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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.

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  • $\begingroup$ Welcome to Biology.SE! Please take the tour and then go through the help pages starting with How to Ask questions effectively on this site and then edit your original question rather than posting a new version. Also, note that "homework"(biology.stackexchange.com/help/homework) can apply to questions even if they are not assigned as homework. ——— We also encourage you to do some research on your own and then, informed by what you have learned, ask any questions you still have (ideally with references to reliable sources). Thanks! 😊 $\endgroup$
    – tyersome
    Apr 13, 2020 at 21:35
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    $\begingroup$ This version of your question is clearer, but part of the reason that previous version was closed was because you don't demonstrate that you have done enough background research. ——— I recommend that you to check out some of the online resources available for learning more about evolution. For example, this a useful introduction to evolutionary theory from UC Berkeley. You should also do reading on "species concepts", which I think may ultimately be what you are trying to understand ... $\endgroup$
    – tyersome
    Apr 13, 2020 at 21:43
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    $\begingroup$ I am going to upvote this back. The species question is not trivial - the above link to species concepts illustrates this in quotations, but this is not truly a matter of species. The questioner's example of U. arctos and U. maritimus is a good one. Typically we apply the species concept to what does interbreed, not what can interbreed. There is no a priori limit to how far interbreeding could take place, and some genera such as Aquilegia are known for looser limitations. Well researched answers here should make interesting reading. $\endgroup$
    – Mike Serfas
    Apr 13, 2020 at 23:42
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    $\begingroup$ Part of the problem here is you are using an overly narrow definition of species, by many schools of thought neanderthals are not a separate species nor are brown bears and polar bears. It all depends on whether you are a splitter or lumper. $\endgroup$
    – John
    Apr 14, 2020 at 15:58

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Based on the biological species concept, you're referring to two different species that experience ecological isolation (also called "habitat isolation") as a prezygotic reproductive isolating mechanism.

See here for a simple explanation of various concepts regarding species.

This, of course, is just one of the many reasons (see here for more) why we have multiple species concepts. Some might argue that these organisms are the same species based on their ability to produce interfertile offspring. If said researcher wanted to do so, they would define these organisms using a different concept.

Recognize that “species” is an artificial construct to allow conceptualization and organization of an incredibly complex, dynamic, and otherwise continuously variable genetic world.

We have not found a perfect way to "cut up" the world neatly/consistently into describable and functionally useful units, and --in fact -- we likely never will. Why? Because the lives of these organisms do not care about our cataloging and description systems -- these concepts are strictly for humankind to functionally relate to and react to their presence. The concept chosen almost inherently relies on the underlying motivation of the describer for naming the organism in the first place.

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  • $\begingroup$ And remember the problem with defining species when you include fossil specimens, where you cannot test for inter-fertility. As I point out to my students if allow a single species to be distributed in time, (Aka a fossilized horse and a living horse are the same species), all of life can be one gigantic complex ring species. $\endgroup$
    – John
    Apr 14, 2020 at 16:03

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