> "In higher mammals the basic property of generating fertile offspring through sexual reproduction is clear-cut and undisputed."
What's undisputed is that there are plenty of species that can and occasionally will interbreed, and yet are still considered separate species. Wolves and coyotes, for example. Horses and donkeys are famous for producing infertile offspring, but occasionally, that offspring is still fertile.
So it's not a clear-cut on/off situation. There's a gradual scale from fertility to infertility. The edges of the old definition of species can get very fuzzy. For this reason, biologists these days tend to talk more in terms of populations than species.
What's undisputed is that there are plenty of species that can and occasionally will interbreed, and yet are still considered separate species. Wolves and coyotes, for example. Horses and donkeys are famous for producing infertile offspring, but occasionally, that offspring is still fertile.
So it's not a clear-cut on/off situation. There's a gradual scale from fertility to infertility. The edges of the old definition of species can get very fuzzy. For this reason, biologists these days tend to talk more in terms of populations than species.