If two sufficiently similar animals can interbreed freely, when are they subspecies and when separate species

In saying "everyone has simplified this answer based upon their species concept", this answer seems to have completely ignored Felicity Titwangle's answer. She has given a clear explanation of why two animals that interbreed are not the same species because of their DNA being different.

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I agree, Felicity Titwangle (great name, BTW) has provided an excellent and well-informed answer.

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It's down to genetics. At a genetic level, an arctic wolf and a European wolf, for example, are the same species. They're only different enough to be considered subspecies of the same species. A coyote is genetically a different animal altogether - a different species.

It used to be stated that two animals were the same species if they could breed and produce fertile offspring. This definition is not used any more because it isn't strictly accurate - wolves and coyotes can breed and produce fertile young, but genetically are different species. They're just very closely related. Some animals are just close enough to be able to breed, but not produce fertile hybrids - horses and donkeys are an example of this. Others cannot breed at all because they are simply too different genetically - that's why you couldn't breed, say, a horse-wolf.

That depends on who is doing the classification. Interbreeding was proposed as a difference between spp and subspp but it is useless for most organisms (those that reproduce asexually and fossils). It also doesn't work very well. There are minnows that interbreed freely across generic lines, not just specific ones.

My personal preference for a species concept is C. Tate Regan's: A species is whatever a competent taxonomist working with the group says is a species. It may sound flip but it works.