Artificial selection
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Artificial selection, also called selective breeding, is selection in which people decide which plants or animals leave offspring, whether or not they mean to change the population. Its mechanism is the mechanism of natural selection: heritable variants differ in reproductive success, so their allele frequencies change from one generation to the next. The two differ in the reason why some individuals breed and others do not.[1] The term covers every way of choosing parents, of which mass selection is one, and the changes that sowing, harvesting and storing seed bring about without anyone intending them.[2]
In Cannabis artificial selection has favoured tall, unbranched stems in hemp and branched plants with large inflorescences in drug-type cultivars, with opposite effects on THC. Robert C. Clarke and Mark D. Merlin judged that human selection during domestication "has had by far the greatest influence on changes in Cannabis phenotypes".[3]: 299–301, 303 How much human selection a landrace has received is disputed.
Deliberate and unconscious selection
Charles Darwin wrote of man's "powers of artificial selection" in 1859.[4] He kept and bred fancy pigeons himself and drew on the methodical selection of their breeders.[1] He described breeders who "try by methodical selection, with a distinct object in view, to make a new strain", and a second kind, "which may be called Unconscious", that comes from everyone trying to own and breed from the best animals. A man keeping pointers breeds from his best dogs with "no wish or expectation of permanently altering the breed", and over centuries the breed still changes.[5] In 1868 he defined unconscious selection as what follows from "preserving the most valued and destroying the less valued individuals, without any thought of altering the breed". He added that it "graduates into methodical, and only extreme cases can be distinctly separated".[6] Later writers call the first kind deliberate, conscious or intentional and the second automatic or unintentional. Most early domestication is thought to have gone by the second.[1][2]
Alfred Russel Wallace objected to Darwin's comparison on the ground that artificial selection needs an intelligent selector and nature has none. Darwin held that unconscious selection did not differ from natural selection, since people who change the conditions in which a crop lives and reproduces impose selection on it with no choice or goal in mind.[2] The historian Bert Theunissen argues that Darwin made the comparison work by playing down crossing and inbreeding, which many breeders of his day thought essential.[7]
Seed that stays on a cannabis plant goes home with the gatherer and seed that drops does not, so people gathering seed for food or sowing selected against shattering without intending to. Cultivators store seed through the winter, which removes the advantage of delayed germination. In densely sown hemp, late seedlings are crowded out by earlier ones, which favours fast, even germination.[3]: 298 One act can carry several effects. Roman farmers who kept big cereal grains for sowing were also keeping grains from healthy, tall and late plants. Anton C. Zeven reads this as unconscious selection for all four characters.[8]: 136
Ways of choosing parents
Darwin wrote that "Man can act only on external and visible characters".[9] In cannabis, potency and aroma can be judged by the senses, which let early farmers raise THC by sowing seed from the strongest plants. Cannabidiol is not psychoactive, so breeding for it depends on laboratory analysis.[3]: 301 Sieved hashish in Afghanistan was collected from whole populations, which left little room for choosing single plants on potency.[3]: 302 Most useful products come from female plants, which makes favourable traits hard to recognise in a male, and selection on economic traits is usually limited to females.[3]: 299, 303
| Method | Basis of the choice | Use in cannabis |
|---|---|---|
| Mass selection | A plant's own phenotype; the seed of chosen plants is sown together | Early hemp and drug landraces. Dioecious European hemp cultivars such as 'Carmagnola' and 'Kompolti', with unsuitable males destroyed before they shed pollen.[3]: 304–305 |
| Family selection | The performance of a plant's offspring | Monoecious hemp: each chosen F1 female is judged by growing out her offspring, and favourable lines are continued.[3]: 305 |
| Selfing with selection | A plant's phenotype over generations of selfing, ending in an inbred line | Single-cannabinoid medical cultivars: chosen females are made to form pollen-bearing flowers and selfed for several generations. Separate lines are then crossed to restore vigour.[3]: 305 |
| Clonal selection | One plant's phenotype, with no further sexual reproduction | Drug cultivars, multiplied by cuttings from plants with the wanted traits.[10] |
| Marker-assisted and genomic selection | A plant's genotype at markers linked to a trait, or a breeding value predicted from markers across the genome | These methods can raise the gain from selection. Genomic selection may serve complex traits in cannabis once marker density and population sizes allow.[10][11] |
Choosing the pollen parent changes the size of the gain. Selection after pollination acts only on the seed parents and gives half the expected gain of selecting both.[12] Hemp offered an early exception to selection on females alone. In work published in 1924, G. Bredemann removed the male plants with low fibre yield before they flowered. The fibre content rose from about 12 per cent to about 24 per cent, on the figures his team gave in 1956.[13]
Response over many generations
The response to selection in one generation is the selection differential multiplied by the narrow-sense heritability.[14] Its behaviour over many generations has been tested at length in maize. In 1896 C.G. Hopkins began selecting one open-pollinated variety in two directions for the share of oil in the kernel, in work continued at the University of Illinois. Each generation the highest or lowest 20 per cent of ears became the parents. By generation 89 oil had moved from 4.7 per cent to 19.3 per cent in the high line and to 1.1 per cent in the low line, where selection stopped because of poor viability and a content too low to measure accurately. The high line showed no sign of a plateau when the results were published in 2004.[15] A cross between the two lines revealed about 50 quantitative trait loci with small, largely additive effects. Their large number accounts for the smooth and sustained response. About a fifth had effects opposite to the direction of selection, which fits hitchhiking and small populations.[15]
Selected populations change in other characters too. Darwin wrote that a man who goes on selecting one peculiarity "will almost certainly unconsciously modify other parts of the structure".[16] The Illinois populations differ in a range of correlated traits and the yield of the high-oil line is poor.[17][15] Selecting for the biggest maize ear each year may also select plants with few stems.[8]: 136 In cannabis, hybrids between narrow-leaflet and broad-leaflet drug plants were selected for THC, early flowering and large, dense inflorescences. Those inflorescences hold moisture and suffer from grey mould (Botrytis cinerea), and the broad-leaflet founders brought little resistance from their dry home climate.[3]: 297 Natural selection can also stop the response before the additive variance is used up, and genetic drift in small breeding populations removes that variance.[18]
Several cannabis populations have lost selected characters when selection stopped or slackened:
- Crops of monoecious hemp include up to 20 per cent true males and females by the third generation from elite seed. Farmers buy new sowing seed each year.[3]: 305
- Chinese hemp in the United States lost fibre quality and yield when farmers multiplied it at random. About 1900 it became relatively uniform after seed production was confined to a few isolated farms under breeders.[3]: 304
- Drug-type seed sown and crossed repeatedly without conscious selection in North America in the 1970s and 1980s lowered the average potency and aroma of domestic crops.[3]: 310
Marks in the genome
Selection for particular alleles leaves less diversity at the selected genes than at others, and genome scans use this to look for its targets, the selective sweeps.[19] Among 774 genes of maize, domesticated from the wild grass teosinte, Stephen Wright and colleagues found evidence of artificial selection in 2 to 4 per cent. The rest kept the signature of a population bottleneck during domestication.[20] Across 75 genomes of wild, landrace and improved maize, selection appeared stronger during domestication than during later breeding, with mean selection coefficients of 0.015 and 0.003 in the regions the scans identified.[21] The "cost of domestication" hypothesis holds that domestication can raise the load of deleterious variants in a crop, which may limit later selection.[22]
In cannabis the scans have so far produced candidate genes. Comparing hemp-type and drug-type samples with the group they call Basal cannabis, Guangpeng Ren and colleagues identified 510 candidate genes in hemp-type plants and 689 in drug-type plants, 253 of them shared. Those in hemp included genes that inhibit branching. Those in drug-type plants included genes that promote branching. Against hemp, drug-type plants also showed selection on a gene of the pathway that supplies substrate for cannabinoid synthesis.[23] Nearly all the marijuana cultivars in the same study carried a complete THCA synthase gene with only pseudogene copies of CBDA synthase, and hemp the reverse. Since the two enzymes compete for one substrate, the authors read this as loss of function under strong selection and said the pattern would need confirming with phenotypic or expression data.[23][3]: 302 Some samples broke this pattern, and Ren and colleagues took that as evidence that synthase inheritance is independent of the other selected traits separating the two groups.[23] Patrick Woods and colleagues found positive selection in drug-type plants on a gene whose nearest match in Arabidopsis regulates trichome formation.[24] In the high-CBD cultivar CBDRx, Christopher Grassa and colleagues traced the cannabinoid ratio to a hemp-derived region of chromosome 7 with signs of recent selection. They suggested that breeders had produced it by introgression followed by artificial selection.[25]
Hemp
In 1956 Bredemann and colleagues wrote that breeding had changed hemp little in most of Europe, that its fibre content was usually around 10 per cent and that until very recently it had been "a wild plant cultivated by man". They credited about 25 years of crossing and selection with making it "a genuine cultivated plant". Monoecious strains and high-fibre strains each yielded poorly at first. The two were then combined by crossing.[13] Bast fibre content is highly heritable, and mass selection for it proved efficient in Dutch fibre hemp.[26] Hemp cultivars have been selected for total fibre yield. Exceptional cultivars such as 'Kompolti TC' can exceed 40 per cent of dry weight.[3]: 299
Selection for very low THC belongs to modern times.[3]: 300 Bredemann and colleagues named it in 1956 as a remaining problem for breeding, and proposed searching chemically for mutants lacking each compound of the resin, on the model of von Sengbusch's sweet lupins.[13] Clarke and Merlin consider that natural selection kept THC fairly low in hemp landraces of northern latitudes, and that modern European cultivars were then selected for very low total cannabinoids.[3]: 301 Most European cultivars descend from the same few single-plant selections.[3]: 310
Drug-type plants
Farmers growing narrow-leaflet drug landraces in Thailand, India, South Africa, Mexico, Colombia and Jamaica kept their varieties by sampling single plants and sowing seed of the most flavourful and potent. The landraces reached North America with fairly high THC and little CBD. Afghan broad-leaflet landraces, seldom selected plant by plant, were high in both.[3]: 302 Within a population of mixed chemotype, seed saved from plants at either end of the THC to CBD range and sown in isolation moves the offspring towards that end, and a few generations of this produce two distinct chemotypes.[3]: 301
From the early 1970s growers in North America removed male plants to raise sinsemilla, then began to keep a few chosen males and pollinate selected branches, which gave seed of known parentage.[3]: 308 Breeders have selected mainly for THC, aroma and flavour.[3]: 309 Commercial crops are now largely grown from cuttings. Clarke and Merlin hold that this, with crossing among close relatives and narrow selection for potency, has brought the drug gene pool to a genetic bottleneck.[3]: 310
Mean THC in cannabis plant material seized in the United States rose from about 4 per cent in 1995 to about 12 per cent in 2014, as production shifted from regular marijuana to sinsemilla.[27] These figures describe seized material and span a change in how the plant is grown, so they do not measure genetic change alone. Selection in general can receive credit for gains that come from better husbandry.[18]
Landraces
Sources differ on how much human selection a landrace receives. Reviews of cannabis breeding say that early landraces arose through mass selection.[3]: 304–305[10] Zeven, reviewing definitions across crops and without discussing cannabis, wrote both that "no, or only limited, human selection is carried out to maintain a landrace" and that human selection "has had a great influence on the development of landraces", though nature may often counteract it. He wrote that Banga was wrong to say a landrace receives no selection or only mild mass selection, and that intensity "will have varied from nil to quite strong selection. Unfortunately, little has been recorded."[8]: 129, 136–137
In the hemp landrace of Tai'an in Shandong, Clarke found in the early 1990s that farmers saved remnant seed each year and made no intentional selection, except possibly for large seed. He attributed the landrace's evolution almost entirely to unconscious selection by farmers and to natural factors.[28]: 61, 63 Narrow-leaflet drug landraces were chosen plant by plant on potency. Even there a chosen female has usually been pollinated by many unchosen males, which restores diversity among her offspring.[3]: 297, 302 Clarke and Merlin state that broad-leaflet landraces have generally not been selected for cannabinoid ratio and that natural selection acts on landraces constantly while cultivation favours the outcomes of human choice.[3]: 295, 301
See also
References
- ↑ 1.0 1.1 1.2 Gregory, T.R. (2009). "Artificial selection and domestication: modern lessons from Darwin's enduring analogy". Evolution: Education and Outreach. 2 (1): 5–27. doi:10.1007/s12052-008-0114-z.
- ↑ 2.0 2.1 2.2 Ross-Ibarra, J.; Morrell, P.L.; Gaut, B.S. (2007). "Plant domestication, a unique opportunity to identify the genetic basis of adaptation". Proceedings of the National Academy of Sciences. 104 (Suppl. 1): 8641–8648. doi:10.1073/pnas.0700643104.
- ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.20 3.21 3.22 3.23 3.24 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.
- ↑ Darwin, C. (1859). On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. London: John Murray. p. 109 – via Darwin Online.
- ↑ Darwin, C. (1859). On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. London: John Murray. pp. 34–35 – via Darwin Online.
- ↑ Darwin, C. (1868). The variation of animals and plants under domestication. Vol. 2. London: John Murray. p. 193 – via Darwin Online.
- ↑ Theunissen, B. (2012). "Darwin and his pigeons: the analogy between artificial and natural selection revisited". Journal of the History of Biology. 45 (2): 179–212. doi:10.1007/s10739-011-9310-8.
- ↑ 8.0 8.1 8.2 Zeven, A.C. (1998). "Landraces: a review of definitions and classifications". Euphytica. 104 (2): 127–139. doi:10.1023/A:1018683119237.
- ↑ Darwin, C. (1859). On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. London: John Murray. p. 83 – via Darwin Online.
- ↑ 10.0 10.1 10.2 Barcaccia, G.; Palumbo, F.; Scariolo, F.; Vannozzi, A.; Borin, M.; Bona, S. (2020). "Potentials and challenges of genomics for breeding cannabis cultivars". Frontiers in Plant Science. 11 573299. doi:10.3389/fpls.2020.573299.
- ↑ Hurgobin, B.; Tamiru-Oli, M.; Welling, M.T.; Doblin, M.S.; Bacic, A.; Whelan, J.; Lewsey, M.G. (2021). "Recent advances in Cannabis sativa genomics research". New Phytologist. 230 (1): 73–89. doi:10.1111/nph.17140.
- ↑ Fehr, W.R. (1991) [1987]. "Maximizing genetic improvement". Principles of cultivar development: Theory and technique. Vol. 1. Ames, Iowa: Walter R. Fehr. p. 239. ISBN 0-9635989-0-2.
- ↑ 13.0 13.1 13.2 Bredemann, G.; Schwanitz, Fr.; von Sengbusch, R. (1956). "Problems of modern hemp breeding, with particular reference to the breeding of varieties of hemp containing little or no hashish". Bulletin on Narcotics (3). United Nations: 31–35.
- ↑ Beavis, W.; Lamkey, K.; Mahama, A.A.; Suza, W. (2023). "Selection response". In Suza, W.P.; Lamkey, K.R. (eds.). Quantitative genetics for plant breeding. Iowa State University Digital Press. doi:10.31274/isdup.2023.146.
- ↑ 15.0 15.1 15.2 Laurie, C.C.; Chasalow, S.D.; LeDeaux, J.R.; McCarroll, R.; Bush, D.; Hauge, B.; et al. (2004). "The genetic architecture of response to long-term artificial selection for oil concentration in the maize kernel". Genetics. 168 (4): 2141–2155. doi:10.1534/genetics.104.029686.
- ↑ Darwin, C. (1859). On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. London: John Murray. pp. 11–12 – via Darwin Online.
- ↑ Moose, S.P.; Dudley, J.W.; Rocheford, T.R. (2004). "Maize selection passes the century mark: a unique resource for 21st century genomics". Trends in Plant Science. 9 (7): 358–364. doi:10.1016/j.tplants.2004.05.005.
- ↑ 18.0 18.1 Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. pp. 447–450.
- ↑ Yamasaki, M.; Tenaillon, M.I.; Vroh Bi, I.; Schroeder, S.G.; Sanchez-Villeda, H.; Doebley, J.F.; Gaut, B.S.; McMullen, M.D. (2005). "A large-scale screen for artificial selection in maize identifies candidate agronomic loci for domestication and crop improvement". The Plant Cell. 17 (11): 2859–2872. doi:10.1105/tpc.105.037242.
- ↑ Wright, S.I.; Bi, I.V.; Schroeder, S.G.; Yamasaki, M.; Doebley, J.F.; McMullen, M.D.; Gaut, B.S. (2005). "The effects of artificial selection on the maize genome". Science. 308 (5726): 1310–1314. doi:10.1126/science.1107891.
- ↑ Hufford, M.B.; Xu, X.; van Heerwaarden, J.; Pyhäjärvi, T.; Chia, J.M.; Cartwright, R.A.; et al. (2012). "Comparative population genomics of maize domestication and improvement". Nature Genetics. 44 (7): 808–811. doi:10.1038/ng.2309.
- ↑ Moyers, B.T.; Morrell, P.L.; McKay, J.K. (2018). "Genetic costs of domestication and improvement". Journal of Heredity. 109 (2): 103–116. doi:10.1093/jhered/esx069.
- ↑ 23.0 23.1 23.2 Ren, G.; Zhang, X.; Li, Y.; Ridout, K.; Serrano-Serrano, M.L.; Yang, Y.; Liu, A.; Ravikanth, G.; Nawaz, M.A.; Mumtaz, A.S.; Salamin, N.; Fumagalli, L. (2021). "Large-scale whole-genome resequencing unravels the domestication history of Cannabis sativa". Science Advances. 7 (29) eabg2286. doi:10.1126/sciadv.abg2286.
- ↑ Woods, P.; Price, N.; Matthews, P.; McKay, J.K. (2023). "Genome-wide polymorphism and genic selection in feral and domesticated lineages of Cannabis sativa". G3. 13 (2) jkac209. doi:10.1093/g3journal/jkac209.
- ↑ Grassa, C.J.; Weiblen, G.D.; Wenger, J.P.; Dabney, C.; et al. (2021). "A new Cannabis genome assembly associates elevated cannabidiol (CBD) with hemp introgressed into marijuana". New Phytologist. 230 (4): 1665–1679. doi:10.1111/nph.17243. PMC 8248131.
- ↑ Hennink, S. (1994). "Optimisation of breeding for agronomic traits in fibre hemp (Cannabis sativa L.) by study of parent-offspring relationships". Euphytica. 78 (1–2): 69–76. doi:10.1007/BF00021399.
- ↑ ElSohly, Mahmoud A.; Mehmedic, Zlatko; Foster, Susan; Gon, Chandrani; Chandra, Suman; Church, James C. (April 2016). "Changes in cannabis potency over the last 2 decades (1995–2014): analysis of current data in the United States". Biological Psychiatry. 79 (7): 613–619. doi:10.1016/j.biopsych.2016.01.004. PMID 26903403.
- ↑ Clarke, Robert C. (1995). "Hemp (Cannabis sativa L.) cultivation in the Tai'an District of Shandong Province, Peoples Republic of China" (PDF). Journal of the International Hemp Association. 2 (2): 57, 60–65 – via Vote Hemp.