As odd as it may be, the ABRSM (Associated Board of the Royal Schools of Music) do not teach the natural minor as such. It is taught in so far as the deviation from it to reach the harmonic and melodic forms, but not covered particularly itself. As I say, I'm not suggesting that the natural minor does not exist, just that for the purposes of this series at this point, any minor key will be harmonic or melodic.

Having been scratching my head to find a reason why this may be, the best I can come up with is that perhaps it helps a student starting out to identify the key a piece of music may be in... major vs relative minor....