section_path: The species problem/Hybridisation
section_name: Hybridisation
paragraph_text: Hybridisation
  
  Natural hybridisation presents a challenge to the concept of a reproductively isolated species, as fertile hybrids permit gene flow between two populations. For example, the carrion crow Corvus corone and the hooded crow Corvus cornix appear and are classified as separate species, yet they can hybridise where their geographical ranges overlap.