@ Phil: As you seem to have worked out, the F precedes the C in the fifths sequence, and in resolving progressions you typically move "left" (backwards to the way the fifths sequence is usually layed out, going from flat keys to sharp keys). In other words, you resolve "down" a fifth (= up a fourth) rather than "up" a fifth.

And while the iii-vi-ii-V-I fifths progression, or some subsection of it, is most prominent in harmony, the IV-vii(dim)-iii part of the progression is seldom used. Note particularly that IV-vii(dim) would move by a tritone rather than by a perfect fifth. So IV typically moves to some other chord (not a resolution, per se). The earlier stages of the fifths progression are, in effect, V-I progressions, but temporarily modulated to a different tonic base and relative mode. The farther you get from I, the weaker is the V-I pull (being a modulation of a modulation of a modulation etc.) So while V mostly moves to I, and ii or II mostly move to V, it's harder to predict where a vi chord might be headed.


@ leeneia: The ascending and descending variants of the melodic minor have to do with the "pull" of the tonic. When leading to the tonic, the major seventh is pulled more strongly than the minor seventh, so when ascending melodically, the seventh was modified to a major seventh, but only when followed by the tonic; when instead it is followed by the sixth or some other note, it tends to remain minor. But the raising of the seventh leaves a gap of an augmented second between the sixth and seventh, so when leading to the tonic both the sixth and seventh may be raised, producing the ascending melodic minor pattern. In other words, it's not a hard and fast rule that one raises one or both of the sixth and seventh in ascending melodic passages, and lowers them descending; it's just a tendency that is strongest when the tonic is the target.

Historically (and Jack Campin will surely correct me), I believe this alteration grew out of the extension of the modes to Aeolian, and occurred much earlier than the Romantic era--even before the Elizabethan. The original modes in liturgical use were Dorian, Mixolydian and Phrygian--surprisingly, Ionian and Aeolian, the bases of our now ubiquitous major and minor modes, were later extensions when each note of the common underlying scale pattern was considered as a tonic base rather than a central range point or a "final". Dorian--the primary minor mode until the development of Aeolian--already had a major sixth, so the occasional raising of the seventh in ascending lines was a relatively small adjustment to make there, and was quite common even from early times.

I hardly think the age of modes has passed. Much rock and country music is based on Mixolydian mode, while blues is based on a decidedly modal scale. Riffing in all of these is often restricted to just a pentatonic subset. Fusion with various ethnic traditions keeps reinvigorating our modal mix. Modal purity may be a thing of the past, with frequent chromatic alterations and modulations to different keys or relative modes, but modal music lives on strong. In traditional music, I gravitate to modal tunes as a refreshing break from the unrelentingly predictable major/minor songs. To each his own. But back to "why does this work in minor?"...