Blogger Abbie Smith at ERV has the story: “Bah bah black sheep, have you any ERVs?”.
The gist of it is that ERVs are viral genomes which read themselves into an individual’s DNA. If the infected cells are in the eggs or sperm, the ERV is passed on to all descendants, who have the same virus at a unique location in their chromosome.
ERVs mutate rapidly, which makes it possible to trace different lineages of the same original ERV and put them into a tree diagram showing who broke off first and who’s descended from the breakaway group vs. the original group.
The evidence thus gathered confirms historical evidence about the domestication of sheep in southwest Asia in two waves. In some remote places, farmers kept on breeding the older types. The details are interesting and the explanation is amusing.