A unifying perspective on pathogen specificity

Linking microevolutionary processes to macroevolutionary patterns

I have always been interested coevolutionary interactions, particularly host-parasite interactions. I have often wondered if the local patterns of interaction between host and parasite (e.g. local adaptation) can scale up and lead to patterns of host specificity. Having a thorough understanding of these selective forces may help us better understand the conditions for disease emergence and perhaps disease virulence evolution.

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Population genetics is concerned with the processes that generate evolutionary change within species and populations. A major question in evolutionary biology is whether these same processes ultimately generate patterns of diversity at higher organizational levels. While interactions between species such as hosts and their parasites (or plants and pollinators, herbivores and plants) have long been implicated as a means of generating patterns of diversification (Ehrlich and Raven, 1964,Thompson, 1994, 2005), the process by which microevolutionary forces generate macroevolutionary patterns is not well understood for coevolutionary systems.

Highly specific interactions between pairs of species can result in population level patterns. Both theoretical and empirical studies show that genetic specificity combined with specific gene flow patterns lead to parasites tracking of local host populations (Dybdahl and Storfer, 2003,Gandon et al., 1996,Gandon and Michalakis, 2002,Kaltz and Shykoff, 1998). In a mutualism between plant and pollinator, the seeming match between the length of a flower corolla and bill may be the result of strong selective pressure. Although we have many good examples of the processes that work at the population level, we have little evidence as to how those processes generate patterns of diversity among interacting species (Thompson, 2005). At the macroevolutionary scale, the processes leading to the observed patterns of host specificity have remained unclear.

The authors of a recent perspective piece in Evolution have addressed this exact issue by asking:

“Can microevolutionary adaptive processes acting at the within-species level explain macroevolutionary patterns across host and pathogen taxa?” (Antonovics et al., 2013)

What processes can explain why most pathogens cannot infect all encountered hosts ?

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