Tallis' Spem in Alium is an example of complex sacred polyphonic music that was designed to fit the architecture of the place of its first performance. So it has a score, and from the evidence of both the score and what is known about the palace where it was first performed (and its intended audience) there is good reason to see it as something other than improvised. It's a highly formal composition of music-for-space.
Relevance to wiki: polyphonic music, whether scripted or improvised, religious or secular, has something to say about the ways in which call-and-response is a way of thinking together.
See also: yoiking.
Wikipedia has this to say:
The motet is laid out for eight choirs of five voices (soprano, alto, tenor, baritone and bass). It is most likely that Tallis intended his singers to stand in a horseshoe shape. Beginning with a single voice from the first choir, other voices join in imitation, each in turn falling silent as the music moves around the eight choirs. All forty voices enter simultaneously for a few bars, and then the pattern of the opening is reversed with the music passing from choir eight to choir one. There is another brief full section, after which the choirs sing in antiphonal pairs, throwing the sound across the space between them. Finally all voices join for the culmination of the work. Though composed in imitative style and occasionally homophonic, its individual vocal lines act quite freely within its fairly simple harmonic framework, allowing for an astonishing number of individual musical ideas to be sung during its ten-to-twelve minute performance time. The work is a study in contrasts: the individual voices sing and are silent in turns, sometimes alone, sometimes in choirs, sometimes calling and answering, sometimes all together, so that, far from being a monotonous mess, the work is continually presenting new ideas.
Whenever it's performed, it changes again. Is it reasonable to think of these changes as forks?
YOUTUBE iT-ZAAi4UQQ The Tallis Scholars, found on YouTube.
One of its most startling forks: in 2012 it was mentioned in Fifty Shades of Grey. So after 600+ years of specialist appreciation, it's found a sudden new following.