boomeronations, there are many closely related species that are able to interbreed but generally do not because of one or more of many factors. In fact there are many definitions of what kind of unit a species is (or should be).
From Wikipedia:
A common definition is that of a group of organisms capable of interbreeding and producing fertile offspring of both genders, and separated from other such groups with which interbreeding does not normally happen. Other debatable definitions may focus on similarity of DNA or morphology. Some species are further subdivided into subspecies, and here also there is no close agreement on the criteria to be used.
Some biologists may view species as statistical phenomena, as opposed to the traditional idea, with a species seen as a class of organisms. In that case, a species is defined as a separately evolving lineage that forms a single gene pool. Although properties such as DNA-sequences and morphology are used to help separate closely-related lineages, this definition has fuzzy boundaries. However, the exact definition of the term "species" is still controversial, particularly in prokaryotes, and this is called the species problem. Biologists have proposed a range of more precise definitions, but the definition used is a pragmatic choice that depends on the particularities of the species concerned.
