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Regional forms and evolutions are a result of a specific species changing and adapting over time to better survive in its new habitat. For example, Alolan Exeggutor are the original exeggutor, so Kantonian ones are just what they're like in less tropical climates.
Those two aren't regional forms because they specifically are not connected to the other species they look like. Toedscool is a mushroom-based species that just happened to end up looking like Tentacool and Wiglett is an eel-based species that just happened to end up looking like Diglett.
The idea of this IRL to the degree they look similar is a little absurd, but think of it like how bats and whales both developed echolocation. Evolution can lead down similar paths even in vastly different circumstances.
