Optimal phenotypic plasticity in a stochastic environment minimises the cost/benefit ratio

This paper addresses the question of optimal phenotypic plasticity as a response to environmental fluctuations while optimising the cost/benefit ratio, where the cost is energetic expense of plasticity, and benefit is fitness. The dispersion matrix Σ of the genes’ response (H=ln|Σ|) is used: (i) in a numerical model as a metric of the phenotypic variance reduction in the course of fitness optimisation, then (ii) in an analytical model, in order to optimise parameters under the constraint of limited energy availability. Results lead to speculate that such optimised organisms should maximise their exergy and thus the direct/indirect work they exert on the habitat. It is shown that the optimal cost/benefit ratio belongs to an interval in which differences between individuals should not substantially modify their fitness. Consequently, even in the case of an ideal population, close to the optimal plasticity, a certain level of genetic diversity should be long conserved, and a part, still to be determined, of intra-populations genetic diversity probably stem from environment fluctuations. Species confronted to monotonous factors should be less plastic than vicariant species experiencing heterogeneous environments. Analogies with the MaxEnt algorithm of Jaynes (1957a,b) are discussed, leading to the conjecture that this method may be applied even in case of multivariate but non multinormal distributions of the responses.