"I'd say it's less a question of "has the species undergone evolution?" & more "how rigidly ingrained is their new behavior?" Because they very clearly have adopted new traits as a result of selective benefits & pressures.
To which I'd say the question is kind of moot because there aren't enough around for there to be any kind of statistical inference, but it feels to me as though Ran & Shaw could be independent if they wanted to, but the dragons seen acting as mounts were pretty thoroughly domesticated.
Of course, there could be a lot of reasons for that. Perhaps young dragons are very susceptible to training. Perhaps egg temperature influences a hatchling's behavior. Perhaps there really is that much difference between those generations, in terms of genes. Certainly, if not natural selection, artificial selection is known to be capable of producing diverse "breeds" of animals in a relatively short period of time."
This sounds right and considering your argument I guess that dragons could be compared with dogs, they can hunt for food themselves, but considering the way they have been brought up they instead wait to be fed by us humans.
"Yeah I meant the specifics of how he got control of the dragon he had in Book 3 of Korra. I don't think the Dragons would give up a baby dragon. And they lived with the Sun Warriors. It could be quite an awesome story. Why did Zuko go back to the Sun Warriors? Are there more than three dragons in the world? Are their wild dragons? It could have been interesting to see.
They used Zuko a lot for Fan service, but I wish we would have gotten a few more answers"
I don't really know how Zuko got his dragon, but no matter how he got it I still think that he didn't take it forcefully, but receive it as a gift and if it was Ran and Shaw's baby, then I think that Zuko either had their "permission" or that they didn't care that he was allowed to raise their baby (like some animal mothers such as cats, dogs or horses).
That other wild dragons exist is probably quite likely since the Avatar world is quite big, and if the Fire Nation didn't manage to find the Sun Warrior tribe, then they most likely did not manage to find all the dragons.
As for why Zuko visited the Sun Warriors, it was probably a quite special moment in his life when he learned how to firebend in the original way, so that is probably a good reason for him to revisit the Sun Warriors. Except for that, he might also want to thank the Sun Warriors for allowing him to learn the "proper" way to firebend and I recently got this idea that Zuko, during some point in his life, had the intention to reveal the Sun Warrior culture for the rest of the Fire Nation (when they are ready) to allow all of its citizens to learn about their cultural heritage.