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Patterns of divergence in the olive sunbird Cyanomitra olivacea (Aves: Nectariniidae) across the African rainforest–savanna ecotone

Authors

  • THOMAS B. SMITH,

    Corresponding author
    1. Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA 90095-1606, USA
    2. Center for Tropical Research, Institute of the Environment and Sustainability, La Kretz Hall, Suite 300, University of California, Los Angeles, CA 90095-1496, USA
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  • HENRI A. THOMASSEN,

    1. Center for Tropical Research, Institute of the Environment and Sustainability, La Kretz Hall, Suite 300, University of California, Los Angeles, CA 90095-1496, USA
    2. Institute of Evolution and Ecology, University of Tübingen, D-72076 Tübingen, Germany
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  • ADAM H. FREEDMAN,

    1. Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA 90095-1606, USA
    2. Center for Tropical Research, Institute of the Environment and Sustainability, La Kretz Hall, Suite 300, University of California, Los Angeles, CA 90095-1496, USA
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  • RAVINDER N. M. SEHGAL,

    1. Center for Tropical Research, Institute of the Environment and Sustainability, La Kretz Hall, Suite 300, University of California, Los Angeles, CA 90095-1496, USA
    2. Department of Biology, San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94132, USA
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  • WOLFGANG BUERMANN,

    1. Center for Tropical Research, Institute of the Environment and Sustainability, La Kretz Hall, Suite 300, University of California, Los Angeles, CA 90095-1496, USA
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  • SASSAN SAATCHI,

    1. Center for Tropical Research, Institute of the Environment and Sustainability, La Kretz Hall, Suite 300, University of California, Los Angeles, CA 90095-1496, USA
    2. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91125, USA
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  • JOHN POLLINGER,

    1. Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA 90095-1606, USA
    2. Center for Tropical Research, Institute of the Environment and Sustainability, La Kretz Hall, Suite 300, University of California, Los Angeles, CA 90095-1496, USA
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  • BORJA MILÁ,

    1. Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA 90095-1606, USA
    2. Center for Tropical Research, Institute of the Environment and Sustainability, La Kretz Hall, Suite 300, University of California, Los Angeles, CA 90095-1496, USA
    3. Museo Nacional de Ciencias Naturales, Consejo Superior de Investigaciones Científicas, Madrid 28006, Spain
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  • DEBRA PIRES,

    1. Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA 90095-1606, USA
    2. Center for Tropical Research, Institute of the Environment and Sustainability, La Kretz Hall, Suite 300, University of California, Los Angeles, CA 90095-1496, USA
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  • GEDIMINAS VALKIŪNAS,

    1. Center for Tropical Research, Institute of the Environment and Sustainability, La Kretz Hall, Suite 300, University of California, Los Angeles, CA 90095-1496, USA
    2. Institute of Ecology, Nature Research Centre, Akademijos 2, Vilnius LT-08412, Lithuania
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  • ROBERT K. WAYNE

    1. Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA 90095-1606, USA
    2. Center for Tropical Research, Institute of the Environment and Sustainability, La Kretz Hall, Suite 300, University of California, Los Angeles, CA 90095-1496, USA
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E-mail: tbsmith@ucla.edu

Abstract

In the debate over modes of vertebrate diversification in tropical rainforests, two competing hypotheses of speciation predominate: those that emphasize the role of geographical isolation during glacial periods and those that stress the role of ecology and diversifying selection across ecotones or environmental gradients. To investigate the relative roles of selection versus isolation in refugia, we contrasted genetic and morphologic divergence of the olive sunbird (Cyanomitra olivacea) at 18 sites (approximately 200 individuals) across the forest–savanna ecotone of Central Africa in a region considered to have harboured three hypothesized refugia during glacial periods. Habitats were characterized using bioclimatic and satellite remote-sensing data. We found relatively high levels of gene flow between ecotone and forest populations and between refugia. Consistent with a pattern of divergence-with-gene-flow, we found morphological characters to be significantly divergent across the gradient [forest versus ecotone (mean ± SD): wing length 60.47 ± 1.81 mm versus 62.18 ± 1.35 mm; tarsus length 15.51 ± 0.82 mm versus 16.00 ± 0.57 mm; upper mandible length 21.77 ± 1.09 mm versus 23.19 ± 0.98 mm, respectively]. Within-habitat comparisons across forest and ecotone sites showed no significant differences in morphology. The results show that divergence in morphological traits is tied to environmental variables across the gradient and is occurring despite gene flow. The pattern of divergence-with-gene-flow found is similar to that described for other rainforest species across the gradient. These results suggest that neither refugia, nor isolation-by-distance have played a major role in divergence in the olive sunbird, although ecological differences along the forest and savanna ecotone may impose significant selection pressures on the phenotype and potentially be important in diversification. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103, 821–835.

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