This is not about cannabis but it shows how a low genetic diversity can be retained for thousands of years.
"It has long been suspected that some grape varieties grown today, particularly well-known types like Pinot Noir, have an exact genetic match with plants grown 2,000 years ago or more, but until now there has been no way of genetically testing an uninterrupted genetic lineage of that age.
Dr. Nathan Wales, from the University of York, said: "From our sample of grape seeds we found 18 distinct genetic signatures, including one set of genetically identical seeds from two Roman sites separated by more than 600km, and dating back 2,000 years ago."
One archaeological grape seed excavated from a medieval site in Orléans in central France was genetically identical to Savagnin Blanc.
This means the variety has grown for at least 900 years as cuttings from just one ancestral plant.
https://phys.org/news/2019-06-ancient-dna-roman-medieval-grape.html
cf. Phylloxera, which wiped out the French vineyards in the 1850s, got most of the way around Europe and as far as Chile
it even reappeared in the 1980s
https://www.smithsonianmag.com/smar...st-wiped-out-frances-wine-industry-180954619/