1.8 Gene Flow Among Domesticates
and Ruderal Populations
Male plants ofC. sativaproduce prodigious amounts of pollen (Faegri et al. 1989 ),
which can be present in very large amounts in the atmosphere in regions where the
species occurs (Stokes et al. 2000 ; Small and Antle 2003 ), and can be carried by
wind for hundreds of kilometers (Aboulaich et al. 2013 ). Indeed, because the pollen
ofCannabisspreads remarkably, an isolation distance of about 5 km is usually
recommended for generating pure-bred seed, exceeding the distance for virtually
every other crop (Small and Antle 2003 ). Because of widespread clandestine cul-
tivation, the pollen can be found, at least in small concentrations, over much of the
planet.
It is indisputable that genes areflowing from domesticates into wild populations.
The domesticated groups ofCannabisnoted previously are of Eurasian origin but,
especially in the last several hundred years, have been transported to and cultivated
in much of the world. In many regions they have escaped, re-evolved characteristics
suited to wild existence, and established as self-perpetuating populations outside of
cultivation. Because both domesticated and wildCannabispopulations are extre-
mely widespread, there are extensive opportunities to interbreed. Biotypes of
C. sativahave a common diploid chromosome number (2n= 20), possess no
biological barriers to interbreeding (Small 1972 ), and indeed the more genetically
different they are the more likely the hybrid progeny will exhibit heterosis, so
wild-growing and domesticated plants can easily produce viable progeny. Indeed,
in nature, onefinds a complete spectrum of intermediate forms, demonstrating
continuity of variation between wild and domesticated forms (Small 1975 ).
Fig. 1.8 Evolutionary relationships and geneflow patterns among the different genetically
distinctive kinds of hemp (non-intoxicatingCannabis sativa), the different genetically distinctive
kinds of marijuana, related weeds and presumed wild ancestral populations. (Compare Fig.1.9)
1 Classification ofCannabis sativaL. in Relation... 35