Pokemon quietly revealed another new addition to its Pokedex coming as a part of gen nine this week, Wiglett. Its name and appearance led many Pokemon fans to believe the long white Pokemon is the Paldean variant of Diglett. Apparently, that is not the case, and someone has used real-life science to explain why the two are unrelated despite their similar appearance.
The key here is something called convergent evolution. Not related to the instant evolution you will have experienced in Pokemon games, but the gradual version that takes place over thousands and millions of years in the real world. Sdrey has drawn up a diagram showing how Pokemon variants happen, and how they differ when compared to convergent evolution, posting the explanation on Reddit.
Convergent evolution begins with two entirely different ancestors who just happen to live in a similar environment, have similar ecological roles, and therefore follow similar evolutionary paths. That's presumably what has happened to Diglett and Wiglett. Both appear to live underground, love to dig, and hence they have become similar-looking Pokemon despite descending from different creatures. That's why even though they look the same, they aren't linked at all.
Now compare that to how regional variants of Pokemon work. Sdrey has used Meowth as an example. Alolan Meowth is fancier, well-kept, and closer to a real-life cat than a regular Meowth. You also have Galarian Meowth, a rougher and scruffier version of the Pokemon due to its more savage surroundings in Galar. Different versions of the same Pokemon as they are all a part of the Meowth species descended from the same regular form of the Pokemon.
Hopefully that clears up why Diglett and Wiglett aren't connected even though they look alike. Convergent evolution can also be used to explain why so many Pokemon look like Pikachu despite a lack of connection between them and the original electric mouse. Wiglett joins the gen nine roster alongside the likes of painting Pokemon Grafaiai and Fidough, a dog Pokemon made out of bread.