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Hypothetical question regarding three animals 'species' {A, B, C}.

Are there any examples in nature where {A and B} cannot regularly produce viable offspring, however {A and C} and {B and C} can regularly produce viable offspring.

I imagine that such a situation could arise if {A, B, C} have a common ancestor, but {A and B} are sufficiently different from each other, but not from C.

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The hybridization situation you describe could be found in ring species, and is partially related to this concept. For instance, the three species A, B and C could have partially overlapping distributions, such as:

enter image description here

In such a situation (assuming that the relative distributions have been stable over evolutionary time), A and C might be able to produce offspring, as well as B and C, while A and B might not produce viable offspring if they would meet.

Here is a picture from the wikipedia page for a similar situation, showing interbreeding between seven gull species in the genus Larus:

enter image description here

However, I also know that the ring species concept has been challanged, mainly by the fact that there are very few good empirical examples of ring species. See for instance the nice blog post "There are no ring species" by Jerry Coyne, which may also include a couple of examples of species groups that are relevant to your question.

Also, the hybridization patterns that you describe can be caused by other processes, for instance specific reproductive barriers between groups of species (molecular or behavioural). My answer is therefore merely mentioning one possible explanation for the hybridization patterns you ask for, but there are many other processes to consider.

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  • $\begingroup$ I don't know what the molecular mechanism behind this phenomenon is but one possibility is presence of a set of lethal mutations that can be compensated in certain crosses. $\endgroup$
    – WYSIWYG
    Oct 7 '15 at 9:55
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    $\begingroup$ Re Jerry Coyne's blog post -- What is it about Darwinian Theory that engenders so many dead-end conjectures and false pedagogical stories? $\endgroup$
    – user23715
    Oct 7 '15 at 18:44
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    $\begingroup$ Maybe it's the underlying theory though and not just a naive understanding of it? See the recent "news" about Great Ape Evolution (Pliobates cataloniae). --- If the underlying theory is sound wouldn't it be driving field and lab work that supports itself instead of "turning it on it's head" as happens so often? --- The Standard Model in particle physics is known to be incomplete yet has truly underpinned the associated research. --- NDE and biology? Meh, not so much. $\endgroup$
    – user23715
    Nov 4 '15 at 15:51
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    $\begingroup$ The Mods will probably move this to chat but, in a nutshell: What is it about NDE that produces ideas like Ring Species or a proposed "tree" for the evolution of the Great Apes, and yet when field work is done in support of the idea, instead of reinforcing it, the theoretical concept gets "turned on its head"? --- Contrast this with Plate Tectonics, a theory that fruitfully guides field work in geology --- Or the aforementioned Standard Model in physics. These latter two have proven themselves time and again. NDE not so much. There is seemingly nothing about NDE that drives fruitful research. $\endgroup$
    – user23715
    Nov 6 '15 at 15:46
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    $\begingroup$ "Ring Species" was an idea birthed by NDE thinking. "Ring Species" do not exist... or maybe one example exists. What's wrong with NDE that it promotes thinking like this? --- Compare with the fruitfulness of the Standard Model in physics or Plate Tectonics in Geology. It just seems odd to me that the fundamental theory in Biology doesn't actually produce anything worthwhile. Advances in Population Genetics, Molecular Biology, etc. have all taken huge leaps irrespective of any potential underlying link to NDE. --- Name any recent (50 yrs +-) advance in Biology needs NDE to be understood. $\endgroup$
    – user23715
    Nov 24 '15 at 1:49
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The term that describes this phenomenon is 'clinal speciation', and the easiest examples to point to are all 'ring species'.

Wikipedia mentions four ring species examples. Three are avian examples (Larus gulls, Song Sparrows, and Greenish Warblers), and one is a plant example (Euphorbia tithymaloides).

The classification of the Larus gull complex as a ring species is contested. Liebers et al (2004) argue that the isolation-by-distance model, with genetic contact at all near points of the species' range, is not well-supported by genetic analyses for this species, and that the species' genetic structure is better explained by a series of 'splitting' events associated with long-distance colonisation followed by genetic isolation of the new colony.

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