Aim  To integrate ecological fitting, the oscillation hypothesis and the taxon pulse hypothesis into a coherent null model for the evolution of complex host–parasite associations. Location  Global. Methods  This paper reviews and synthesizes literature that focuses on phylogenetic analyses and reciprocal mapping of a model system of hosts and their parasites to determine patterns of host–parasite associations and geographical distributions through time. Results  Host‐switching and geographical dispersal of parasites are common phenomena, occurring on many temporal and spatial scales. Diversification involving both co‐evolution and colonization explains complex host–parasite associations. Across the expanse of Earth history, the major radiations in host–parasite assemblages have been preceded by ecological disruption, ecological breakdown and host‐switching in a context that can be defined by the concept of ecological fitting. This cyclical process sets the stage for co‐diversification during periods of relative stability, punctuated by host‐switching during episodes of regional to global environmental disruption and climatological change. Main conclusions  Most observed host–parasite associations can be explained by an historical interaction between ecological fitting, oscillation (episodes of increasing host range alternating with isolation on particular hosts) and taxon pulses (cyclical episodes of expansion and isolation in geographical range). Major episodes of environmental change appear to be the main drivers for both the persistence and diversification of host–parasite systems, creating opportunities for host‐switching during periods of geographical expansion and allowing for co‐evolution and co‐speciation during periods of geographical isolation.

Global climate change is altering the ecology of infectious agents and driving the emergence of disease in people, domestic animals, and wildlife. We present a novel, empirically based, predictive model for the impact of climate warming on development rates and availability of an important parasitic nematode of muskoxen in the Canadian Arctic, a region that is particularly vulnerable to climate change. Using this model, we show that warming in the Arctic may have already radically altered the transmission dynamics of this parasite, escalating infection pressure for muskoxen, and that this trend is expected to continue. This work establishes a foundation for understanding responses to climate change of other host-parasite systems, in the Arctic and globally.

Parasites are integral components of the biosphere. Host switching correlated with events of episodic climate change is ubiquitous in evolutionary and ecological time. Global climate change produces ecological perturbations, which cause geographical and phenological shifts, and alteration in the dynamics of parasite transmission, increasing the potential for host switching. The intersection of climate change with evolutionary conservative aspects of host specificity and transmission dynamics, called ecological fitting, permits emergence of parasites and diseases without evolutionary changes in their capacity for host utilization.