lets call this term "collection" Example: group of humans.
what is not a "collection" horses and donkey: can't produce full interfertile offspring, lions and tigers: can't produce full interfertile offspring, Camels and Lama: can't produce full interfertile offspring.
And no this is not "species". Polar bear is a species grizzly bear is a species they can produce interfertile offspring, they are of the same "collection" Two species can belong to the same "collection". Two different "collections" can't be of the same species.
And no this is not a homework question.
Why is this not defined in biology?
Why is this down-voted? I know what ring species is. First and last generation do not interbreed naturally. But it does not say they can not interbreed at all or can not produce full interfertile offspring. The question is about having the ability of intebreeding and producing full interfertile offspring.
I will add more details: The term species is a fuzzy term and makes a lot of confusion. I'm not asking about it. There is a clean cut for all living things that intebreed and produce. For a given group of living things they either can intebreed and produce full interfertile offspring or not. There are many example. Why is this clean cut not have a term in biology?
Again. This is not a "species" and from my point of view the term species should not be used in biology. Anything is a species... Homo sapiens, Neanderthals two different species although they had interbred and produced full interfertile offspring.....
Different dogs "races" are sometimes called different speices although they can intebreed and produce full interfertile offspring.
I already know about the fly experiment where the author claims that he got two different "species" of flies after some year of combination of sexual isolation and interbreeding. But no, all flies during the whole experiment of the same "collection" even if the author says they are different "species". The author did not make sure that they intebreed, he let them naturally decided whether they want to interbreed or not. They did not because they are not attacted to each other, but this does not mean they can not interbreed and produce full interfertile offspring.