Because plants and their insect enemies are strikingly species-rich groups, understanding their interactions has been a key issue in ecology and evolution. Coevolutionary theory has long predicted that the arms race between plants and insects may drive diversification in both groups and has led to evolutionary conservatism in plant defenses and in host use by insect herbivores. We tested these expectations using the Neotropical genus of trees Inga (Fabaceae) and its associated lepidopteran herbivores in the Peruvian Amazon.
Results/Conclusions
We constructed multilocus phylogenies for both plants and insects and collected data on host associations and plant defensive traits. Contrary to the coevolutionary model, our results showed that host defensive traits were not phylogenetically conserved. Similarity in herbivore assemblages between Inga species pairs was correlated with similarity in defensive traits, but not phylogenetic proximity. Furthermore, host defensive traits explained 40% of observed herbivore community similarity. Analysis at finer taxonomic scales showed that different lepidopteran families select hosts based on different anti-herbivore traits, suggesting taxon-specific histories of herbivore-host plant interactions. Finally, we compared the phylogeny of Inga, the defenses of Inga, and the phylogeny for the major lepidopteran families. We found that closely related herbivores fed on Inga species with similar defenses rather than on closely related plants, a pattern more consistent with ecological resource tracking than with the arms race model of coevolution.
Together, these results suggest that defenses evolve rapidly and that traits related to host choice evolve more slowly. Hence, there is an apparent asymmetry in the evolutionary interactions between Inga and its herbivores. Specifically, although divergence in herbivores might not be driven solely by their interactions with plants, herbivores may be an important factor driving the divergence among plant species.