Colour polymorphism exemplifies extreme morphological diversity within populations1,2. It is taxonomically widespread but generally rare. Theory suggests that where colour polymorphism does occur, processes generating and maintaining it can promote speciation but the generality of this claim is unclear1. Here we confirm, using species-level molecular phylogenies for five families of non-passerine birds, that colour polymorphism is associated with accelerated speciation rates in the three groups in which polymorphism is most prevalent. In all five groups, colour polymorphism is lost at a significantly greater rate than it is gained. Thus, the general rarity and phylogenetic dispersion of colour polymorphism is accounted for by a combination of higher speciation rate and higher transition rate from polymorphism to monomorphism, consistent with theoretical models where speciation is driven by fixation of one or more morphs3. This is corroborated by evidence from a species-level molecular phylogeny of passerines, incorporating 4,128 (66.5%) extant species, that polymorphic species tend to be younger than monomorphic species. Our results provide empirical support for the general proposition, dating from classical evolutionary theory2,4,5,6, that colour polymorphism can increase speciation rates.
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We thank M. Elgar, J. Endler, L. Joseph, A. Moussalli, A. Phillimore, S. Pryke and D. Rabosky for critical comments. This work was supported by the Australian Research Council (DP1092908).
This file contains Supplementary Tables 1-7, Supplementary Figures 1-7, Supplementary Text and Supplementary Methods. Supplementary Tables 1 and 2 show sampling and summary statistics from diversification analyses; Supplementary Figure 1 and the Supplementary Text show the effects of sampling; and Supplementary Figure 1 also shows phylogenetic uncertainty. The Supplementary Methods contain detailed phylogenetic methods; Supplementary Tables S3-S7 and Supplementary Figure S2 contain summary statistics and additional information for phylogeny reconstruction; and Supplementary Figures S3-S7 contain phylogenies for the Accipitridae, Strigiformes, Caprimulgiformes, Falconidae and Galliformes.
This file contains newick format trees with node support values for the Strigiformes, Caprimulgiformes, Falconidae, Galliformes and four passerine trees: Lower Oscines-Corvoidea, Passeroidea, Suboscines; Genus-level tree.
About this article
BMC Biology (2015)