1. Department of Biology, University of Oslo, P.O. Box 1050 Blindern, N-0316 Oslo, Norway
2. Department of Molecular Cell Biology, Institute of Medical Biology, University of Tromsø, N-9037 Tromsø, Norway
3. Palacký University, Laboratory of Ornithology, T. Svobody 26, 77146 Olomouc, Czech Republic
4. Forestry Commission, 78386 Dlouhá Louka, Czech Republic
5. Zoological Museum, P/Sevaka 7, Jerevan 44, Armenia
6. Museo Nacional de Ciencias Naturales, J. Guiterrez Abascal 2, 28006 Madrid, Spain

Correspondence to: Glenn-Peter Stre¹ Correspondence and requests for materials should be addressed to G.P.S. (e-mail: Email: glennp@darwin.uio.no).

Abstract

Theory suggests that natural selection against the production of unfit hybrids may reinforce barriers to gene flow, eventually leading to reproductive isolation of differentiated populations^{1, 2, 3, 4}. This mode of speciation may be achieved by female choice selecting for a divergence in male secondary sexual traits that facilitates species recognition. Although intuitively appealing, conclusive evidence for such reinforcement is generally lacking^{5, 6, 7, 8}, and serious doubts have been raised about its validity^{9, 10, 11}. We have tested key predictions of the reinforcement hypothesis on the European, black-and-white, Ficedula flycatchers, using molecular techniques, field observations and mate choice experiments. In populations where two species coexist, we show that female choice selects for a divergence in male plumage colour and that the resulting character displacement reduces the frequency of hybridization.