It is impossible for a mammal to be more closely related to a reptile than to all other mammals. Unless evolution works in a completely different way on other planets (which is almost impossible), the different species of Xindi are not closely related. A Xindi primate and a Xindi insectoid would be about as closely related as you are to a praying Mantis.
Unless the terms 'primate', 'reptilian' and 'insectoid' are basically meaningless human-centric descriptions that concern only their superficial characteristics. Without looking deeper at more internal characteristics, it would be just as easy to declare that whales and dolphins clearly can't be closely related to elephants and gorillas - but it wouldn't be entirely accurate.
Not unless you have no comprehension of what "insect" means scientifically compared to "reptile" or "mammal".
Your understanding of those terms doesn't guarantee that the people describing the Xindi as insectoid or primate are using the terms literally.
There are several problems with the Xindi. But also with evolution in Star Trek in general. It is deterministic, it produces abundant intelligent life, mostly humanoid. But, what is the issue? That they are related? Or that they are evolved? To their relation: Just from a statistical DNA view-> If they are from the same planet, the should have some 40-60% similarity. If we add the percentage of similarity that exist between advanced humanoids (the fact that different alien species are genetically compatible, think on all those hybrids) we could come up with a greater number. If we take a look on the point that the Xindi evolved on the same planet: Yes, their different environments could be the answer-> Xindi-Aquatics - sea Xindi-Arboreals - ??? Xindi-Avians - ??? Xindi-Insectoids - ??? (rain forests, some place humid) Xindi-Primates - ??? Xindi-Reptilians - warm regions As most races in Star Trek have a small drive to expand, this could be similar in the Xindi and they did not have the opportunity to wipe each other in their Neanderthal stage, but only met each other in their Antiquity, when they were on equal footing. We should also note that they did not have to have evolved, but could have been uplifted by some old race...
Yes thank you I did manage to grasp the gist of the story. I just thought a pocket of them would be found
I'd say it's not realistic. I was peeved when Phlox said the replitian and primate as closely related as we are to the neanderthals. We look very similar to the neanderthals...
The problem is that these writers aren't even high school biology teachers. They don't know what they're talking about, half the time ... especially when it comes to how closely related primates, reptiles and insects are. They just include what "sounds good," and figure "what the balls, if this isn't hard science?" It's all make-believe, folks.
The Reptilian Xindi looks an awful lot like the Primate Xindi. The skin's different and there's some minor anatomy changes, but, heck, black wolves and bichons frise are the same species, and they barely share much in appearance besides ``quadruped animal''.
The fact remains that the different species of Xindi have differences in their anatomy and biology that are just too far different for them to be closely related. We see no closely related species on Earth that are so wildly different. As I said before, it would be impossible for the different species of Xindi to be closely related unless evolution worked in a very different way on Xindus. And, as I also said, that is practically impossible.
It's far more likely than a transporter accident is two cause two radically different species to reconstitute into one stable hybrid with the union of their memories and skills. Come on, this is Star Trek! In the Star Trek universe, a black hole has the gravity of several solar masses even if the mass of the black hole is only a few ounces. You could argue that a good deal of the genes they have in common make up the prefrontal cortex and the upright skeletal structure. Maybe reptilian is just the human translation of a Xindi word and the similarities to earth reptiles are superficial. Also maybe the aquatics and insectoids are farther removed genetically than the other three.
No. They don't. Not even similar enough to be in the same genus. You mentioned those "minor anatomy changes". They shouldn't all occur within the same genus. If you shaved both wolves and Bichons, they'd look a lot alike. Bichons are just morbidly inbred dwarf wolves with genes that encourage wooly white hair and floppy ears. Floppy ears are often found in domesticated animals.
I nevertheless feel confident in saying that if one shaved off the outer later of latex appliqués one would see compelling anatomical similarities between a Xindi-Reptilian and a Xindi-Primate.