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Nature. 2014 May 8;509(7499):222-5. doi: 10.1038/nature13272. Epub 2014 Apr 30.

Niche filling slows the diversification of Himalayan songbirds.

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Department of Ecology and Evolution, University of Chicago, Chicago, Illinois 60637, USA.
1] Department of Ecology and Evolution, University of Chicago, Chicago, Illinois 60637, USA [2] Department of Zoology, Swedish Museum of Natural History, 10405 Stockholm, Sweden.
1] Department of Ecology and Evolution, University of Chicago, Chicago, Illinois 60637, USA [2] Institute of Pharmacy and Molecular Biotechnology, University of Heidelberg, Im Neuenheimer Feld 364, 69120 Heidelberg, Germany.
1] Key Laboratory of Zoological Systematics and Evolution, Institute of Zoology, Chinese Academy of Sciences, 1 Beichen West Road, Chaoyang District, Beijing 100101, China [2] Swedish Species Information Centre, Swedish University of Agricultural Sciences, Box 7007, 75007 Uppsala, Sweden.
Systematics and Biodiversity, Department of Biology and Environmental Sciences, University of Gothenburg, 40530 Gothenburg, Sweden.
Wildlife Institute of India, PO Box 18, Chandrabani, Dehradun 248001, India.
Institute of Zoology, Johannes Gutenberg University, Mainz 55099, Germany.
Max Planck Institute for Evolutionary Biology, August Thienemannstrasse 2, 24306 Plön, Germany.


Speciation generally involves a three-step process--range expansion, range fragmentation and the development of reproductive isolation between spatially separated populations. Speciation relies on cycling through these three steps and each may limit the rate at which new species form. We estimate phylogenetic relationships among all Himalayan songbirds to ask whether the development of reproductive isolation and ecological competition, both factors that limit range expansions, set an ultimate limit on speciation. Based on a phylogeny for all 358 species distributed along the eastern elevational gradient, here we show that body size and shape differences evolved early in the radiation, with the elevational band occupied by a species evolving later. These results are consistent with competition for niche space limiting species accumulation. Even the elevation dimension seems to be approaching ecological saturation, because the closest relatives both inside the assemblage and elsewhere in the Himalayas are on average separated by more than five million years, which is longer than it generally takes for reproductive isolation to be completed; also, elevational distributions are well explained by resource availability, notably the abundance of arthropods, and not by differences in diversification rates in different elevational zones. Our results imply that speciation rate is ultimately set by niche filling (that is, ecological competition for resources), rather than by the rate of acquisition of reproductive isolation.

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