I saw a very good evolutionary explanation for "irreducibly complex" systems a few weeks ago: First, a structure arises from a combination of simple underlying pieces -- irreducible but not complex. After later mutation/selection, additional pieces/functions are piled "on top of" the simple underlying pieces, superseding some of those underlying pieces; the structure becomes complex but is no longer irreducible. Still later, mutation/selection knocks out one or more of the superseded underlying bits -- now, suddenly, the whole structure is irreducibly complex. I wish there were more people out there who could devote their time to explaining these sorts of things as clearly as that.