How do you tell one species of turtle from another?

Hi, Percy. It's basically this. If you look at Fig. 2A, you can see that the two populations from Santa Cruz island (the orange and red blocks) resolve at very different locations on the phylogenetic tree.They're apparently not even sister groups: the Cerro Fatal population's is most closely related to a population from Cristobal Island, which is considered a separate species (Chelonoidis chathamensis); and the La Reserva population is most closely related to a population from Fernandina Island, which is also considered a separate species (C. phantastica).When you classify species by genetic analysis, the only real criterion is consistency: if the Cristobal and Fernandina populations are considered separate species, then, to be consistent, these two Santa Cruz populations should also be considered separate species. The alternative is to reshuffle all the currently-accepted species into an alternative arrangement (for example, looking at Fig 2A, you could argue that there are really only 3-4 different "primary" lineages), but elevating one population to a new species is less work and less confusion than trying to reassign everything.

Hi, Percy. It's just a matter of deciding whether the product you get from using rigorous, experimentally-derived standards like interfertility would be substantially more useful and informative than a product based on something less intellectually pure but more logistically viable.Ultimately, a species designation is a hypothesis about the population's evolutionary history and genetic isolatoin. Experimentation to test those hypotheses would be the intellectually pure thing to do, but when you consider how resource-intensive it would be, and the high likelihood that it will only be really useful to a very narrow range of academics, you can kind of guess that it's a low priority.