What is artificial selection?
Selection is a process through which organisms with particular genetic characteristics leave more offspring than do organisms with alternative genetic forms. This may occur because the genetic characteristics confer upon the organism a better ability to survive and ultimately produce more offspring than individuals with other characteristics (natural selection), or it may be caused by selective breeding of individuals with characteristics valuable to humans (artificial selection). Natural and artificial selection may act in concert, as when a genetic characteristic confers a disadvantage directly to the organism. Dwarfism in cattle, for example, not only directly reduces the survival of the affected individuals but also reduces the value of the animal to the breeder. Conversely, natural selection may act in opposition to artificial selection. For example, a genetic characteristic that results in the seed being held tightly in the head of wheat grass is an advantage to the farmer, as it makes harvesting easier, but it would be a disadvantage to wild wheat because it would limit seed dispersal.
Artificial selection was probably conducted first by early farmers who identified forms of crop plants that had characteristics that favored cultivation. Seeds from favored plants were preferentially kept for replanting. Characteristics that were to some degree heritable would have had the tendency to be passed on to the progeny through the selected seeds. Some favored characteristics may have been controlled by a single gene and were therefore quickly established, whereas others may have been controlled by a large number of genes with individually small effects, making them more difficult to establish. Nevertheless, seeds selected from the best plants would tend to produce offspring that were better than average, resulting in gradual improvement in the population. It would not have been necessary to have knowledge of the mechanisms of genetics to realize the favorable effects of selection.
Likewise, individuals who domesticated the first animals for their own use would have made use of selection to capture desirable characteristics within their herds and flocks. The first of those characteristics was probably docile behavior, a trait known to be heritable in contemporary livestock populations.
Technology to improve organisms through selective breeding preceded an understanding of its genetic basis. Recording of pedigrees and performance records began with the formal development of livestock breeds in the 1700s. Some breeders, notably Robert Bakewell, began recording pedigrees and using progeny testing to determine which sires had superior genetic merit. Understanding of the principles of genetics through the work of Gregor Mendel enhanced but did not revolutionize applications to agricultural plant and animal improvement.
Development of reliable methods for testing the efficiency of artificial selection dominated advances in the fields of plant and animal genetics during the first two-thirds of the twentieth century. Genetic merit of progeny was expected to be equal to the average genetic merit of the parents. More effective breeding programs are dependent on identifying potential parents with superior genetic merit. Computers and large-scale databases have greatly improved selection programs for crops and livestock. However, selection to improve horticultural species and companion animals continued to rely largely on the subjective judgment of the breeder to identify superior stock.
Plant and animal genome-mapping programs facilitated the next leap forward in genetic improvement of agricultural organisms. Selection among organisms based directly on their gene sequences promised to allow researchers to bypass the time-consuming data-recording programs upon which genetic progress of the 1990s relied. Much effort has gone into identifying quantitative trait loci (QTLs), which are regions of chromosomes or genes that may play a role in the diversity of a trait. QTL analysis is often used along with marker-assisted selection (MAS), where markers associated with the gene of interest are used as surrogates for the actual gene. An example of a biochemical marker is a protein that is encoded by a specific gene. An example of a gene marker is a single nucleotide polymorphism (SNP). Regarding SNPs, there are some cases in which a trait may be controlled by a gene that has a different allele (also known as an alternative DNA sequence) depending on a change in one single nucleotide, and this change may or may not result in a different characteristic. A benefit to using QTL and MAS is that plants and animals can undergo genetic screening to determine whether a desired trait has been artificially selected into the new progeny, rather than waiting until the plant or animal has matured to see if the trait has been passed on.
In addition to identifying different alleles, SNP analysis can be used to learn more about the genetic history of plants and animals and to determine which genes have remained consistent over time and which have varied due to artificial selection. For example, one evolutionary study, conducted by Masanori Yamasaki, Stephen I. Wright, and Michael D. McMullen and published in 2007 in the Annals of Botany, found that approximately 1,200 genes in the modern maize genome were affected by artificial selection during the domestication from wild grass teosinte.
The ultimate limit to what can be achieved by selection is the exhaustion of genetic variants. One example of the extremes that can be accomplished by selection is evident in dog breeding: some of the heaviest breeds, such as mastiffs, can weigh more than fifty times as much as the lightest breeds, such as chihuahuas. Experimental selection for body weight in insects and for oil content in corn has resulted in variations of similar magnitudes.
However, most modern breeding programs for agricultural crops and livestock seek to decrease variability while increasing productivity. Uniformity of the products enhances the efficiency with which they can be handled mechanically for commercial purposes. As indigenous crop and livestock varieties are replaced by high-producing varieties, the genetic variation that provides the source of potential future improvements is lost. Widespread use of uniform varieties may also increase the susceptibility to catastrophic losses or even extinction from an outbreak of disease or environmental condition. The lack of biodiversity in the wake of such species loss could threaten entire ecosystems and human beings themselves.
As the genomes of different species are sequenced and analyzed, databases of gene mapping are becoming available. These gene maps help to link the QTLs to specific genes by sequencing and functional analysis and help to connect different alleles to different SNPs. It is predicted that this enhanced genetic knowledge will improve artificial selection by facilitating the selective breeding of animals and plants with greater valued commercial traits. However, many traits are controlled by several genes, making QTL analysis quite complex.
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