User blog comment:WhiteArmor/Anything Espada/@comment-1755153-20100413164925

Well, I think that before Aizen the Espada were Baraggan's chosen, the eilte but still loyal.

Once Aizen showed up, he showed his power to the super powerful but unloyal guys so they'd respect him,

and then made the Espada that were already there privaron and the ones he chose the new Espada.

and using the newly aquired Hogyoku, he sent his new army out looking for Vasto Lordes to make more new Espada. And with a lot of "dead weight" Aizen killed off a few privarons.

But as for how they're ranked, I think it has to do with the most powerful (kind of a lame and obvious guess but that's just how I see it)