Read the following scenario to answer the following question. 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. The production of sterile male ligers is an example of ________.