Dorian mode is often considered the "fourth minor scale" in music theory. While it is technically a mode of the major scale, it can be used effectively as a minor scale in certain contexts.
First, let's review the three traditional minor scales: natural minor, harmonic minor, and melodic minor. Each of these scales has a flattened third, which is what gives them their "minor" quality. However, they differ in the sixth and seventh degrees of the scale.
Natural minor has a flattened sixth and seventh, which gives it a distinct sound and is the basis for minor key signatures. Harmonic minor has a flattened sixth and an unaltered seventh, which creates a leading tone and allows for stronger resolution to the tonic. Melodic minor has an unaltered sixth and seventh.
So, where does Dorian mode fit in? Dorian mode has a flattened third, like all minor scales, but it has an unaltered sixth and a flattened seventh. While Dorian mode is technically the second mode of the major scale, it can be used as a minor scale in certain contexts, like when mi7 is used as the tonic. This can be found in modal tunes like "So What". In this case it makes much more sense to think of Dorian Mode in the tonic key, rather than as chord ii.
In conclusion, while Dorian mode is technically a mode of the major scale, it can be used effectively as a minor scale in certain contexts. Its unique sound, created by a natural sixth and flattened seventh, sets it apart from the other minor scales and can be a useful tool for adding variety and interest to a piece of music.