Wednesday, October 1, 2008

Macroevolutionary mosaics – revealing the history for complex host–parasite systems

An emerging synthesis linking biogeography, ecology and coevolution provides a new framework to explore the structure and his-tory of intricate biological associations such as those represented by host and parasite systems. A general model, established by Dr. Eric Hoberg from the US National Parasite Collection, USDA and Dr. Daniel Brooks from the University of Toronto, for the evolution of parasite biotas emerges from empirical evidence describing a complex mosaic in which host switching and geographic colonization have served as determinants of diversity.
Complex assemblages of hosts and parasites are explained through coevolution and colonization and by integrating aspects of three hypotheses – ecological fitting, oscillation (episodes of increasing host range alternating with isolation on particular hosts) and taxon pulses (cyclical episodes of geographical expansion of ancestral species followed by isolation of populations producing descendant species, occasionally accompanied by ecological divergence, setting the stage for the next episode of expansion) – to establish a context for host and geographical distri-bution across varying temporal and spatial scales. Concepts are examined and framed by equating colonization with a breakdown in mechanisms for ecological isolation such as those driven by periodic global extinction, or episodic and cyclical climate fluctuation and environmental perturbation that have characterized marine and terrestrial systems in evolutionary and ecological time. Major radiations for assemblages of hosts and parasites, across nearly all taxa, have their roots in episodic events of extinction and biotic expansion in Earth history.
The synthesis signifies a conceptual shift from a mechanistically simplistic view of diversification through long-term mutual association and mutual modification of lineages to one involving an intricate historical mosaic involving host switches resulting from change in ecological context and geographic distribution. This view suggests that major episodes of climate change can trigger multiple rapid host switches, including those we call emerging diseases. This provides an appropriate ecological and evolutionary dimension for understanding patterns of introduction and dissemination of invasive species, and emergence of pathogens, parasites and disease in the current regime of global climate change with attendant disruption of ecological continuity.

Source paper: Hoberg, E.P. & Brooks, D.R. (2008) A macroevolutionary mosaic: episodic host-switching, geographical colonization and diversification in complex host–parasite systems. Journal of Biogeography, doi: 10.1111/j.1365-2699-2008-01951.x.

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