Stireman, collaborator Patrick Abbot, and Lab technician Hilary Devlin have just published a paper analyzing the extraordinary radiation of Asteromyia carbonifera gall midges on their Solidago hosts across North America. We show that this nominal species is actually comprised of a multitude of distinct evolutionary lineages that differ in host plant use and gall morphology. However, there is very little genetic structure related to geography.
These results suggest that A. carbonifera has undergone a very recent, rapid adaptive radiation, driven by ecological selective pressure from both lower trophic levels (host plant suitability and defenses) and higher ones (parasitoids, which select on gall morphology). Interestingly, divergence in gall morphology and host-plant associations appear to evolve relatively rapidly, with similar gall forms evolving multiple times, and host plants being repeatedly colonized by different lineages.
This is a fascinating example of very recent, even current, adaptive radiation in which we can still perceive the selective forces driving phenotypic divergence. However, this paper primarily focuses on patterns observed in the genetic structure of populations, and much more needs to be done confirming the processes responsible.