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Correlated evolution of morphology and vocal signal structure in Darwin's finches


Speciation in many animal taxa is catalysed by the evolutionary diversification of mating signals1. According to classical theories of speciation, mating signals diversify, in part, as an incidental byproduct of adaptation by natural selection to divergent ecologies2,3, although empirical evidence in support of this hypothesis has been limited4,5,6. Here I show, in Darwin's finches of the Galápagos Islands, that diversification of beak morphology and body size has shaped patterns of vocal signal evolution, such that birds with large beaks and body sizes have evolved songs with comparatively low rates of syllable repetition and narrow frequency bandwidths. The converse is true for small birds. Patterns of correlated evolution among morphology and song are consistent with the hypothesis that beak morphology constrains vocal evolution, with different beak morphologies differentially limiting a bird's ability to modulate vocal tract configurations during song production. These data illustrate how morphological adaptation may drive signal evolution and reproductive isolation, and furthermore identify a possible cause for rapid speciation in Darwin's finches.

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Figure 1: Beak morphology (sketches reprinted15) and representative sound spectrograms of songs from eight Darwin's finch species on Santa Cruz Island (from top to bottom: G. magnirostris, G. fortis, G. fuliginosa, G. scandens, C. parvulus, C. psittacula, C. pallida , C. olivacea).
Figure 2: Acoustic analyses.
Figure 3: Vocal deviation as a function of beak depth in G. fortis.
Figure 4: Interspecific analyses.

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Field work was coordinated through the Charles Darwin Research Station and the Galápagos National Park Service. I thank M. Rossi-Santos, M. Moreano, H. Vargas, H. Snell and M. Hau for assistance in the field; S. Nowicki, L. Baptista, R. Bensted-Smith, R. Bowman, P. & R. Grant, A. Hendry, W. Hoese, S. Hopp, J. Jaenike, J. Lundberg, W. Maddison, L. McDade, C. Nufio, D. Papaj, S. Patek and R. Prum for discussion and feedback; and the National Science Foundation, the Univ. Arizona Foundation, the Univ. Arizona Office of the Vice President for Research and TAME airlines for financial support.

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Podos, J. Correlated evolution of morphology and vocal signal structure in Darwin's finches. Nature 409, 185–188 (2001).

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