Ecological character displacement in the face of gene flow: Evidence from two species of nightingales
Abstract
Ecological character displacement plays an important role in species differentiation; however, its role in establishing reproductive isolation during the process of speciation remains controversial. Here we studied patterns of morphological variation in sympatric and allopatric populations of two hybridizing species of birds, the Common Nightingale and the Thrush Nightingale.
We found that nightingales converged in overall body size and diverged in relative bill size in sympatry. The convergence in body size can be attributed largely to increasing body size with increasing latitude.
The divergence in relative bill size is better explained by interspecific interactions than by geographical gradients and most likely reflects segregation of feeding niches between the species in sympatry. Our results are consistent with the idea that ecological character displacement could facilitate species divergence even in the face of interspecific gene flow and may therefore contribute to speciation.