How do you tell one species of turtle from another?

It was reported today that a New Tortoise Species In The Galapagos Islands Went Unidentified For More Than A Century, based upon the recently published paper Description of a New Galapagos Giant Tortoise Species (Chelonoidis; Testudines: Testudinidae) from Cerro Fatal on Santa Cruz Island. What was thought to be a single species of turtle is actually two different species.The species identification was performed using DNA analysis, but I'd like to understand how they were able to tell from DNA samples alone that they were different species. Yes, they identified different evolutionary lineages, but do they actually know that this new species of turtle does not and perhaps cannot breed with the old? I mean, even if one turtles' DNA was made of ooblick, if they could freely interbreed it would mean they're the same species, but I could not find in the paper where they ever address this issue ( I did not carefully read the paper - I confess to scanning it and doing a number of searches for word fragments like "inter", "breed", "egg", "fertil", etc. -). Or are they perhaps using some DNA bookkeeping criteria for determining species that ignores interfertility of populations?I put this here in Coffee House because it isn't related to creation/evolution, and because it's really just a question, though probably not a simple one.--Percy

This is very interesting. It tells me that at least in some realms of biology they might not know the degree of genetic interfertility when they make the determination of different species. These two turtle species could, as far as the scientists know, be 100% genetically interfertile, with the low introgression due to other factors like geography, behaviors, etc.I expected that genetic interfertility would have counted for more. I'm a little surprised. But maybe it makes sense. It must be hard to measure objectively and accurately. And whether the DNA molecules of two separate populations continue to play nicely together must include large elements of chance, so maybe it's reasonable that genetic interfertility not play a significant role.Thanks.--Percy

Thanks. That's even more detailed and makes it more clear.This is still sort of beside the point, but it still doesn't feel right to me to ignore genetic interfertility. Say we had some standard measure of genetic interfertility. Say we took 100 sperm of one species and 100 eggs of another. Then we mixed one sperm and one egg and see if we achieved fertilization. We repeat this for the remaining 99 sperm and eggs. Then we do the same again, but this time with 100 sperm of the other species and 100 eggs of the other other species. From this we get a measure of genetic interfertility. Or imagine any other way you like of obtaining this number, the point is that we develop a way of measuring the genetic interfertility between two different species.So we've classified C. chathamensis and C. phantastica as two different species, but if we don't know to what degree they're genetically interfertile, how much sense does it make to say that?--Percy

Sure, the species problem, as NoNukes reminds us.So interfertility is one way of looking at species, as in a population. A population, a group of mutually interbreeding organisms, is all the same species. And the overalll population of tortoises on Santa Cruz island is mutually interbreeding (unfortunately, to some unknown extent), and therefore they're all the same species. By that way of looking at it.But by the genetic way of looking at it, they're different species.These two ways (and more) of defining species have been discussed here before, but that article seemed to be saying, "They *are* two different species," and hence my question, which I might rephrase like this: When did genetics win out as *the* determining factor in determining species?It didn't, of course, and even though they're geneticists writing from a genetics perspective it still doesn't seem right that they should completely ignore interfertility.--Percy

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. Moving back and forth between threads discussing hypothetical scenarios and threads discussing the real world can result in a cognitive whiplash.But beyond that, even after I read through all the details of genetic differences in the paper about the Santa Cruz tortoises (again, in the interest of full disclosure, I've only skimmed through much of it) I'm still left wondering what threshold of degree of difference in the two turtle populations was surpassed? We get diagrams like this chock full of information: These provide a strong impression of how different the proposed species are, but what are the objective criteria for the amount of difference that makes a different species? Looking at the paper's Discussion section, a key part of their rationale for the Santa Cruz tortoises being two different species involves genetic distance, but they qualify that approach by saying, "Using genetic distance values for species delimitation is not a practice we ascribe to because of its several flaws." But they use it anyway because "the two Santa Cruz taxa are as divergent for both types of genetic markers as other named Giant Galapagos tortoise species." Well, okay, but how good is the scientific support for the classification as separate species for those other Giant Galapagos tortoise populations?--Percy