The terms Tonic and Dominant refer to the the 1st and Fifth tones in the diatonic scale--and the scales and chords that are based on them--Classical music uses major and minor scales (not really but...) and a harmonic system that is based on the intervals in what pretty much works out to be the tempered scale--Harmonies music of the sort that we know, as well as the circle of fifths (or fourths, depending on which way you want to go around) will not work in modal music...neither will that tempered scale convention that allows transposition of anything melody or harmonic part into any of the chromatic keys--
Peter T--you yourself restricted this discussion, in the opening post, to the use of these so called modes--now your bringing in all this speculation about"if someone from a non-" stick to the subject, or all hope is lost!!!!
Let's restrict our conversation to contemporary use of modes--because the rest of this discussion is just going to confuse everything--
Let's assume that we are in the key of C, and that the words mode and scal are interchangeable--A Dorian mode simply would be D-E-F-G-A-B-C-D. the D major scale would be D-E-F#-G-A-B-C#-D--The Dominant scale would be G-A-B-C-D-E-F-G--when you play over a vamp of |Dm7-G7|. you can wail your little heart our using either the dominant or the dorian--even though this is in the key of C-if you try to play a C-scale, it won't sound that good--
The obvious question is, when the notes are the same, what is the difference? The difference is betweeen where you start, move toward, and end-which would be the fundamental and the fifth, in each scale--
The Dm, and the Dorian scale are part of the Dominant harmony, so your ear fits them together with the G7,as unresolved sounding--