The definition of "species" varies based on the biological specialty you're talking about (it's the least arbitrary of the taxa, but it still is kind of arbitra

The definition of "species" varies based on the biological specialty you're talking about (it's the least arbitrary of the taxa, but it still is kind of arbitrary). Cladists/geneticists tend to have a view close to the one you mention ("interbreeding with viable non-sterile offspring defines a species"), but even they will say that definition gets too simplistic. Behavioral biologists generally have a more stringent definition of "species" that involves the population's ecological niche. But, either way, surrogacy is not breeding; it would be a northern egg and sperm implanted in a southern rhino.