Ecological and evolutionary determinants for the adaptive radiation of the Madagascan vangas

Abstract

Adaptive radiation is the rapid diversification of a single lineage into many species that inhabit a variety of environments or use a variety of resources and differ in traits required to exploit these. Why some lineages undergo adaptive radiation is not well-understood, but filling unoccupied ecological space appears to be a common feature. We construct a complete, dated, species-level phylogeny of the endemic Vangidae of Madagascar. This passerine bird radiation represents a classic, but poorly known, avian adaptive radiation. Our results reveal an initial rapid increase in evolutionary lineages and diversification in morphospace after colonizing Madagascar in the late Oligocene some 25 Mya. A subsequent key innovation involving unique bill morphology was associated with a second increase in diversification rates about 10 Mya. The volume of morphospace occupied by contemporary Madagascan vangas is in many aspects as large (shape variation)-or even larger (size variation)-as that of other better-known avian adaptive radiations, including the much younger Galapagos Darwin's finches and Hawaiian honeycreepers. Morphological space bears a close relationship to diet, substrate use, and foraging movements, and thus our results demonstrate the great extent of the evolutionary diversification of the Madagascan vangas.daptive radiation is the rapid diversification of a single lineage into many species that inhabit a variety of environments or niches and differ in the morphological and/or physiological traits required to exploit these environments (1-4). Well-known examples of adaptive radiations include Galapagos finches (5), Hawaiian honeycreepers (6), Hawaiian lobeliads (7), and Caribbean anoles (8). Although evolutionary biologists do not understand why some lineages undergo adaptive radiation and others do not, ecological opportunity appears to be a common feature. Opportunity might arise as a new food resource, a mass extinction of competitors and/or predators, and the colonization of new land or environments (4, 9, 10). Adaptive radiation is ultimately the outcome of divergent natural selection arising from occupation of different environments, use of different resources, or resource competition (4). The progressive filling of ecological space, and the accompanying decrease in ecological opportunity, is expected to result in a decrease in rates of diversification and morphological evolution over time (11,12).The bird family Vangidae (15 genera, 22 species) is endemic to Madagascar and considered an extraordinary example of adaptive radiation. This is due particularly to the wide range of foraging strategies as well as to the evolution of striking differences in bill morphology that have allowed vangid species to exploit diverse foraging niches (13,14). However, the evolutionary history of the group remains poorly understood. Previous systematic analyses have not included all members of the group (15)(16)(17), and these studies have not investigated morphological traits in a comparative phylogenetic framework, pre...

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