But if the species adapted to Alola, why wouldn't the other members have adapted as well? Just like in real life, when animals start to adapt in a new environment, typically all of the members do, otherwise the species would die out there.
Because they are on a different island with different areas? You really think ice vulpix is gonna be anywhere without ice? I am NOT saying that this PROVES anything. But if we want to get all sciencey, then, yeah, you would have several kinds of animals adapting it different climates/biomes/niches. Look at Oricorio and Darwin's finches... Or the several kinds of squirrel, deers, wild dogs and cats, foxes, snakes and crocodiles, etc...
If there is a swamp on one island, and no where else, then if something adapted to live in a swamp it isn't going to have the SAME swamp variation in a non swamp place. I'm not even talking Pokémon just in general. Like, yes, I'd imagine that ice vulpixes that didn't adapt to the cold on that particular island would die out with no other alternative. But we also have animals like foxes and big cats that adapted to different areas. We don't only have snow leopards and then all the other big cats died out on the planet because they couldn't adapt to snow. They adapted to their area.
Alola has a lot of variety in its idlands. In real life hawaii, there isn't that much diversity. But Pokémon regions are often trying to mimic multiple biomes in one place. So we get desserts, and snow cities, and forests, and swamps, and beaches, and lakes, and valleys and mountains ...
Like yes we can apply real world logic but we also have to apply game logic. Which kind sometimes spits in the face of actual logic. Like Alolan Raichu and his pancakes.
Certainly doesn't prove or disprove alolan forms and regular forms appearing, but it's a circular argument. No one can win here cause no one is wrong or right at the same time. Pokémon is gonna mix real science, with video game science, with pure marketability and profit. So they loosely follow real world logic, but also their own video game canon, etc.