The minor scale is commonly thought of as the major mode’s moodier, darker brother. As you learned from the article on Church Modes, what’s the easiest way to make the major scale darker? Just rotate through its pitches so that its root shifts up by a few fifths. As it turns out, the natural minor scale is the same series of notes as the Aeolian mode.
When composers started using tonality instead of modality – that is, a shift away from counterpoint using modes and toward a style of music revolving around scales and keys – the Aeolian mode brought some problems with it. The biggest of these problems was that composers liked the confidence of the resolution between scale degrees seven and one. That key-defining half-step pull was missing between the G and A. The rules of counterpoint and harmony that had developed until that point mandated that to reach a conclusion on the tonic pitch (the root of the scale), it had to be approached by half step from below and by whole step from above. The minor scale wasn’t compatible with that mandate, so they changed it. By raising the seventh scale degree, and in doing so leaving the key signature, composers gave birth to the harmonic minor scale.
This, however, left a huge gap between scale degrees six and seven. This augmented second was intolerable in melodies, which required smooth stepwise passage between pitches. A jump like that where there should have been a step was unacceptable. The solution was to raise the sixth scale degree, as well, so that passage upward from the fifth scale degree to the first was just a series of perfectly normal whole steps. This created the melodic minor scale, used only in upward motion. (The descending melodic minor scale is the same as the natural minor scale.)
Interestingly, this melodic minor differs from the major scale only by one note: the third scale degree. That one pitch is sufficient to anchor the tonality of any composition because the raised sixth and seventh scale degrees are only used for resolutions upward from the fifth scale degree to the first.