Climate change and emerging infectious diseases: Evolutionary complexity in action

Abstract More than a century of coevolutionary thought failed to anticipate the current crisis of emerging infectious diseases. This produced the Parasite Paradox : parasites are resource specialists with restricted host ranges, and yet shifts onto relatively unrelated hosts are common in the diversification of parasite lineages and directly observable in ecological time. An emergent framework, called the Stockholm Paradigm , resolves the paradox: ecological fitting - phenotypic flexibility and phylogenetic conservatism in traits related to resource use, most notably host preference - provides substantial opportunities for rapid host switching in changing environments, in the absence of the evolution of novel host-utilization capabilities. Host shifts via ecological fitting provide the fuel for the expansion phase of the recently proposed oscillation hypothesis of host range and speciation, and, more generally, the generation of novel combinations of interacting species within the geographic mosaic theory of coevolution. Large-scale environmental perturbations associated with climate change catalyze new oscillation cycles. The Stockholm Paradigm integrates emergent (Darwin's nature of the organism) and systemic (Darwin's nature of the conditions) properties characteristic of complex systems dynamics. The crisis of emerging diseases will continue so long as global climate change occurs.