Theory suggests that natural selection against the production of unfit
hybrids may reinforce barriers to gene flow, eventually leading to re
productive isolation of differentiated populations(1-4). 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 reinforc
ement is generally lacking(5-8), and serious doubts have been raised a
bout its validity(9-11). We have tested key predictions of the reinfor
cement hypothesis on the European, black-and-white, Ficedula flycatche
rs, using molecular techniques, field observations and mate choice exp
eriments. In populations where two species coexist, we show that femal
e choice selects for a divergence in male plumage colour and that the
resulting character displacement reduces the frequency of hybridizatio
n.