Adaptive radiation from the top down

Finally, the last paper stemming from Jeremy Heath’s PhD research has been published in The American Naturalist! This is a super-cool paper illustrating the importance of natural enemies (parasitoids in this case) in driving the adaptive diversification in their gall midge hosts. We suggest that such top-down driven diversification (rather than resource-driven) may be much more widespread than is currently appreciated.  I could explain further, but I think the abstract below sums it up nicely.  You can access the paper here.

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Aprostocetus tesserus laying an egg in an Asteromyia carbonifera gall. (J.J. Heath)

Abstract

Most studies of adaptive radiation in animals focus on resource competition as the primary driver of trait divergence. The roles of other ecological interactions in shaping divergent phenotypes during such radiations have received less attention. We evaluate natural enemies as primary agents of diversifying selection on the phenotypes of an actively diverging lineage of gall midges on tall goldenrod. In this system, the gall of the midge consists of a biotrophic fungal symbiont that develops on host-plant leaves and forms distinctly variable protective carapaces over midge larvae. Through field studies, we show that fungal gall morphology, which is induced by midges (i.e., it is an extended phenotype), is under directional and diversifying selection by parasitoid enemies. Overall, natural enemies disruptively select for either small or large galls, mainly along the axis of gall thickness. These results imply that predators are driving the evolution of phenotypic diversity in symbiotic defense traits in this system and that divergence in defensive morphology may provide ecological opportunities that help to fuel the adaptive radiation of this genus of midges on goldenrods. This enemy-driven phenotypic divergence in a diversifying lineage illustrates the potential importance of consumer-resource and symbiotic species interactions in adaptive radiation.

 

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