Beyond R0 Maximisation: On Pathogen Evolution and Environmental Dimensions

A widespread tenet is that evolution of pathogens maximises their basic reproduction ratio, R0. The breakdown of this principle is typically discussed as exception. Here, we argue that a radically different stance is needed, based on evolutionarily stable strategy (ESS) arguments that take account of the ‘dimension of the environmental feedback loop’. The R0 maximisation paradigm requires this feedback loop to be one-dimensional, which notably excludes pathogen diversification. By contrast, almost all realistic ecological ingredients of host–pathogen interactions (density-dependent mortality, multiple infections, limited cross-immunity, multiple transmission routes, host heterogeneity, and spatial structure) will lead to multidimensional feedbacks. Contrary to established wisdom, selection in the long run rarely favours parasites that maximise their epidemiological basic reproduction ratio, R0. R0 maximisation only occurs in models with simple forms of environmental feedback. In realistic host–parasite interactions, ecological processes will commonly preclude R0 maximisation. The dimension of the environmental feedback loop here emerges as a unifying concept.