In some zoos, rare crosses between a male lion and a female tiger have produced hybrid offspring called ligers. Male ligers are sterile but some female ligers are fertile. In the wild, lion and tiger ranges do not naturally overlap, making such a cross unlikely. Furthermore, the solitary behavior of tigers and the social organizations of lions create behavioral differences.
Applying the biological species concept, the production of ligers reveals that ______.
tigers and lions are separate species