The natural minor scale simply conforms to its key signature. Any sharps or flats in a natural minor scale are those found in the key signature.
Historically, when musicians began using chords to harmonize a minor key melody, they soon developed a preference for the major V chord. In A minor, for example, they liked the sound of an E major chord, which has a G sharp, rather than the E minor chord, which has a G natural. Notice, however, that this G sharp isn’t in the key signature. Theorists, rule bound pendants that they are, decided to account for this oft used sharp. So they hit on the harmonic minor scale. In harmonic minor the seventh degree is sharped, which accounts for the sharped third in a V chord. (For example, the G sharp in the E major V chord in A minor.) Because this sharp is for harmonic reasons, hence the name “harmonic minor.”
Now we have two minor keys: natural minor, and harmonic minor. And that’s where it might have stayed. But then singers, those trouble making prima donnas, got into the act.
Try singing a harmonic minor scale. You’ll find the augmented second between the sixth and seventh degrees is awkward to sing. So singers began sharping the sixth degree to smooth out this awkward interval. It worked so well that sharping the six and seventh degrees of a natural minor scale became a thing. And theorists, doing their fussy thing, concocted a third scale: “melodic minor.” It’s called melodic minor because the sharped sixth degree is for melodic smoothness.
So why, you may ask, are the sixth and seventh scale degrees sharped when ascending, but revert to natural minor when descending? It’s a good question with a good answer. Play a descending A melodic minor scale in which the seventh and sixth degrees are still sharped (A, G#, F#, E, D, C, B, A). What does it sound like? From A through D it’s an A major scale. It doesn’t sound minor until you hit the C, when the scale is almost finished. Obviously it makes no sense for a minor scale to sound minor when ascending but major when descending. So singers instinctively reverted to natural minor when descending.
So there you have it: three different minor scales, each with its own justification. This is why music students the world over will gladly practice major scales, but run screaming into the night at the prospect of minor scales.
Be sure to hug a music theorist today.
South Euclid, OH