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.

Fig. 1.

(A) Watercolor by J.F. illustrating the Madagascan vanga species and morphological diversity. From the bottom moving clockwise: Mystacornis crossleyi, Cyanolanius (two species), Calicalicus (two species), Euryceros, Schetba, Vanga, Xenopirostris (three species), Oriolia, Falculea, Artamella, Leptopterus, and Newtonia (four species). (B) Bayesian topology of the Vangidae and other closely related core corvoids obtained from the combined dataset of six genes (Myo, ODC, GAPDH, Fib-5 c-mos, and ND2). Bayesian posterior probabilities >0.90 (except for the Madagascan vanga clade, pp = 0.88 are indicated to the left of the nodes (asterisks indicate posterior probabilities of 1.00) followed by maximum-likelihood bootstrap values ≥70% from 100 pseudoreplicates. (C) Map of Madagascar depicting the main habitat zones.

Fig. 2.

(A) BEAST chronogram of the Vangidae pruned to only include one individual per species. (B) Lineage-through-time plot (bold line) with error margins (shaded gray) and speciation events pr million y (thin line). (C) Disparity-through-time plot of PC axes 1–4, PC axis 1 and PC axes 2–4. Average extant disparity at a given point in time is the average disparity of subclades whose ancestral lineages were present at that time relative to the disparity of the entire taxon. The higher the value of relative disparity, the greater the average volume of morphological space occupied by subclades relative to the morphological disparity of the taxon as a whole. The dotted line shows mean simulated disparity under a Brownian-motion model (35).

Fig. 3.

The morphological distribution of 22 Madagascan vangid species, and 11 continental species in the genera Philentoma, Hemipus, Tephrodornis (Asia), Prionops, and Bias (Africa), on one size and five shape principal components axes, compared with identical measurements for species in two other monophyletic island passerine bird radiations (Hawaiian honeycreepers and Galapagos finches), against a background of 1,590 additional species of passerine birds sampled globally.