Taxonomy
The genus Cannabis was formerly placed with nettles in the family Urticaceae or with mulberries in the family Moraceae, but is now considered along with hops (Humulus sp.) to belong to the family Cannabaceae. All strains of Cannabis can interbreed, and produce fertile offspring, which means that all known Cannabis plants satisfy one criterion for a single species type called Cannabis sativa L. .[please verify the credibility of this source] But whether the different strains of Cannabis actually do constitute a single species or multiple species has been a contentious issue for more than two centuries.

Classifications
Cannabis was first classified by Carolus Linnaeus in 1753 as a genus comprising a single species, called Cannabis sativa, describing hemp, which was widely cultivated at the time. In 1785, famous French biologist Jean-Baptiste Lamarck described C. indica, a type of Cannabis from India having morphological distinctiveness from the C. sativa previously described, and in particular not suitable for use as hemp because of poor fiber quality, but having greater potential as an inebriant. Numerous additional species were proposed in the 19th century, but by the beginning of the 20th century, the single-species concept was widely accepted. , except within the Soviet Union, where Cannabis continued to be the subject of active taxonomic study.

In 1924, Janischevsky described the low-intoxicant type of wild (ruderal) Cannabis found in central Russia as either a species or a variety, proposing C. ruderalis or C. sativa var. ruderalis as alternative names. In 1929, Vavilov described wild populations of drug-type Cannabis as the variiety C. indica var kafiristanica, and recognized the wild populations previously described by Janischevsky as the variety C. sativa L subsp sativa var. spontanea Vav. In 1940, Serebriakova and Sizov proposed a classification of Cannabis recognizing C. sativa L and C. indica Lam. as distinct species, describing "common hemp" and "Indian hemp" respectively. Within common hemp, they recognized cultivated and wild populatins as distinct subspecies, C. sativa var. culta and C. sativa var spontanea. They additionally recognized a total of 17 varieties within C. sativa, including 4 distinct groups within the cultivated subspecies, but did not describe variation within C. indica. In the late 1960's and 1970's, the question of scientific classification of Cannabis took on legal importance in North America. Laws prohibiting Cannabis in the United States and Canada specifically named products of Cannabis sativa L. as the prohibited substances. Several persons charged with violating these laws claimed that the material was in fact derived from C. indica or C. ruderalis, and was therefore not prohibited by the laws. Legal defenses on this basis were not often successful. In the early 1970's, botanists Richard E. Schultes and Loran Anderson conducted taxonomic studies of Cannabis, and concluded that sufficient evidence exists to support recognition of three species, C. sativa L, C. indica Lam., and C. ruderalis Janisch. For Schultes, this was a reversal of his previous opinion that Cannabis is a monotypic genus. According to their species descriptions, C. sativa is tall and laxly branched with relatively narrow leaflets, C. indica is shorter, conical in shape, and has relatively wide leaflets, and C. ruderalis is short, branchless, and grows wild in central Asia. This concept was embraced by Cannabis aficionados who commonly distinguish narrow-leafleted "sativa" drug strains from wide-leafleted "indica" drug strains.
In 1976, Ernest Small and Arthur Cronquist proposed a classification comprising a single species Cannabis sativa L, with two subspecies: C. sativa L subsp. sativa and C. sativa L subsp. indica (Lam). According to this concept, C. sativa subsp. sativa was selected for traits that enhance fiber or seed production and has low levels of the psychoactive delta-9-tetrahydrocannabinol (THC), whereas C. sativa subsp. indica was primarily selected for drug production and has relatively high levels of THC. Within these subspecies they described C. sativa L subsp. sativa var. spontanea (Vav.) as a wild or escaped type of low-intoxicant Cannabis, and described C. sativa L subsp. indica var. kafiristanica (Vav.) as a wild or escaped variety of the high-intoxicant type. . This classification was based on several factors, including interfertility, chromosome uniformity, systematic chemotype analysis, and numerical analysis of diagnostic characters. . Ernest Small was awarded the G. M. Cooley Prize by the American Association of Plant Taxonomists for his work involving Cannabis.

Ongoing research

Molecular analytical techniques developed in the late twentieth century are being brought to bear on questions of scientific classification. This has resulted in many reclassifications based on evolutionary systematics. Scientific classification of Cannabis has been investigated using molecular biology and genetic techniques. Several groups conducted genetic analyses of RAPD markers among drug and fiber cultivars. These analyses showed an extremely high degree of genetic polymorphism between and within populations, suggesting a very high degree of potential variation, even in heavily selected cultivars. Long-time Cannabis researcher EPM de Meijer described these analyses as confirming the continuity of the gene pool throughout the studied accessions, and as further confirmation that the genus comprises a single species. In 2004, Karl W. Hillig, then a postgraduate student at Indiana University teamed with Cannabis expert Paul Mahlberg to conduct a chemotaxomic analysis of Cannabis, using analytical chemistry of cannabinoid content to determine allele frequencies within studied populations. They concluded that observed chemotypes support recognition of C. sativa and C. indica as distinct species, but concluded that recognition of C. ruderalis was not supported but rather was consistent with classification of C. ruderalis as C. sativa var spontanea Vav. The authors categorized fiber/seed landraces and feral populations from Europe, central Asia, and Asia Minor in C. sativa., and categorized narrow-leaflet drug (NLD), wide-leaflet drug (WLD) cultivars, southern and eastern Asian hemp cultivars, and feral Himalayan populations as C. indica. In 2005, Hillig published another paper based on the same research, with additional statistical analyses, and this time proposed a 3-species classification, recognizing C. sativa, C. indica, and C. ruderalis. In his doctoral dissertation, published the same year, Hillig stated that principal components analysis failed to differentiate the putative species, but that canonical variates analysis resulted in a high degree of discrimination of the putative species and infraspecific taxa. Hillig stated that the patterns of variation support a two-species concept, but not recognition of C. ruderalis as a separate species from C. sativa. His conclusion was that taxonomic revision of Cannabis is warranted, but further research is needed to substantiate his proposed taxonomic treatment. As of 2006, the single-species concept of Cannabis continues to be widely accepted.