UC Berkeley

Large species radiations provide exceptional opportunities for understanding the processes involved in the formation of new species. This research is focused on a species-rich lineage of ecologically diversified planthoppers in the Hawaiian islands, Nesosydne (Hemiptera: Delphacidae). In order to examine the factors promoting diversification in this lineage at multiple slices through evolutionary time, I used an integrative approach combining three classes of methods. First, I used molecular phylogenetics and comparative methods to characterize the features of the Nesosydne adaptive radiation. I found that the Hawaiian Nesosydne are ecologically divergent and have undergone substantial within-island diversification. Second, I used population genetics and phylogeographic approaches in order to characterize within-species genetic structure. I found that the species Nesosydne chambersi comprises a very recently diverged, highly structured set of populations that I hypothesize to be in the early stages of diversification. Geographic isolation due to natural fragmentation from volcanic activity, not ecological adaptation to different host plant species, appears to be associated with population structure in N. chambersi. In addition, I documented a stable zone of secondary contact, which suggests that partial reproductive isolation develops rapidly between populations. Finally, using methods from animal communication, I established that: a) vibratory sexual signals in N. chambersi vary among populations on small temporal and spatial scales, b) sexual signal divergence occurs both in the absence and presence of ecological shifts, and c) signal divergence is evident in the zone of secondary contact in a pattern consistent with reproductive character displacement.

Based on my findings, I propose that the Hawaiian Nesosydne radiation represents an unusually clear case study in which the initial divergence within species is decoupled from the ecological diversification observed in the phylogeny. Species within the lineage first fracture into multiple genetic pools under the influence of geographic isolation. Sexual signals then shift among populations and they become reproductively isolated. This leads to the formation of populations that are set on independent evolutionary trajectories where they are free to either differentiate through adaptive means, differentiate through non-adaptive means, or go extinct. In other words, the formation of multiple reproductively isolated genetic pools results in a set of incipient species that are primed to radiate, given the appropriate conditions - but ecological forces are not responsible for the initial divergence. In a dynamic landscape such as the Hawaiian islands, plant and arthropod communities are both assembling and diversifying in concert. Consequently, the diversity of host plants used by the Hawaiian Nesosydne may best be explained by speciation first followed by a process of ecological fitting during a period of ecological opportunity as plant communities establish in novel terrain. Despite repeated observations of ecological diversification from classic adaptive radiations and plant-insect systems representing a wide variety of taxonomic groups, it is difficult to pinpoint with certainty the initial causes of divergence in any of these groups. The Hawaiian Nesosydne offer a rare vantage point into this enigmatic phase of diversification.