The “introduced species” link is helpful, thanks! A bit anthropocentric, though, defining “indigenous” as “whatever was there before we got there”, much like white people defined humans around the world as “indigenous” even though they had obviously migrated there from somewhere else once upon a time, with a common humanity origin from somewhere in Africa :)
I think you’re arguing with yourself though, stating on the one hand you must save species from extinction, while also stating that natural selection is an important part of our reality. It’s not clear to me that there is any real justification for favoring an arbitrarily defined “indigenous” species against an “alien” one, or protecting species from going extinct, other than our own hedonistic pleasure of seeing ourselves as “good” people. I know folks make the the claim that there might be some sort of magic cure in some genetic sequence, but in that case, the only important part is to sequence the genome of the species before it dies out, rather than preserving it for all time.
I guess it all depends on whether you believe people are a part of nature, or apart from nature :)