my thoughts at the moment
Well, I think that you can approach this one of two ways (or maybe even a combination of the two).
First, I could see you selecting vast areas of expertise that are mostly occupied by one species. Such as in D&D, where the halflings are the nomadic thieves, but with a modern twist. For example, you could say that halflings, taking their nomadic roots, and their stereotypical thievery, have set themselves up as a series of competing, global, criminal organizations (The Mob, the Yakuza, etc.).
Second, you can just say that, while each species is basically separate, they have evolved much the way that the various human "races" have done. While there is "inter-racial" breeding, they mostly keep within the same species, with some exceptions based and family, and region. For example, in America, instead of the Europeans invading the Native Americans, it was the European Humans invading the American Elves (who are themselves variants of the Asian Elves). This would require templates for each of the various inter-species variations (though this could work with just feats, such as "Dwarven Heritage" on an elf, they'd be slightly shorter, stockier, and have advanced low-light vision, or short-ranged darkvision. A good example of this is the Half-Elf, which would use the "Elven Heritage" feat, which grants +1 to Listen, Search, and Spot, Secret Door Sense, etc.).
Though, you'd have to change these examples for your world.