What is one type of organism that is closely related and can mate to produce fertile offspring?

One type of organism that is closely related and can mate to produce fertile offspring is the domestic dog (Canis lupus familiaris) and the gray wolf (Canis lupus). Both belong to the same species, Canis lupus, and can interbreed, producing viable and fertile offspring known as wolf-dog hybrids. Their close genetic relationship allows for successful reproduction, highlighting their shared ancestry.