The evolutionary divergence of a single species into two has never beendirectly observed in nature, primarily because speciation can take a longtime to occur. A ring species, in which a chain of intergrading populationsencircles a barrier and the terminal forms coexist without interbreeding,provides a situation in which variation in space can be used to infer variationin time1,2,3. Here we reconstruct the pathway to speciationbetween two reproductively isolated forms of greenish warbler (Phylloscopustrochiloides). These two taxa do not interbreed in central Siberia butare connected by a long chain of intergrading populations encircling the TibetanPlateau to the south4. Molecular data and climatic history implythat the reproductively isolated taxa came into contact following expansionsnorthward around the western and eastern sides of the plateau. Parallel selectionpressures for increased song complexity during the northward expansions havebeen accompanied by divergence in song structure. Playback experiments showthat the two Siberian forms do not recognize each other's songs. Our resultsshow how gradual divergence in a trait involved in mate choice leads to theformation of new species.
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We thank J. Kohn for the use of his laboratory; P. Alström, K. Marchetti,U. Olsson, A. Richman, J. Tiainen, the British Museum and the Burke Museumfor samples; Z. Benowitz-Fredericks, J. Gibson, S. Gross, J. Irwin, G.Kelberg, A. Knorre, K. Marchetti and B. Sheldon for help in the field; A.Asbeck, M. Bouvier, H. Neville, K. Petren, R. Radtkey and A. Richmanfor technical assistance; T. Case, J. Coyne, M. Dantzker, D. Holway,J. Irwin, J. Kohn, T. Pärt, A. Qvarnström, A. Suarez, N. Tsutsui,M. Turelli, A. Uy and S. Vehrencamp for comments on the manuscript. Forfinancial support we thank the American Ornithologists’ Union, The ExplorersClub, the Jeanne Messier Memorial Fund, the National Geographic Society, theNational Science Foundation and Sigma Xi.
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Irwin, D., Bensch, S. & Price, T. Speciation in a ring. Nature 409, 333–337 (2001). https://doi.org/10.1038/35053059
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