Natural selection
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Natural selection is the change in the genetic make-up of a population that follows when individuals of different genotypes differ in survival, in fertility or in both, so that some leave more offspring than others.[1] Charles Darwin gave the process its name in 1859 "in order to mark its relation to man's power of selection".[2] In a cultivated crop natural and human selection act on the same plants. The grower chooses the seed parents, as in mass selection; survival and fertility under the conditions where the plants grow select among them too. The two can favour different plants.[3]
Selection is one of the processes that change allele frequencies from one generation to the next, with mutation, gene flow and genetic drift.[4] In Cannabis it has been studied in the plant's response to day length across latitudes, in the seed traits of plants growing outside cultivation and in hemp that escaped from farms in the United States and has reproduced on its own since the 1940s.[5][6][7]
Natural and human selection
Darwin set out the principle in On the Origin of Species. A variation that helps an individual in its relations with other organisms and with its surroundings tends to preserve that individual and is generally inherited by its offspring, which then have a better chance of surviving, since only a small share of those born can survive. He set this beside the selection practised by breeders, who adapt plants and animals to their own uses by accumulating slight useful variations.[2] In population genetics the term has a narrow and a broad sense. Narrowly it covers every case in which viability or fertility depends on genotype. Broadly it is the main force that makes evolution adaptive.[1]
A breeder shapes a crop to human requirements, while natural selection acts on survival and fertility. Among the parents chosen, fertility may be lowest in those that look best to the breeder, and among their offspring those that best fit the breeder's aim may survive least well. Natural selection then works against artificial selection and can halt the response to it before the genetic variation is used up. A character that has not been under strong artificial selection is more likely to be held at intermediate values by stabilising selection than pushed in one direction. When the breeder stops selecting, natural selection can draw the character back towards its former value.[3]
Cannabis populations have been shaped by both kinds of selection.[8] In situ conservation of crops on farms, landraces among them, rests on the idea that populations kept in cultivation stay exposed to selection pressures, some arising in nature and some from farmers, which change with farming practice and with what people want from the crop. In a review of 2017, Mauricio Bellon and colleagues judged that this rationale had not been examined systematically and that its support was fragmented.[9]
Fitness and the pace of change
The fitness of a genotype is the expected number of offspring left by an individual of that genotype from birth: its chance of surviving to adulthood multiplied by the average number of offspring of a survivor.[1] Selection acts on differences in fitness between genotypes. If one genotype has a relative fitness of 1 + s against another, s is its selection coefficient. The time selection needs to raise an allele from a frequency of 0.01 to 0.99 is roughly inversely proportional to s: about 96 generations when s is 0.1 and about 924 when it is 0.01. In a very large population a small advantage still changes allele frequencies, only more slowly. Change is slow while the favoured allele is rare or nearly fixed and fastest at intermediate frequencies.[10]
A rare recessive allele changes in frequency very slowly, whether it is favoured or disfavoured.[11] A rare allele occurs mostly in heterozygotes, where a recessive allele has no effect on the phenotype.[12] For a measured character, whether selection brings a response in the next generation depends on how much of its variation is genetic and passed on, the character's heritability.[13]
Selection against drift and migration
While a new favourable allele exists in only a few copies, chance can remove it. In a large population an allele with a small advantage s survives this stage with a probability of about 2s: with s at 0.01, one new allele in 50 goes on to spread; with s at 0.1, about one in six. Once it has about 1/(2s) copies, its loss is very unlikely.[14] Whether selection outweighs genetic drift depends on the product of the population size N and the selection coefficient. Selection begins to have a marked effect when 4Ns exceeds about 1, which corresponds to at least one individual every two generations dying or failing to breed for genetic reasons at the locus. Where the population departs from the ideal model, its effective population size Ne takes the place of N.[15] In a population with separate sexes in unequal numbers, the effective size is set mainly by the scarcer sex. In a dioecious crop such as cannabis, a field left with few male plants therefore breeds as a small population.[16]
Migration makes neighbouring populations alike, while selection that differs from place to place keeps them apart. In the simplest model, a population favours one allele and each generation receives a fraction m of its members from a source where the other allele is fixed. The locally favoured allele is lost when m is as large as s. When s exceeds m it is kept, at a frequency of 1 − m/s.[17]
Selection that keeps variation
Some selection holds two or more alleles in a population. Where the heterozygote is fitter than both homozygotes, known as overdominance, each allele increases when it is rare, because it is then carried mostly by heterozygotes. Both alleles stay at a stable intermediate frequency, a protected polymorphism. With a fitness of 0.85 for one homozygote, 1 for the heterozygote and 0.70 for the other homozygote, the frequency of the first homozygote's allele settles at about 0.67 from any starting point between 0 and 1.[12]
Genome scans detect balancing selection from genes that carry unusually high diversity. Patrick Woods and colleagues compared the whole genomes of feral plants from Colorado, Kansas and Nebraska with those of European hemp and drug-type cannabis. In each of the three groups they identified 23 genes with the signature of balancing selection. In the feral plants six of them were genes of response to stresses such as wounding, pathogens, oxidation, salt and osmotic stress. The authors suggested that natural selection keeps variation at these genes through an advantage to heterozygotes as the plants adapt to life outside cultivation. One gene, MTERF9, which in Arabidopsis acts in chloroplast gene expression and in the response to abiotic stress, met the criteria in all three groups.[7]
Adaptation to latitude and climate
Cannabis is a short-day plant: shortening days bring on flowering and long days delay it.[5] Qingying Zhang and colleagues grew 43 wild and domesticated accessions from China, collected between 23° and 50° N, side by side at Kunming in 2016. The time to flowering and to seed maturity, stem diameter, plant height and the number of branches all varied closely with the latitude of origin. Plants of the high-latitude chloroplast lineage, found mainly north of about 40° N, matured seed in a mean of 77.2 days, grew to 99.2 cm and bore 3.2 branches. Plants of the low-latitude lineage, found mainly south of about 30° N, took 133.6 days, reached 238.0 cm and bore 11.0 branches. Of 20 climatic factors tested against the distribution of the lineages, mean day length over the growing season was the most significant. The authors read the pattern as adaptation to latitude. They suggested that sensitivity to day length keeps the low-latitude lineage, whose northern limit lies near 43° N, from spreading further north.[5]
Xuan Chen and colleagues grew wild and cultivated Chinese accessions under 18-hour days. The wild plants formed flower buds within 50 days, one accession within 31. The cultivated accessions from south-western and north-western China were still vegetative after 100 days. Under long days the wild plants expressed more of the flowering gene FT-like. Its expression rose with the latitude of each accession's origin. Cultivated plants from high latitudes, grown at Kunming, flowered and matured early, stayed short and branched little. The authors concluded that cultivated cannabis has adapted to different day lengths through the regulation of FT-like. Varieties from low latitudes grown further north have a longer growing period and risk losing immature fibre or seed to an early frost, while varieties from high latitudes grown further south have a shorter one and yield much less fibre and seed.[6]
John McPartland and Ernest Small traced the divergence of the cannabis of Central Asia and South Asia to natural selection under the different climates of the two regions, with domestication widening it later. Central Asia has cooler and drier climates and shorter growing seasons; South Asia is warmer and wetter, with longer seasons. Central Asian plants grown in South Asian conditions suffer reduced fitness. Their heavily flowered branches can snap under monsoon rain. Their dense, leafy inflorescences resist the grey mould Botrytis cinerea poorly in high humidity. The looser, less leafy inflorescences of South Asian plants tolerate such fungi better. McPartland and Small proposed that these adaptations isolated the two populations by habitat. The wild-type plants of the two regions have adjacent ranges that barely overlap, which they took to support that view.[8]
Seed traits outside cultivation
Wild-type cannabis has small fruits with an elongated base that breaks away easily and a perianth that stays on and is mottled for camouflage. Domesticated plants have larger seeds that do not shatter and a perianth that comes away.[8] In the accessions of Chen and colleagues, a thousand seeds of the wild accessions weighed 3.15 to 9.80 g, against 17.40 to 63.13 g for the cultivated ones. Ripe wild seed fell easily from its stalk, most of it with a fleshy outgrowth at the base. Fewer than 2 per cent germinated at room temperature without a period of cold, moist treatment. The authors listed nondormant seed, large seed that does not shatter, tall plants with few branches for fibre and high cannabinoid content among the traits people selected in domesticating the crop.[6] Zhang and colleagues described the wild forms in the same terms, with long seed dormancy.[5]
Domesticated cannabis escapes cultivation easily. In Canada it returned to a wild-type form within 50 generations of the prohibition of its cultivation. Such rapid change makes truly wild plants hard to tell from cultivated ones that have reverted. McPartland and Small considered that cannabis may have no surviving populations unaltered by domestication.[8]
Feral hemp in the United States
The feral cannabis of the United States descends largely from hemp grown there between 1942 and 1945 and has evolved under natural selection since.[7] In the genome study of Woods and colleagues the feral plants were most closely related to European hemp, yet had diverged from it. Of the three groups they carried the largest share of derived alleles at high frequency. Two broad regions, on chromosomes 4 and 10, set them apart; the authors took these to be possible chromosomal inversions. One gene with a derived allele at high frequency in the feral plants, SRFR1, acts in defence against bacterial pathogens in Arabidopsis. The authors read these results as local adaptation to new climates and to the organisms the plants meet, together with natural selection against the traits bred into their domesticated ancestors. In drug-type cannabis the same scan found selection on a homologue of a gene that regulates the number of trichomes. The authors attributed it to human selection for more of the glandular trichomes that make tetrahydrocannabinol.[7]
See also
References
- ↑ 1.0 1.1 1.2 Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. pp. 51–52.
- ↑ 2.0 2.1 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. 61 – via Darwin Online.
- ↑ 3.0 3.1 Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. pp. 447–450.
- ↑ Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. chs. II–VII.
- ↑ 5.0 5.1 5.2 5.3 Zhang, Q.; Chen, X.; Guo, H.; Trindade, L.M.; Salentijn, E.M.J.; Guo, R.; Guo, M.; Xu, Y.; Yang, M. (2018). "Latitudinal adaptation and genetic insights into the origins of Cannabis sativa L." Frontiers in Plant Science. 9 1876. doi:10.3389/fpls.2018.01876.
- ↑ 6.0 6.1 6.2 Chen, Xuan; Guo, Hong-Yan; Zhang, Qing-Ying; Wang, Lu; Guo, Rong; Zhan, Yi-Xun; Lv, Pin; Xu, Yan-Ping; Guo, Meng-Bi; Zhang, Yuan; Zhang, Kun; Liu, Yan-Hu; Yang, Ming (2022). "Whole-genome resequencing of wild and cultivated cannabis reveals the genetic structure and adaptive selection of important traits". BMC Plant Biology. 22 (1) 371. doi:10.1186/s12870-022-03744-0.
- ↑ 7.0 7.1 7.2 7.3 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.
- ↑ 8.0 8.1 8.2 8.3 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.
- ↑ Bellon, M.R.; Dulloo, E.; Sardos, J.; Thormann, I.; Burdon, J.J. (2017). "In situ conservation: harnessing natural and human-derived evolutionary forces to ensure future crop adaptation". Evolutionary Applications. 10 (10): 965–977. doi:10.1111/eva.12521.
- ↑ Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. pp. 71–72.
- ↑ Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. p. 80.
- ↑ 12.0 12.1 Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. pp. 88–91.
- ↑ Beavis, W.; Lamkey, K.; Espinosa, K.; Mahama, A.A.; Suza, W. (2023). "Components of Variance". In Suza, W.; Lamkey, K. (eds.). Quantitative Genetics for Plant Breeding. Iowa State University Digital Press. doi:10.31274/isdup.2023.146.
- ↑ Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. pp. 317–319.
- ↑ Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. pp. 326–328.
- ↑ Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. pp. 266–268.
- ↑ Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. pp. 186–187.