Modal Jam Theory
When the plain symmetry of the tritone interval is intentionally avoided... the spiral symmetry of the golden ratio (and its reciprocal) is revealed as the catalyst for modulation.
When we are improvising with the tones of 'C' Ionian (melody) --- the harmony we are hearing (considering only the dominant [most audible] overtone) is the tone of the perfect fifth interval above each note. These are the tones of the 'Key Of The Dominant' - 'G' Ionian. This means there is an 'F#' in the natural (Dominant) harmony of the 'C' Major Scale. This 'F#' is the result of the dominant overtone (perfect fifth) of the 'leading tone' in 'C' Ionian: 'B.'
Considering the 'Harmony' as a 'palette' of tones (rather than a series of ascending or descending notes) -- might we - just as well - call this 'Harmonic Scenario' by the name of 'C' Lydian?
Now - we can see that 'C' Lydian is the natural harmony of 'C' Ionian.
We will return to the idea of these two scales being identified as the first modal pair... but before we explore how the natural overtone series can lend itself to concepts of natural modulation -- we'll make a general distinction between traditional 'Diatonic Harmony' and 'Modal Jam Theory.'