Well, if AxB and BxC can mate, then the off spring has genes from A and C so by some second degree derivative they can mate. iff. Not my field of interest but

Well, if AxB and BxC can mate, then the off spring has genes from A and C so by some second degree derivative they can mate. iff. Not my field of interest but I feel reminded of dialects and how difficult it is to define language. The dialect with a military meme has some truth to it, when variet are measured against a standard or prestige dialect. B obviously has an advantage so unless other disadvantages apply, it would be dominant. A societal (?) or socio-cultural definition is interesting as well. Even genetically identical specimen couldn't mate if on opposite sites of the globe, and they are bound to drift eventually. I do kindle with the idea that groups have to be understood as organisms in their own right, but I never read anything specific about it. About language families, it is startling how anthropologists reason about language by proxy via archaogenetics. Lots of open questions there, too.