Remnants of the earlier sheep breeds tend to have lower-quality wool, and this suggests that the secondary dispersal of sheep was associated with the beginning of the exploitation of the sheep's fur, rather than only its meat, resulting in the popularity of the new breeds.
It will be extremely exciting to track this dispersal archaeologically. We ought to have a few diagnostic SNPs of the various breeds of all the major domesticated animals very soon, which would be relatively easy to amplify in archaeological bone samples.
From the paper:
Collectively, the data we obtained indicate that relicts of the first migrations are still present in the Mouflon of Sardinia, Corsica, and Cyprus and in breeds in peripheral north European areas.
The homogeneous retrotypes (R2 only, or both R2 and R4) that we observed in the sheep of modern-day Turkey, Iran, Saudi Arabia, Syria, Israel, and Egypt, combined with available archaeological evidence, suggest that selection of domestic sheep with the desired secondary characteristics common to the modern breeds occurred first in Southwest Asia and then spread successfully into Europe and Africa, and the rest of Asia. [...] The primitive breeds survived the second migrations of improved breeds from Southwest Asia by returning to a feral or semiferal state in islands without predators or by occupying inaccessible areas less prone to commercial exchanges and associated introgression.
Our study also provides genetic evidence supporting the anecdotal origin of some less common sheep breeds. For example, one of the 10 populations analyzed from the British Isles, the Jacob sheep, displayed a homogeneous R2 retrotype very different from that of the other British populations and more similar to that of the southwestern Asiatic and African breeds [...] Our study also firmly links the Soay sheep with the Mediterranean and Asiatic Mouflon.
Revealing the History of Sheep Domestication Using Retrovirus Integrations
Bernardo Chessa et al.
The domestication of livestock represented a crucial step in human history. By using endogenous retroviruses as genetic markers, we found that sheep differentiated on the basis of their "retrotype" and morphological traits dispersed across Eurasia and Africa via separate migratory episodes. Relicts of the first migrations include the Mouflon, as well as breeds previously recognized as "primitive" on the basis of their morphology, such as the Orkney, Soay, and the Nordic short-tailed sheep now confined to the periphery of northwest Europe. A later migratory episode, involving sheep with improved production traits, shaped the great majority of present-day breeds. The ability to differentiate genetically primitive sheep from more modern breeds provides valuable insights into the history of sheep domestication.