I guess the conclusion to draw from this short thread is that one must keep clearly in mind how vague a term is species. Not only is there the species problem where science attempts to describe the real and messy world, but many of our discussions focus on an idealized conception of species where we concoct scenarios that evolve them in unambiguous directions of our own invention. The reality is very messy.
And there is the problem of "lumpers" and "splitters". The authors are clearly in the later category. My impression from my initial read through is that this is a pretty well done paper, but I am not convinced that they should name this population of tortoises as a new species. My major contention is they have no distinguishing morphological characters by which to distinguish this population; genetic distance is not sufficient in and of itself to establish a new species.
Based on the phylogeny they presented, I would group this population with
C. chathamensis from San Cristobal Is. Unless they can establish some morphological or reproductive reason why they don't belong to that group, they are closely enough related to call them the same species. They are not so isolated geographically that it would not be expected that a population could have migrated from San Cristobal to Santa Cruz.
Their whole discussion centers around the idea that the two populations on Santa Cruz are two distinct species, which I agree with. However, I don't see where they address the idea that the Santa Cruz population might be the same species as the San Cristobal population (
C. chathamensis). IMO, they need a morphological study that demonstrates that Santa Cruz population is significantly different morphologically from the San Cristobal population.
I'm still left wondering what threshold of degree of difference in the two turtle populations was surpassed? ... what are the objective criteria for the amount of difference that makes a different species?
That's the problem with using genetic distance to separate species; there can be no standard threshold that applies across all situations. It depends on the genes chosen and the histories of the populations in question. They make the case that this population is genetically distinct, but that is different than being a different species. There is no objective criteria that can establish how far apart species should be genetically.
Well, okay, but how good is the scientific support for the classification as separate species for those other Giant Galapagos tortoise populations?
They could probably reduce the number of species to 8 rather than 15, but again, morphology needs to come into play here.
But you're right, this shows just how difficult and contentious defining species is. Everyone wants to name a new species, right?
HBD
Edited by herebedragons, : correction
Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for... I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca
"Nothing is easier than to persuade people who want to be persuaded and already believe." - another Petrarca gem.
Ignorance is a most formidable opponent rivaled only by arrogance; but when the two join forces, one is all but invincible.
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