Answer:
The postzygotic isolating mechanisms
Explanation:
The postzygotic isolating mechanisms refer to the mechanisms that isolate members of different species reproductively after the process of fertilization.
One of these mechanisms is hybrid sterility wherein the hybrid produced by a cross between two organisms of different species is sterile and can not reproduce.
In the given example, the mule is an interspecific hybrid of a female horse and male donkey but is sterile. This is an example of postzygotic isolating mechanisms that prevent gene flow between horses and donkeys.
