Abstract
Adaptive radiation is the rapid evolution of morphologically and ecologically diverse species from a single ancestor. The two classic examples of adaptive radiation are Darwin's finches and the Hawaiian honeycreepers, which evolved remarkable levels of adaptive cranial morphological variation. To gain new insights into the nature of their diversification, we performed comparative three-dimensional geometric morphometric analyses based on X-ray microcomputed tomography (µCT) scanning of dried cranial skeletons. We show that cranial shapes in both Hawaiian honeycreepers and Coerebinae (Darwin's finches and their close relatives) are much more diverse than in their respective outgroups, but Hawaiian honeycreepers as a group display the highest diversity and disparity of all other bird groups studied. We also report a significant contribution of allometry to skull shape variation, and distinct patterns of evolutionary change in skull morphology in the two lineages of songbirds that underwent adaptive radiation on oceanic islands. These findings help to better understand the nature of adaptive radiations in general and provide a foundation for future investigations on the developmental and molecular mechanisms underlying diversification of these morphologically distinguished groups of birds.This article is part of the themed issue ‘Evo-devo in the genomics era, and the origins of morphological diversity’.
Highlights
Adaptive radiation, or the rapid evolution of morphologically and ecologically diverse species from a single ancestor [1,2], was first described as an important phenomenon in organismal evolution in The major features of evolution by George Gaylord Simpson [3]
We evaluate how the whole-skull shape covaries with the shapes of each cranial subdivision in Darwin’s finches, Hawaiian honeycreepers and all remaining taxa using the twoblock partial least-squares (PLS) analysis [45], to address whether the mode of skull shape evolution through adaptive radiation, which is characterized by rapid morphological diversification, is in large part shared or divergent in the two songbird lineages and whether it matches the trends reported for other case studies on avian morphological evolution [40,47,48]
A phenotypic resemblance between the Kauai creeper and the Hawaii creeper (M. mana) has been reported based on bill shape, the absence of a tubular tongue, tree-creeping foraging and anti-predator mobbing behaviours, and juvenile plumage pattern [15,69,72,73,74,75]. Their similarity is so strong that the two species were often treated taxonomically as congeneric or even conspecific in earlier classifications. Their phenotypic resemblance is strongly thought to be a product of evolutionary convergence [12,69], our current comparative morphometric study newly reveals that when we focus on the whole skull shape, that of the treecreeping, insectivorous O. bairdi is similar to those of the tree-creeping, insectivorous M. mana and the insectivorous/nectarivorous anianiau (Magumma parva, a species that possesses a tubular tongue and does not exhibit treecreeping behaviour) based on interspecific Euclidian distances calculated from the shape space
Summary
The rapid evolution of morphologically and ecologically diverse species from a single ancestor [1,2], was first described as an important phenomenon in organismal evolution in The major features of evolution by George Gaylord Simpson [3]. The most important goal of this study is to gain new insights into the magnitude and modes of morphological diversification of the skulls in avian adaptive radiations on oceanic islands by quantitatively analysing and comparing the shapes of skulls in both Darwin’s finches and Hawaiian honeycreepers (including some extinct/fossil taxa), their respective relatives and shared outgroup taxa (figure 1; electronic supplementary material, table S1). A total of 23 landmarks that cover the entire structure were placed on digital images of the skulls (figure 2; electronic supplementary material, table S2) and the coordinates of those landmarks were compared among the representative species of both Darwin’s finches and Hawaiian honeycreepers (for detail, see §2c). PLS analyses were conducted using MORPHOJ [52]
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