I typed a pretty long explanation, but accidentally clicked outside of the text box and hit the delete button, changing my browser page and deleting all of what I had typed... so here's a summary of what I had written:
Arceus created the nature Pokemon (Dialga, Palkia, Kyogre, the cover legendaries), but had no Pokemon to inhabit its universe that wasn't one of a kind. Arceus then created Mew, containing a huge amount of DNA (maybe with a bit of DNA from the other legends, but probably not), to inhabit this universe. Arceus basically got bored with the lack of diversity and split Mew's DNA into all of the varieties of Pokemon that exist now, leaving just one (or however many) mew to stay as representative of the Pokemon origin.
It could also be that Mew represents the original molecular life on planet Earth that, over the course of millions of years, became the varied population that we have today.
It depends, really, on how long the Pokemon world existed. At least several million, according to the fossil Pokemon, but I'd say that for a realistically diverse Pokemon world, it would probably have taken several hundred million years for Mew to diversify into what we have today.
meow