Ecological character displacement in the face of gene flow: Evidence from two species of nightingales
- Equal contributors
1 Department of Zoology, Faculty of Science, Charles University, Prague, Czech Republic
2 Institute for Environmental Studies, Faculty of Science, Charles University, Prague, Czech Republic
3 Department of Zoology and Laboratory of Ornithology, Faculty of Science, Palacký University, Olomouc, Czech Republic
4 Department of Behavioural Ecology, Adam Mickiewicz University, Poznan, Poland
5 Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, USA
BMC Evolutionary Biology 2011, 11:138 doi:10.1186/1471-2148-11-138Published: 24 May 2011
Ecological character displacement is a process of phenotypic differentiation of sympatric populations caused by interspecific competition. Such differentiation could facilitate speciation by enhancing reproductive isolation between incipient species, although empirical evidence for it at early stages of divergence when gene flow still occurs between the species is relatively scarce. Here we studied patterns of morphological variation in sympatric and allopatric populations of two hybridizing species of birds, the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (L. luscinia).
We conducted principal component (PC) analysis of morphological traits and found that nightingale species converged in overall body size (PC1) and diverged in relative bill size (PC3) in sympatry. Closer analysis of morphological variation along geographical gradients revealed that the convergence in body size can be attributed largely to increasing body size with increasing latitude, a phenomenon known as Bergmann's rule. In contrast, interspecific interactions contributed significantly to the observed divergence in relative bill size, even after controlling for the effects of geographical gradients. We suggest that the divergence in bill size most likely reflects segregation of feeding niches between the species in sympatry.
Our results suggest that interspecific competition for food resources can drive species divergence even in the face of ongoing hybridization. Such divergence may enhance reproductive isolation between the species and thus contribute to speciation.