Abstract
Trapliners are pollinators that visit widely dispersed flowers along circuitous foraging routes. The evolution of traplining in hummingbirds is thought to entail morphological specialization through the reciprocal coevolution of longer bills with the long-tubed flowers of widely dispersed plant species. Specialization, such as that exhibited by traplining hummingbirds, is often viewed as both irreversible and an evolutionary dead end. We tested these predictions in a macroevolutionary framework. Specifically, we assessed the relationship between beak morphology and foraging and tested whether transitions to traplining are irreversible and lead to lower rates of diversification as predicted by the hypothesis that specialization is an evolutionary dead end. We find that there have been multiple independent transitions to traplining across the hummingbird phylogeny, but reversals have been rare or incomplete at best. Multiple independent lineages of trapliners have become morphologically specialized, convergently evolving relatively large bills for their body size. Traplining is not an evolutionary dead end however, since trapliners continue to give rise to new traplining species at a rate comparable to non-trapliners.
Highlights
For plants that are widely dispersed across a landscape, there is a premium in attracting high-fidelity long-range pollinators and excluding low-fidelity shortrange pollinators [1,2,3,4,5,6,7]
To generate a null expectation of classification accuracy based on observed phylogenetic similarity among species, we simulated the evolution of traits randomly under the Brownian motion model of evolution 1000 times using the fastBM function of the R package phytools (Revell [42]) on rate-scaled trees inferred using BayesTraits v3 with default settings [43] and used these randomly simulated traits as predictors of foraging behaviour
The classification accuracy remains 75% when using phylogenetic PC analysis (PCA)
Summary
For plants that are widely dispersed across a landscape, there is a premium in attracting high-fidelity long-range pollinators and excluding low-fidelity shortrange pollinators [1,2,3,4,5,6,7]. Through coevolution with the flowers of the various species they pollinate, trapliners may become more morphologically and ecologically specialized than their non-traplining counterparts This hypothesis on the coevolution between guilds of widely dispersed flowers and traplining pollinators inspires several macroevolutionary predictions, which we test in this study of hummingbirds. Clades of specialist trapliners may be able to diversify in the specific flowers on which they feed, supporting high rates of diversification, the opposite of what the evolutionary dead ends hypothesis would predict. Evidence in the literature for specialization being an evolutionary dead end is currently mixed [26,27] We test these ideas on hummingbirds using phylogenetic comparative methods to characterize diversification and rates of morphological evolution in relation to evolutionary transitions in foraging ecology
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