Genes make an individual unique. Organisms which form a single species can, by definition, breed and leave viable offspring. Those from similar, yet different species, may breed and may even produce offspring, but they are, with few exceptions, sterile. Examples include mules, tiger x lions, donkey x zebras. Such crosses rarely occur in nature as the species involved live in different environments.
Reproductive effort resulting in sterile hybrids represents a wastage of biological resources.
Nature usually will not tolerate such inefficiencies, and the normal biotic response to, say, the sudden reunion of two previously isolated species which are still closely related is that they do not attempt to breed. Over periods of time, this social isolation acts to even further increase differences between them. Eventually, gross reproductive isolating mechanisms can include having reproductive organs which are mechanically mismatched. An example of how this process might arise can be seen through artificial selection in the dog, by considering such widely divergent breeds as, say, the great dane and the chihuahua. Because they both remain members of the same species they will probably try to breed.