Abstract
Explaining the exceptional diversity of herbivorous insects is an old problem in evolutionary ecology. Here we focus on the two prominent hypothesised drivers of their diversification, radiations after major host switch or variability in host use due to continuous probing of new hosts. Unfortunately, current methods cannot distinguish between these hypotheses, causing controversy in the literature. Here we present an approach combining network and phylogenetic analyses, which directly quantifies support for these opposing hypotheses. After demonstrating that each hypothesis produces divergent network structures, we then investigate the contribution of each to diversification in two butterfly families: Pieridae and Nymphalidae. Overall, we find that variability in host use is essential for butterfly diversification, while radiations following colonisation of a new host are rare but can produce high diversity. Beyond providing an important reconciliation of alternative hypotheses for butterfly diversification, our approach has potential to test many other hypotheses in evolutionary biology.
Highlights
Explaining the exceptional diversity of herbivorous insects is an old problem in evolutionary ecology
Ever since Ehrlich and Raven[2] argued for interactions between herbivorous insects and their host plants as being central to the diversification of both, and in the process formalising the concept of coevolution, evolutionary ecologists have searched for evidence of how such interactions could drive diversification[3,4]
If most of the diversity of butterflies was generated by adaptive radiations on new host plants, the resulting network should be highly modular
Summary
Explaining the exceptional diversity of herbivorous insects is an old problem in evolutionary ecology. The adaptive radiation scenario hypothesises that herbivorous insects quickly radiate into many species following a shift from an old to a novel plant taxon, by overcoming their host defences As such, this is consistent with the idea of a key innovation by Ehrlich and Raven, though it does not require subsequent coevolution[7]. Novel statistical approaches to study state-dependent diversification have been developed recently[20,21], but have so far produced divergent results and, different explanations for the effect of host range on diversification[10,11,17] Part of this problem arises from the classification of host range, which is a complex trait, into two opposing states (specialist vs generalist) or multiple states. The mechanisms underlying these patterns can be assessed using independent sources of information, such as phylogenetic relationships[24,25,26,27]
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