedback process can be initiated in which a differentiated group enhances physical fluctuation beyond the tolerance of any "cheat", and in so doing enhances the selection pressure it experiences for homeostasis. This may help explain the persistence of transitions in individuality, and the coincidence of some such transitions with periods of change and oscillation in global scale environmental variables.