There is a sister species to the worldwide common Blacktip shark living solely off the northern half of Australia: the Australian blacktip shark. The two requiem shark species look similar (but with different sizes, vertebrae counts and mitochondrial DNA), but genetics suggests that the other sister species, the smoothtooth blacktip shark (living solely off Arabia) and the graceful shark (with a habitat between Arabia and Australia) are closer related to the Australian blacktip shark (Carcharhinus tilstoni).
Nevertheless: several hybrids (some of them with a hybrid parent, what means it has to be fertile) between the Australian blacktip shark and the common blacktip shark have been discovered all along the eastern coast of Australia. This is the first confirmed case of hybridization among cartilaginous fishes – and no easy feat, even if more common in fishes, because of the internal fertilization of sharks.
The numbers suggest that shark hybrids may be more common than previously assumed, possibly between other sister shark species as well, and only haven’t been discovered yet. Even if hybridization is no key to prevent sharks extinction it is a fascinating part of evolution.