if different species attempt to procreate, the zygotes will most likely fail due to a differing number of chromosomes. if the zygotes are able to form and develop, the result will be sterile and unable to breed on its own, and will not be classified as a specie due to its inability to procreate. but sometimes they are force bred - such as the mule, which is the result of a male donkey and a female horse. Mules are typically hardier than horses, live longer - while being smarter and more patient than donkeys.