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I was motivated to ask based on an answer to another question that I read on SE-Biology:

You can differentiate this species [Ashok's Bronzeback (Dendrelaphis ashoki)] from closely related D. girii based on it having a longer "postocular" black line than D. ashoki -- in D. ashoki it stops shortly caudal to the eye...

When the differences are so minor (e.g. "longer "postocular" black line") what's the reason for designating them as two different species?

Is there a objective reason when to call them different species? Would those two not interbreed?

Note: I'm not just asking "If they interbreed why is it a different species?"

I'm more in the vein of asking: "If two species so similar are still different species why are they different? Is there an objective metric to draw the line at which point we differentiate as different species."

Edit:

I've asked a related question over here:

Can Eskimos be regarded a distinct species from Kalahari Bushmen based on morphological differences & geographic isolation?

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    $\begingroup$ While it is not obvious from the title of the other post, your question is possibly a duplicate of How could humans have interbred with Neanderthals if we're a different species?. $\endgroup$ – Remi.b Feb 27 '17 at 14:34
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    $\begingroup$ @Remi.b I disagree. That question asks specifically about how interbreeding could take place between 2 species. This question is focused differently and asks why differentiate two species that look so similar.Though your own answer from the other question could fill in the OP immensely about the concept of a species, I still think more specifics could be said about this instance. $\endgroup$ – theforestecologist Feb 27 '17 at 14:56
  • $\begingroup$ @theforestecologist Fair enough! There are two questions though; 1) what's the reason for designating them as two different species? 2) I'm more in the vein of asking: "If two species so similar are still different species why are they different? Is there an objective metric to draw the line at which point we differentiate as different species.". IMO, the second question is a duplicate, the first question is not. If you agree, I let you edit the question to narrow it down to a single question. $\endgroup$ – Remi.b Feb 27 '17 at 15:39
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    $\begingroup$ Remi.b Yup, I agree. But I encourage @curious_cat to edit the question on their own in a way that narrows it down appropriately but still satisfies their original curiosity. $\endgroup$ – theforestecologist Feb 27 '17 at 15:47
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    $\begingroup$ @Remi.b I feel like your answer to that question is comprehensive enough to apply to this question, but the questions themselves don't really seem to be duplicates - I'm not sure what the agreed SE meta is in that situation. I feel like the title of this question conveys a broader curiosity so it has some value beyond the value of the Neanderthal question. $\endgroup$ – Bryan Krause Feb 27 '17 at 17:17
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Short explanation

According to the papers I could find, Dendrelaphis ashoki and D. girii aren't as closely related as you suggest: D. ashoki appears to be closely related to D. pictus, a Southeast Asian snake species, while D. girii appears to be closely related to D. bifrenalis, a Sri Lankan/South Indian snake species. They were distinguished from their sister species largely on the basis of morphological differences and geographical isolation.

How do we know D. ashoki is a distinct species?

The key to this answer is in Vogel and van Rooijen, 2011a, the scientific article in which Dendrelaphis ashoki was first described. Before their work, these snakes were thought to belong to the species Dendrelaphis pictus. To test whether this was in fact the case, they compared 9 individuals of D. ashoki with 34 individuals of D. pictus, and found statistically significant ($p < 0.05$) differences in five morphological characters: SUBC (number of subcaudals), TAIL (tail-length), EYED (horizontal diameter of the eye), SUBL (number of infralabials touched by the first sublabial, left and right added) and STRIPE1 (ventrolateral stripe bright/dull/absent). Here's the statistical analyses they ran:

The south Indian form that has previously been referred to as D. pictus (e.g., Wall, 1921; Smith, 1943; Whitaker and Captain, 2004) differs from Indochinese D. pictus in the characters SUBC ($F_{1,25} = 17.7, P = 0.0003$), TAIL ($F_{1,25} = 30.0, P = 0.00003$), EYED ($F_{1,35} = 22.5, P = 0.00003$), SUBL (Mann-Whitney U, $Z = −4.6, N=40, P < 0.00001$) and STRIPE1 ($\chi^2_1 = 42, P < 0.00001$).

They also compared the 9 individuals of D. ashoki with 48 individuals of D. proarchos, and found statistically significant ($p < 0.05$) differences in five morphological characters: VENT (number of ventrals), SUBC (number of subcaudals), TAIL (tail-length), EYED (horizontal eye diameter), SUBL (number of infralabials touched by the first sublabial, left and right added) and STRIPE1 (ventrolateral stripe bright/dull/absent).

[The south Indian form] differs from D. proarchos in VENT ($F_{1,49} = 24.1, P = 0.00001$), SUBC ($F_{1,28} = 13.7, P=0.001$), TAIL ($F_{1,28} = 40.1, P < 0.00001$), EYED ($F_{1,47} = 16.8, P=0.0001$), SUBL (Mann-Whitney U, $Z = −4.7, N=52, P < 0.00001$), and STRIPE1 ($\chi^2_1 = 53.0, P < 0.00001$).

The authors didn't rely just on morphological evidence: they note that D. ashoki, which is found in the Western Ghats, is also geographically isolated from other populations of D. pictus, which are found in Northeast India, Myanmar and South East Asia. Therefore, they concluded (emphasis mine):

The distinct morphology and coloration as well as strong geographic isolation of the population from the Western Ghats imply a unique evolutionary history that does not allow application of either the name D. pictus or the name D. proarchos. Consequently, we describe this form, previously referred to as D. pictus, as a new species, D. ashoki sp. nov., endemic to the Western Ghats.

How do we know that D. girii is a distinct species?

For this information, we need to look up another paper published that same year, Vogel and van Rooijen, 2011b. Interestingly, the authors don't compare this snake with D. ashoki at all, but with Dendrelaphis bifrenalis:

Due to their double loreal shield, D. girii sp. nov. and D. bifrenalis occupy a unique position within the genus. A congeneric species with a double loreal shield has been described in the past, namely Dendrelaphis biloreatus (Wall, 1908). However, the presence of a double loreal in D. biloreatus appears to be based either on an anomalous specimen or on a misjudgement (Vogel & Van Rooijen, 2011b). Furthermore, D. biloreatus differs from D. girii sp. nov. and D. bifrenalis in its number of dorsal scale rows (13) and its number of ventral scales (190-199).

There are several differences between Dendrelaphis girii sp. nov and D. bifrenalis. The most obvious one is the light ventrolateral line, which is present in D. bifrenalis and missing or very faint in Dendrelaphis girii sp. nov. (see Fig. 3, 4).

The postocular stripe is much broader in D. bifrenalis. In D. bifrenalis there are black oblique bars in the neck region of some, but not all specimens (8 specimens out of 12 in our data) (see Fig. 3-5). This pattern is not sex related. It is absent in D. girii sp. nov. For Dendrelaphis girii sp. nov. we had only 1 male, so it is not possible to compare the sexual dimorphism in this character. The snout of Dendrelaphis girii sp. nov. is much broader than that of D. bifrenalis. Dendrelaphis girii sp. nov. has, on average, fewer subcaudals and more ventrals than D. bifrenalis, but the ranges broadly overlap.

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  • $\begingroup$ Thanks Gaurav. But to play the devils advocate suppose I analyzed 9 individuals of the Eskimo tribe and another 34 belonging to the Kalahari bushmen I'm sure we could find a statistically significant morphological difference in at least five characteristics. e.g. Hair Color, Eye Color, Heights etc. Would that make the bushmen a distinct species from the Eskimos? Why not? Also, we wouldn't be relying on purely morphological evidence. The Eskimos are found in regions of the world geographically isolated from the parts where the Bushmen are found. $\endgroup$ – curious_cat Mar 4 '17 at 4:04
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    $\begingroup$ Even shorter, if perhaps overly cynical, explanation: if you've discovered a new species (or can 'split' an existing species), you can get a paper published. Might even get grants if your new species are now rare enough to be considered endangered. $\endgroup$ – jamesqf Mar 4 '17 at 4:59
  • $\begingroup$ @jamesqf Actually, that's exactly the sort of explanation my reading is pointing to. But I just want to get other perspectives on things. Maybe I (and you) are overly cynical & we are missing something? I am no expert on this. $\endgroup$ – curious_cat Mar 4 '17 at 5:39
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    $\begingroup$ I've asked a related question here: biology.stackexchange.com/questions/56945/… $\endgroup$ – curious_cat Mar 4 '17 at 5:39
  • $\begingroup$ @curious_cat You could, but "Eskimos" (e.g. the Iñupiat) have only been separated from the Bushmen (San people) for around 15,000 years; D. bifrenalis and D. pictus are different enough that several species can be found in between, and are likely separated by millions of years of evolution -- so in some way these 5/6 snake characters are more revealing of evolutionary-scale patterns than human populations. IF that is your ultimate goal. $\endgroup$ – Gaurav Mar 4 '17 at 5:45

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