my attempt at the primary u.s. continuum would go something like this... (but please feel free to correct me i know relatively little compared to some, its as much to try to get discussion going as anything)
gospel / blues
early soul / rhythm & blues
motown / chess era
70s soul
funk / disco
hip-hop
electro
garage / house (im very hazy as to how exactly this stage went... ive always been intrigued, are there any good books about it yet?)
but the problem is that there are breakaways and then points where it may feed back in again... and really a lot of the time forms would co-exist and develop seperately... look at jazz, bebop, big band through to quincy jones then again into hip-hop.
also, there clearly has to be a separate (although with occassional intersections) thread of evolution which splits off from rhythm and blues, into rock n roll, into rock, into the myriad forms of guitar based music. but that also has a relationship with americana, bluegrass, country and folk, which fed into that strain in the 60s and beyond