The pokedex entry for Oshawott specially mentions it being from Unova, a detail that would be somewhat odd if it was already native to the region
This Pokémon from the Unova region uses the shell on its belly as a weapon to cut down its foes. Thus, I’ve conferred upon this shell the name “scalchop.”
A similar thing happens with the entry for Cyndaquil
Hails from the Johto region. Though usually curled into a ball due to its timid disposition, it harbors tremendous firepower.
The pokedex entry for Hisui Samurott mentions that the form is not common, which could be hinting at how the new starters having a very small population in Hisui, which would be evidence towards them being in Hisui due to the distortions rather than being naturally found in Hisui.
Hard of heart and deft of blade, this rare form of Samurott is a product of the Pokémon’s evolution in the region of Hisui. Its turbulent blows crash into foes like ceaseless pounding waves.
The entry for Dartix explicitly mentions it struggling with the cold climate of Hisui, which would make more sense if the species only recently moved to the region.
Regularly basks in sunlight to gather power—presumably due to the frigid climate. Nonetheless, the edges of the blade quills set into its wings are keen as ever.
While all three entries for the final evolutions of the starters mentions them being due to the influence of the environment of Hisui, it is never explicitly stated that this is due to natural selection
The air stored inside the rachises of Decidueye’s feathers insulates the Pokémon against Hisui’s extreme cold. This is firm proof that evolution can be influenced by environment.
The usage of the word evolution in this context could be referencing more so to the evolution mechanic than the process of evolution the word refers to normally.
The reasoning given for how the environment of Hisui causes Hisui Typhlosion feel more fitting for the possibly that these new forms are odd occurrences rather than the result of adaptations to the environment.
Said to purify lost, forsaken souls with its flames and guide them to the afterlife. I believe its form has been influenced by the energy of the sacred mountain towering at Hisui’s center.
along with the example of other regional variants, there are some other Pokémon that have evolutions caused by being in different environmental conditions
The evolutions of Magnazone, Probopass, and Vikavolt are caused by exposure to magnetic field found in certain areas
this method feels similar to how Hisui Typhlosion was described, just with a different phenomenon causing the evolution
Eevee evolving into Glaceon or Leafeon by being close to either the ice rock or moss rock.
The evolution of Crabominable by being in a cold area