Genetic erosion
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Genetic erosion is the loss of genetic diversity from a crop, a population or a collection over time. The Food and Agriculture Organization defines it as "the loss of individual genes and the loss of particular combinations of genes" and notes a wider use of the term for the loss of varieties. It does not necessarily mean the extinction of a species or a population.[1]
Writers on Cannabis report heavy losses. Robert Clarke and Mark Merlin wrote in 2016 that most traditional landraces are no longer grown, naming prohibition, neglect and economic priorities among the causes.[2] Most reports of this kind concern varieties gone from fields or from stored collections. A variety can go out of cultivation while its alleles live on elsewhere, and a campaign can destroy plants without showing which alleles went with them, so neither by itself shows that alleles were lost. Surveys of Moroccan and Iranian populations have measured diversity by comparing populations with one another, which gives a level and no trend.[3][4]
Genes, varieties and crops
In its narrow sense the term refers to genes or alleles and in its wider sense to varieties. Diversity among alleles has two sides: richness (the number of alleles present) and evenness (their relative frequencies).[1] Mark van de Wouw and colleagues separated three levels at which erosion may occur: crop, variety and allele. They judged a fall in allelic evenness and richness the most useful definition, read together with events among varieties.[5]
Under genetic swamping the pure genome of a population ends, but its alleles and traits need not, since they can live on in the hybrids that replace it.[6] In the opposite case a variety keeps its name while it loses variation. Each regrowing of a stored accession from a limited number of plants may lower its allelic variation.[7] In a small population genetic drift removes alleles by chance, at a rate set by the effective population size.[8]
Otto Frankel wrote in 1970, of landraces that keep evolving in the field, that "the difficulty is to find the border line between 'adaptive change' and 'genetic erosion'".[9] Genetic vulnerability is a separate condition: a widely planted crop that is uniformly susceptible to a pest, a pathogen or an environmental hazard.[1]
Measuring a change over time
Erosion is a change over time, so a measure of it needs two dates. Collecting missions went through Albania in 1941 and 1993 and through southern Italy in 1950 and the 1980s. Hammer and colleagues compared the number of landraces still cultivated with the earlier number and put erosion at 72.4 per cent in Albania and 72.8 per cent in southern Italy.[10] Their figures count landraces.
Molecular studies compare samples of different dates. Flint maize landraces collected in central Italy in the 1950s were genotyped alongside those grown 50 years later, after hybrids had come into wide use. The later landraces carried some introgression from the hybrids, with no sign of genetic erosion.[11] A meta-analysis of 44 papers on varieties released by breeders in eight field crops found a fall of 6 per cent in the 1960s against the 1950s and no substantial long-run reduction in regional diversity over the 20th century.[12] Van de Wouw and colleagues took the likeliest pattern to be a fall when modern cultivars replace landraces, with no further fall once the replacement is complete.[5]
A project in nine developing countries found that crop diversity in farmers' fields had been maintained overall, while studies of pearl millet in Niger and sorghum in Cameroon reported genetic shifts. The Food and Agriculture Organization concluded in 2010 that general statements quantifying the overall amount of erosion over the previous decade were not warranted. Agreement was greater on erosion from the complete shift to modern production systems. No practical and generally accepted indicator was available.[1] A synthesis by Colin Khoury and colleagues found marked losses, maintenance and increases on farms, among modern cultivars, among crop wild relatives and in gene banks, varying with species, scale, region and method.[13]
Causes
Countries reporting to the Food and Agriculture Organization named these major causes: replacement of local varieties, land clearing, overexploitation, population pressures, environmental degradation, changing agricultural systems, overgrazing, inappropriate legislation and policy, and pests, diseases and weeds.[1] The mechanisms by which a small population loses alleles are treated under genetic drift and population bottleneck. Alleles arriving from outside are treated under gene flow and genetic contamination.
Climate change
Kristin Mercer and Hugo Perales wrote in 2010 that climate change will alter landraces kept in the field, through adaptive evolution on one side and the extinction of populations on the other. In their account farmers retire a landrace's seed for economic and social reasons in addition to its yield. For Mexican maize they suggested that the greatest erosion might come in the highlands, where farmers would discard populations unless those populations could evolve quickly.[9]
For cannabis, Anna Halpin-McCormick and colleagues modelled in 2025 where the plant could grow. The model used 137 photographed occurrences in the eastern hemisphere, of plants judged wild or escaped from cultivation. Averaged across two emission scenarios and the years 2050 and 2070, the area suitable for natural growth fell by about 43 per cent worldwide, from about 13.8 million to 7.8 million square kilometres. The fall was about 29 per cent in Asia and Russia, 15 per cent in Europe and 81 per cent in the United States. The authors held that conserving wild relatives and naturalised populations will be critical.[14] The projections rest on climate variables alone and sample no genes.
Reports of lost cannabis varieties
Clarke and Merlin wrote that "the majority of traditional fiber, seed, and drug landraces are no longer grown" and that many of those left have been diluted by crossing with introduced cultivars.[2]: 317 In the same review they wrote that "the vast majority of landraces may already be extinct".[2]: 318 They named prohibition, custodial neglect and economic priorities as routes of loss. They also quoted a list by Watson and Clarke from 1997: suppression and eradication, neglect by agricultural officials and industry, anti-hemp propaganda and the decline of hemp breeding and research.[2]: 317–318 These are the authors' judgements, given without a count.
Hemp cultivars
In Clarke and Merlin's account, Japanese hemp introduced to California was lost. They know of no North American hemp cultivar still alive, "only their feral offspring".[2]: 304 Most of the dioecious Italian cultivars, they write, "are now believed extinct".[2]: 306
The last legal hemp crops in the United States were planted in 1958. Escaped hemp has formed feral populations across regions where it was grown. Ademola Aina and colleagues genotyped 760 feral plants from twelve states and offered them as material for new hemp cultivars.[15] Ernest Small and Marcus held in 2002 that decades of eradication had "exterminated many of the naturalized populations in North America", which they called a natural genetic reservoir.[16]
Drug-type landraces
John McPartland and Small concluded in 2020 that Central and South Asian landraces "face extinction through introgressive hybridization". For landraces in Afghanistan, McPartland and Small cite a 2005 thesis calling them "disappearing".[17] Clarke and Merlin wrote that original narrow-leaflet landraces have become rare in Jamaica, Mexico, Morocco and Thailand.[2]: 324 The introductions of foreign seed that McPartland and Small describe are listed under genetic contamination.
Growers and seed sellers
ACE Seeds gives the reasons it considers pure varieties to be disappearing: the loss of their habitats, wars, the loss of older cultures, genetic contamination and, above all, prohibition.[18] Derg Corra Collective says that its main aim is to save old, rare or wild strains from disappearing.[19] Sellers have also taken up the names of past crops for their own lines, among them the Colombian names of the bonanza marimbera.[20] Whether a line on sale descends from the older population is the seller's claim.
Losses from stored collections
The hundreds of seed collections acquired for Small's studies were destroyed in 1980, because Canadian government policy then saw no prospect of hemp becoming a legitimate crop. Asked about the 56 United States Department of Agriculture hemp collections supplied to Small and Beckstead for their 1973 study, the department replied that it held no hemp collections and that any found would have to be destroyed.[16]
Matthew Welling and colleagues concluded in 2016 that the international narcotics conventions had been influential over more than 50 years in the "fractionation and erosion of publicly accessible Cannabis ex situ genetic resources".[7] At the Vavilov Institute, lack of funding is thought to have compromised regrowing and cost unique diversity.[7] Clarke and Merlin, citing a 2004 study by Hillig, wrote that low population sizes and incomplete isolation there brought "considerable loss of genetic diversity and purity".[2]: 318 Regrowing and the holdings of public collections are treated under ex situ conservation.
Prohibition and eradication as causes
Welling and colleagues called the status of cannabis as a source of an illicit drug the major factor in the erosion of its genetic resources, writing of stored collections and hemp breeding.[7] Plants destroyed in a campaign are counted as plants, and the count does not say which alleles went with them. Pierre-Arnaud Chouvy and Kenza Afsahi wrote that it is "not clear what role forced eradication might have played" in the turn to hybrids in the Rif Mountains, since it mostly took place outside the historic growing zone.[21] Afsahi and Salem Darwich wrote that eradication limited cultivation "for a short time but not in any sustainable way", naming the return of growing in Lebanon and the replacement of the local variety by hybrids in Morocco.[22]
McPartland and Small list among the uses of Central Asian genes in hybrids that they shorten the plants, which are then more easily concealed outdoors and grown indoors.[17] In Iran, where growing the plant carries strict legal penalties and no breeding programme exists, Aboozar Soorni and colleagues wrote that "the loss of this unique germplasm is of great concern".[4]
Clarke and Merlin also expected legal markets and monocropping to leave only the most commercially viable clones in production, narrowing the gene pool further.[2]: 317
Measured diversity in cannabis
Chouvy and Afsahi observed dozens of fields across the Rif in July 2013 and saw very little kif, an estimate they called a "guesstimate" based on visual observation.[21] Fatima Bachir and colleagues wrote in 2022 that the local varieties are becoming contaminated and rapidly disappearing.[23] Chaymae Benkirane and colleagues genotyped plants from four Moroccan regions at six microsatellite markers. The local population, which they call Beldia, showed higher diversity than the introduced cultivar Critical. The two belonged to different gene pools with gene flow between them. The authors called for protection from "potential genetic erosion".[3] Bachir and colleagues propose sampling herbarium material.[23] Chouvy holds that the kif landrace remains distinct and warns against strict conservation approaches that would fix it in place.[24]
In Iranian populations 93 to 96 per cent of the molecular variance lay within populations.[4] Several drug-type varieties in a microsatellite survey were clones of identical genotype.[25] Jason Sawler and colleagues warned in 2015 that the variation they described was in danger of being lost, given the few public hemp collections, the patchwork of private drug-type collections and the short life of stored seed.[26] Clarke and Merlin hold that clones and inbreeding have lowered the diversity of drug-type cannabis.[2]: 316 Welling and colleagues hold that domestication has likely reduced genetic and allelic diversity.[7] Ryan Lynch and colleagues, with 193 genomes, found global diversity still undersampled, with Asian material underrepresented.[27]
These surveys compare populations and varieties with one another, so each gives a level of diversity and no rate of loss. The design used for maize in central Italy and barley in Sardinia, in which collections of two dates are genotyped together, is set out under genetic contamination.
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 Food and Agriculture Organization of the United Nations (2010). "The state of diversity" (PDF). The Second Report on the State of the World's Plant Genetic Resources for Food and Agriculture. Rome: Commission on Genetic Resources for Food and Agriculture, FAO. pp. 15–17. ISBN 978-92-5-106534-1.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 Clarke, Robert C.; Merlin, Mark D. (2016). "Cannabis domestication, breeding history, present-day genetic diversity, and future prospects". Critical Reviews in Plant Sciences. 35 (5–6): 293–327. doi:10.1080/07352689.2016.1267498.
- ↑ 3.0 3.1 Benkirane, C.; Charif, M.; Müller, C.M.; Taaifi, Y.; Mansouri, F.; et al. (2024). "Population structure and genetic diversity of Moroccan cannabis (Cannabis sativa L.) germplasm through simple sequence repeat (SSR) analysis". Genetic Resources and Crop Evolution. 71 (5): 2037–2051. doi:10.1007/s10722-023-01754-x.
- ↑ 4.0 4.1 4.2 Soorni, A.; Fatahi, R.; Haak, D.C.; Salami, S.A.; Bombarely, A. (2017). "Assessment of genetic diversity and population structure in Iranian Cannabis germplasm". Scientific Reports. 7 (1) 15668. doi:10.1038/s41598-017-15816-5.
- ↑ 5.0 5.1 van de Wouw, M.; Kik, C.; van Hintum, T.; van Treuren, R.; Visser, B. (2010). "Genetic erosion in crops: concept, research results and challenges". Plant Genetic Resources. 8 (1): 1–15. doi:10.1017/S1479262109990062.
- ↑ Todesco, M.; Pascual, M.A.; Owens, G.L.; Ostevik, K.L.; Moyers, B.T.; Hübner, S.; Heredia, S.M.; Hahn, M.A.; Caseys, C.; Bock, D.G.; Rieseberg, L.H. (2016). "Hybridization and extinction". Evolutionary Applications. 9 (7): 892–908. doi:10.1111/eva.12367. PMC 4947151.
- ↑ 7.0 7.1 7.2 7.3 7.4 Welling, M.T.; Shapter, T.; Rose, T.J.; Liu, L.; Stanger, R.; King, G.J. (2016). "A belated green revolution for Cannabis: virtual genetic resources to fast-track cultivar development". Frontiers in Plant Science. 7 1113. doi:10.3389/fpls.2016.01113.
- ↑ Davinack, D. (2024). Molecular Ecology & Evolution: An Introduction. Norton, Massachusetts: Wheaton College. pp. 4, 111.
- ↑ 9.0 9.1 Mercer, K.L.; Perales, H.R. (2010). "Evolutionary response of landraces to climate change in centers of crop diversity". Evolutionary Applications. 3 (5–6): 480–493. doi:10.1111/j.1752-4571.2010.00137.x. PMC 3352508.
- ↑ Hammer, K.; Knüpffer, H.; Xhuveli, L.; Perrino, P. (1996). "Estimating genetic erosion in landraces — two case studies". Genetic Resources and Crop Evolution. 43 (4): 329–336. doi:10.1007/BF00132952.
- ↑ Bitocchi, E.; Nanni, L.; Rossi, M.; Rau, D.; Bellucci, E.; Giardini, A.; Buonamici, A.; Vendramin, G.G.; Papa, R. (2009). "Introgression from modern hybrid varieties into landrace populations of maize (Zea mays ssp. mays L.) in central Italy". Molecular Ecology. 18 (4): 603–621. doi:10.1111/j.1365-294X.2008.04064.x.
- ↑ van de Wouw, M.; van Hintum, T.; Kik, C.; van Treuren, R.; Visser, B. (2010). "Genetic diversity trends in twentieth century crop cultivars: a meta analysis". Theoretical and Applied Genetics. 120 (6): 1241–1252. doi:10.1007/s00122-009-1252-6. PMC 2839474.
- ↑ Khoury, C.K.; Brush, S.; Costich, D.E.; Curry, H.A.; de Haan, S.; et al. (2022). "Crop genetic erosion: understanding and responding to loss of crop diversity". New Phytologist. 233 (1): 84–118. doi:10.1111/nph.17733.
- ↑ Halpin-McCormick, A.; Maaz, T.M.; Kantar, M.B.; Barton, K.E.; Masalia, R.R.; Batora, N.; Law, K.; Kuntz, E.J. (2025). "Species distribution of Cannabis sativa: past, present and future". PLOS ONE. 20 (3) e0306007. doi:10.1371/journal.pone.0306007. PMC 11906060.
- ↑ Aina, Ademola; Wenger, Jonathan P.; Stanton, Eliot; Majumdar, Chandrani Gon; ElSohly, Mahmoud; Weiblen, George D.; Ellison, Shelby (1 July 2025). "Genetic diversity, population structure, and cannabinoid variation in feral Cannabis sativa germplasm from the United States". Scientific Reports. 15 (1) 20423. doi:10.1038/s41598-025-07912-8. PMC 12215616.
- ↑ 16.0 16.1 Small, E.; Marcus, D. (2002). "Hemp: a new crop with new uses for North America". In Janick, J.; Whipkey, A. (eds.). Trends in New Crops and New Uses. Alexandria, Virginia: ASHS Press. pp. 284–326.
- ↑ 17.0 17.1 McPartland, J.M.; Small, E. (2020). "A classification of endangered high-THC cannabis (Cannabis sativa subsp. indica) domesticates and their wild relatives". PhytoKeys. 144: 81–112. doi:10.3897/phytokeys.144.46700.
- ↑ "About Us". ACE Seeds. Archived from the original on 23 February 2024. Retrieved 23 September 2026.
- ↑ "Derg Corra Collective". SeedFinder. Retrieved 8 October 2026.
- ↑ Moreno, Fidel. "Las aventuras de Gato y su misión marimbera". Cáñamo (in español). Retrieved 2 November 2026.
- ↑ 21.0 21.1 Chouvy, Pierre-Arnaud; Afsahi, Kenza (2014). "Hashish revival in Morocco". International Journal of Drug Policy. 25 (3): 416–423. doi:10.1016/j.drugpo.2014.01.001.
- ↑ Afsahi, Kenza; Darwich, Salem (May 2016). "Hashish in Morocco and Lebanon: A comparative study". International Journal of Drug Policy. 31: 190–198. doi:10.1016/j.drugpo.2016.02.024. PMID 27178873.
- ↑ 23.0 23.1 Bachir, Fatima; Eddouks, Mohamed; Arahou, Mohamed; Fekhaoui, Mohammed (2022). "Origin, Early History, Cultivation, and Characteristics of the Traditional Varieties of Moroccan Cannabis sativa L.". Cannabis and Cannabinoid Research. 7 (5): 603–615. doi:10.1089/can.2021.0020.
- ↑ Chouvy, Pierre-Arnaud (2024). "International property rights for Cannabis landraces and terroir products. The case of Moroccan Cannabis and hashish". International Journal of Drug Policy. 129 104479. doi:10.1016/j.drugpo.2024.104479.
- ↑ Dufresnes, C.; Jan, C.; Bienert, F.; Goudet, J.; Fumagalli, L. (2017). "Broad-scale genetic diversity of Cannabis for forensic applications". PLoS ONE. 12 (1) e0170522. doi:10.1371/journal.pone.0170522.
- ↑ Sawler, Jason; Stout, Jake M.; Gardner, Kyle M.; Hudson, Darryl; Vidmar, John; Butler, Laura; Page, Jonathan E.; Myles, Sean (2015). "The genetic structure of marijuana and hemp". PLOS ONE. 10 (8) e0133292. doi:10.1371/journal.pone.0133292.
- ↑ Lynch, R.C.; Padgitt-Cobb, L.K.; Garfinkel, A.R.; Knaus, B.J.; Hartwick, N.T.; et al. (2025). "Domesticated cannabinoid synthases amid a wild mosaic cannabis pangenome". Nature. 643 (8073): 1001–1010. doi:10.1038/s41586-025-09065-0. PMC 12286863.