In cases where a wild progenitor species is reasonably well established, one may use the approach pioneered by de Candolle and Vavilov of identifying the geographical region in which the progenitor is most common and in which its populations are the most diverse (see Wilkes 2004). According to Evans (1984), “Darwin’s recognition of the power of selection in changing organisms was almost entirely due to what he learned of plant and animal breeding.” Certainly, Darwin himself wished to convey that his discovery of natural selection grew from his studies of artificial selection. Genetics of adaptation and domestication in livestock. These novel characteristics greatly alter the means by which plants grow and reproduce. Plant Biotechnol J 2005;3:363–70. By contrast, Darwin dedicated several pages to a discussion of domesticated pigeons in the Origin in 1859 and expanded this to a treatment spanning two entire chapters in Variation in 1868. approaches zero (Coyne and Orr, 2004). Vollbrecht E, Sigmon B. Amazing grass: developmental genetics of maize domestication. Many other genes of smaller effect are expected to have influenced features such as the size of the ear, growth conditions, and the nutrient content of kernels. Trends Plant Sci 2001;6:127–34. (2006) noted, “most members of our modern industrial societies have never seen and would not recognize the unpromising wild plants that are the progenitors of our remarkably productive crops.” Beginning with the agricultural revolution around 10,000 years ago, hundreds of plants were domesticated on several continents (Fig. doi:10.1038/nature07446. Chloroplast DNA variation confirms a single origin of domesticated sunflower (Helianthus annuus L.). Once such regions have been identified, the next step is to investigate their phenotypic impacts, for example by experimentally disrupting their function and observing the consequences (Vollbrecht and Sigmon 2005; Doebley et al. Department of Integrative Biology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada, You can also search for this author in All of these considerations apply in the study of evolution by natural selection as well. Indeed, it is clear that certain groups of animals and plants are far less amenable to domestication than others (Diamond 1997, 2002; Gepts 2004; Zeder et al. Artificial selection during domestication and crop improvement involved selection of specific alleles at genes controlling key morphological and agronomic traits, resulting in reduced genetic diversity relative to unselected genes. Doebley J. Unfallen grains: how ancient farmers turned weeds into crops. 2008). Contrasting effects of selection on sequence diversity and linkage disequilibrium at two phytoene synthase loci. 4). The plants should become more maize-like for branch length (and perhaps seed size and tillering), but for most other traits they should remain true to the teosinte condition. (2005) put it, “In a sense, maize domestication involved turning the teosinte ear inside out.” A QTL known as teosinte glume architecture 1 (tga1) has been identified as playing an important role in this transition (Doebley 2004). In fact, as far as is known, the genes of major effect at the heart of domestication in many species are functional (Doebley 2006; Doebley et al. Olsen KM, Caicedo AL, Polato N, McClung A, McCouch S, Purugganan MD. Examples include: grain size3 (GS3), which influences grain length and weight (Fan et al. Planta 2007;226:11–20. doi:10.1093/molbev/msn177. Fitzgerald MA, Sackville Hamilton NR, Calingacion MN, Verhoeven HA, Butardo VM. doi:10.1038/nature03863. PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association 1986;1986:502–13. doi:10.1016/j.applanim.2005.11.015. In some cases, human farming practices may not only be unconscious but may engender unintended and undesirable consequences. As a notable early example, Darwin (1868) compared the features of various breeds of pigeons and used this information to construct a diagram of their hypothetical relationships (Fig. Though minor at the genetic level, these changes result in a substantial morphological effect, namely the loss of fruit cases around grains (Fig. Genetic control of rice plant architecture under domestication. Though many of their genes have been lost or have migrated to the nucleus, these organelles retain their own genomes which evolve largely independently of the nuclear genome. parviglumis) to see if I can change the population to be more maize-like, i.e. For example, treatment with antibiotics instigates a process of natural selection among bacteria in which mutants that happen to be resistant survive and reproduce more than individuals lacking resistance. Is there a second fragrance gene in rice? The horns of domesticated livestock can therefore be lost not because humans select directly for their reduction but because a lack of sexual selection allows alleles specifying smaller horns to increase in frequency indirectly or by chance (Mignon-Grasteau et al. In this case, archeologists may discover small remnants of plants that provide information about the time of origin of particular features. In fact, an understanding of domestication and artificial selection provides significant clarification on these points. Proc Natl Acad Sci U S A 2002;99:12959–62. Proc Natl Acad Sci U S A 2008;105:17855–60. doi:10.1016/j.tig.2006.01.004. In some cases, only microscopic remains are available, as with starch grains or phytoliths (“plant stones”; tiny silica particles formed in living cells of plants as protection from predators, reservoirs of carbon dioxide, and structural support). For instance, Drake and Klingenberg (2008) were able to use museum specimens to track the gradual changes in skull shape among dogs in the St. Bernard breed that occurred as a result of selective breeding over the past 120 years. On the contrary, the details of when, where, and how this occurred are becoming increasingly clear. 1998; Doebley et al. p. 1–12. doi:10.1038/ng.247. 2006; Cruz et al. Ancient DNA is coming to play a significant role in broader evolutionary studies as well. The discovery of preserved prehistoric remains is important in the study of domestication (archeology), just as the fossil record is in the study of large-scale evolution (paleontology) (see Smith 2006). Eubanks MW. Documenting the presence of maize in Central and South America through phytolith analysis of food residues. From Salamini et al. The process of domesticating animals began in early prehistory where species such as rice, wheat and dogs were selected and domesticated. doi:10.1098/rspb.2007.1169. Gene 2007;390:39–51. Nor is every intermediate stage preserved such that a step-by-step transition can be determinedFootnote 5, and in many cases the wild progenitor is extinct. Wilkes (2004) suggested that “it takes 25 corn plants per person per day to support the American way of life.” In another assessment, Buckler and Stevens (2006) pointed out that “if 10 people derive 10% of their calories from maize, it is estimated that roughly 250,000–350,000 plants would have to be grown annually.”, With the advent of genetic engineering, it has now become possible to add specific characteristics to crops and livestock at will, including by transplanting genes for desirable traits from other species. Ross-Ibarra J, Morrell PL, Gaut BS. Alu elements and hominid phylogenetics. PLoS Biol 2006;4:e72. In: Motley TJ, Zerega N, Cross H, editors. But if the result is that individuals bearing them are statistically more successful in passing on their traits, then these will increase in frequency under artificial selection. The process of domestication is called artificial selection. Some authors have suggested that it was not new mutations per se that resulted in the unique phenotype of maize but selection on standing variation and the bringing together of several rare variants for the first time (e.g., Vollbrecht and Sigmon 2005; Doebley et al. No reliable observation has yet been made to refute the notion that livestock, pets, and crops evolved from wild predecessors. Vorzimmer P. Darwin’s Questions About The Breeding of Animals (1839). Drake AG, Klingenberg CP. The plausibility of this hypothesis is strengthened by the example of “domesticated” silver foxes, which have been the subject of a decades-long breeding experiment. Selection is a process by which the frequencies of existing variants changes from one generation to the next, and it is not the only one. Retrotransposons, in particular, are very abundant in animal and plant genomes and, like retroviruses, spread by being transcribed into RNA and then reinserting back into the DNA of the “host” chromosomes. His primary focus in this case was on the extensive variation observed among individual plants and animals under domestication, which paralleled that witnessed in nature. Domestication has been investigated using comparative morphology, biogeography, archeology, and numerous independent sources of genetic data ranging from chromosome number to gene sequences to complete genomes to ancient DNA. Motley TJ. doi:10.1007/s12052-008-0035-x. doi:10.2307/2406971. Sweet corn results from naturally occurring recessive mutations that may include the sugary (su), sugar enhanced (se), or shrunken (sh2) genes which control the conversion of starch into sugar in the endosperm of kernels. The gene for fragrance in rice. 2). Princeton: Princeton University Press; 2008. doi:10.1266/ggs.82.217. Artificial selection explains how these rare variants became ubiquitous (“fixed”) within modern domesticated populations. Thus, the phrase “survival of the fittest,” which was coined by Herbert Spencer (a contemporary of Darwin’s), is misleading if it implies physical fitness or other measures of survival ability. Hull DL. doi:10.1016/j.livprodsci.2004.11.001. doi:10.1007/BF00356983. doi:10.1105/tpc.105.038430. 2007; Fig. Annu Rev Genet 2004;38:37–59. Such questions can almost never be answered with certainty because, a few examples notwithstanding (e.g., strawberries, pecans, sugar beets, rubber), most plants and animals were first domesticated long before the beginning of recorded history (Diamond 2002; Wilkes 2004). California Privacy Statement, Cruz F, Vilà C, Webster MT. Nonetheless, citing additional evidence such as Darwin’s notebooks and his use of other published works, historians and philosophers have continued to debate both the role that artificial selection played (or did not play) in Darwin’s development and justification of the concept of natural selection and the nature of the analogy that he intended to draw between artificial and natural selection (e.g., Vorzimmer 1969b; Herbert 1971; Ruse 1973, 1975; Schweber 1977; Kohn 1980; Cornell 1984; Evans 1984; Rheinberger and McLaughlin 1984; Waters 1986; Bartley 1992; Richards 1997; Sterrett 2002; Gildenhuys 2004). Beyond the general interest in animal domestication, numerous genetic loci potentially selected for by humans have been detected to contribute to a full understanding of early domestication and artificial selection. Eyre-Walker A, Gaut RL, Hilton H, Feldman DL, Gaut BS. Notably, selective breeding in the wild cabbage species Brassica oleracea has produced a wide variety of distinct cultivars, each representing a modification of a particular part of the plant (Purugganan et al. Another approach involves scanning the genome for sequences that show signs of having been under selection (see Wright and Gaut 2005; Nielsen et al. Such a protracted rate of change under artificial selection appears to be typical among cereal crops, with changes in grain size taking 500–1,000 years and alleles for loss of shattering becoming fixed after another 1,000–2,000 years (Fuller 2007). Cytokinin oxidase regulates rice grain production. J Hist Biol 1969a;2:269–81. They believed that studying domesticated animals and plants could illuminate the mechanisms of natural evolution. Jin J, Huang W, Gao JP, Yang J, Shi M, Zhu MZ, et al. Panaud O. doi:10.1093/molbev/msm077. Artificial selection of corn. As an interesting example of this, Diamond (2002) considers why almonds were domesticated but other trees such as oak were not. A shift to agriculture from hunting and gathering created a new environment for humans just as it did for plants and livestock. Cereal genomics. In this regard, some important points about the operation of artificial selection are outlined in the following sections. J Hered 2006;97:403–8. However, on the scale of individual human lifetimes, it may have been so gradual as to be nearly imperceptible. London: Allen and Unwin; 1969. J Hist Biol 1984;17:345–68. Specifically, individual foxes have been selected purely on the basis of tameness, but major changes in behavior, development, and appearance have arisen as a result (see Trut 1999). Trends Genet 1992;8:302–7. 2004a, b; Panaud 2008). This makes it possible to harvest the grains but not impossible to remove them from the stalk (Doebley 2006; Doebley et al. Trends Genet 2006;22:126–31. Strong selective pressure on beneficial phenotypes could cause nucleotide fixations in the … 2008; http://www.genomesonline.org). Darwin C. An account of the fine dust which often falls on vessels in the Atlantic ocean.
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