This week, Pokemon stealthily unveiled Wiglett as a brand-new Pokemon to be included in generation 9. Many Pokemon fans mistakenly thought the extended white Pokemon was the Paldean version of Diglett due to its name and appearance. However, that is not the case, and real-world science has been utilized to explain why the two are unrelated despite looking identical.
Convergent evolution is an essential concept in this situation. Not to be confused with the quick evolution you may have seen in Pokemon video games, but rather the gradual process that occurs over thousands or even millions of years in the real world. Sdrey posted an explanation on Reddit, along with a diagram outlining how Pokemon variations differ from convergent evolution and how they arise.
Beginning with two completely different ancestors who just so happen to share a habitat, have comparable ecological roles, and have similar evolutionary histories, convergent evolution can occur. Diglett and Wiglett have likely experienced this. Despite deriving from separate organisms, they have similar looks since they both seem to reside underground and enjoy digging. Because of this, while having a similar appearance, they are not at all connected.
Now contrast that with how regional Pokemon variations operate. Sdrey gave the example of Meowth. An Alolan Meowth is more acceptable, cleaner, and more like a genuine cat than a standard Meowth. Galarian Meowth is another option, a rougher and dirtier variation of the Pokemon because of its harsher environment in Galar. They are all Meowth species members descended from the same ordinary form of the Pokemon. Hence they are all different variations of the same Pokemon.
That should explain why they are unrelated despite the fact that Diglett and Wiglett resemble one another. Another benefit of convergent evolution is that it can explain why so many Pokemon resemble Pikachu despite being unrelated to the original electric mouse. Wiglett enters the Gen 9 lineup alongside Pokemon like the bread-dog Fidough and the artist Pokemon Grafaiai.