All credits due to Robert Clark, bought his Marijuana Botany book at 14, thanks Tattered Cover :) (Screw the 'grow bibles ,ect' if you spend money on a good weed book, make sure Robert Clarke is in the author spot. Perhaps for the more science oriented though)
"Geographical Distribution of Cannabinoid Phenotypes:
Utilizing GC, Samll and Beckstead (1973b) analyzed 350 sample of Cannabis from police seizures and germplasm collections representing approximately 50 countries that were grown outdoors during the summer of 1971 at Ottawa, Canada. Graphs of d9-THC percentage versus CBD percentage produced "three large modally distinct groups with respect to cannabinoid content."
Type 1 samples contained larger amounts of THC (>0.3 percent) and limited amounts of CBD (<0.5 percent). Type 1 samples typically originated from south of 30 degrees north latitude. Type 2 varieties contained relatively high levels of both THC (>0.3 percent) and CBD (>0.5 percent), and Type 3 varieties contained very little THC (<0.3 percent). Types 2 and 3 usually originated north of 30 degrees north latitude....
They (Small and Beckstead) attributed the difference in varieties to origin north or south of 30 degrees north latitude and to selection for fiber and seed varieties at northern latitudes and drug varieties at southern latitudes.
....
North America: North American self-sown popluations have the lowest chemotype ratios (THC etc, to CBD etc.) since varieties were originally introduced for fiber use. NI.H was introduced from W.Europe in early 17th century and BLH from China in the late 19th century. This accounts for the similarity between cannabinoid ratios for N.America and those for W.Europe and China and reflects the primary uses for Cannabis in these regions for fiber and seed."
Cannabis: Evolution and Ethnobotany
By Robert Clarke, Mark Merlin
Sooo...It seems that is may be more Human induced trait than anything. Just how long we been growing this shit?
Or read and answer for yourself.
http://books.google.com/books?id=poenY6QMq8UC&printsec=frontcover&dq=cannabis evolution and ethnobotany&hl=en&sa=X&ei=uDBxU_mPAcGhyASRo4C4DA&ved=0CDYQ6AEwAA#v=onepage&q=cannabis evolution and ethnobotany&f=false