[skrebbel]

That's logically equivalent to (A + B can breed) AND (B + C cannot breed), not a very exciting question.

Did you mean AND (A + C can breed)?

[quickthrower2]

Still not exciting. I think we are looking for non transitivity: A+B & B+C but !A+C

[irishsultan]

That's exactly what a ring species is.

[ethiclub]

Thanks for fixing. Yes that was the intended question.