selection

(sĭ-lĕk'shən)

1. 1. The act or an instance of selecting or the fact of having been selected.
   2. One that is selected.
2. A carefully chosen or representative collection of people or things. See synonyms at choice.
3. A literary or musical text chosen for reading or performance.
4. Biology. A natural or artificial process that favors or induces survival and perpetuation of one kind of organism over others that die or fail to produce offspring.

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For more information on selection, visit Britannica.com.

Extraction of a sample or subset of a group based upon certain selection criteria. See also list selection; parameter; screen.

noun

The act of choosing: choice, election, option, preference. See choice.

Definition: preference from among choices
Antonyms: refusal, rejection

Selection is a process in which members of a population reproduce at different rates, due to either natural or human-influenced factors. The result of selection is that some characteristic is found in increasing numbers of organisms within the population as time goes on.

Types of Selection

Artificial selection, which is even older than agriculture, refers to a conscious effort to use for future breeding those varieties of a plant or animal that are most useful, attractive, or interesting to the breeder. Artificial selection is responsible for creating the enormous number of breeds of domestic dogs, for instance, as well as high-yielding varieties of corn and other agricultural crops.

Selection also occurs in nature, but it is not conscious. Charles Darwin called this natural selection. Darwin saw that organisms constantly vary in a population from generation to generation. He proposed that some variations allow an organism to be better adapted to a given environment than others in the population, allowing them to live and reproduce while others are forced out of reproduction by death, sterility, or isolation. These genetic variations gradually replace the ones that fail to survive or to reproduce. This gradual adjustment of the genotype to the environment is called adaptation. Natural selection was not only Darwin's key mechanism of evolution for the origin of species, it is also the key mechanism today for understanding the evolutionary biology of organisms from viruses to humans. Natural selection leads to evolution, which is the change in gene frequencies in a population over time.

The concept of selection plays an increasingly important role in biological theory. New fields such as evolutionary psychology rely heavily on natural selection to explain the evolution of human behavioral traits, such as mate choice, aggression, and other types of social behavior. A great difficulty in such a theoretically based science is the paucity of experimental or direct evidence for presumed past environments and presumed behavioral responses that were genetically adaptive.

Variation

The variation that selection requires arises from two distinct sources. The ultimate sources of variation are gene mutation, gene duplication and disruption, and chromosome rearrangements. Gene mutations are randomly occurring events that at a molecular level consist mostly of substitutions or small losses or gains of nucleotides within genes. Gene duplication makes new copies of existing genes, while gene disruptions destroy functional copies of genes, often through insertion of a mobile genetic element. Chromosome rearrangements are much larger changes in chromosome structure, in which large pieces of chromosomes break off, join up, or invert. Individually, such mutations are rare. Most small mutations are either harmful or have no effect, and they may persist in a population for dozens or hundreds of generations before their advantages or disadvantages are evident.

The second source of variation arises from the shuffling processes undergone by genes and chromosomes during reproduction. During meiosis, maternally and paternally derived chromosome pairs are separated randomly, so that each sperm or egg contains a randomly chosen member of each of the twenty-three pairs. The number of possible combinations is over eight billion. Even more variation arises when pair members exchange segments before separating, in the process known as crossing over. The extraordinary variety in form exhibited even by two siblings is due primarily to the shuffling of existing genes, rather than to new mutations.

The Importance of the Environment

A disadvantageous trait in one environment may be advantageous in a very different environment. A classic example of this is sickle cell disease in regions where malaria is common. Individuals who inherit a copy of the sickle cell gene from both of their parents (homozygotes) die early from the disease, whereas heterozygotes (individuals who inherit only one copy of the gene) are favored in malarial areas (including equatorial Africa) over those without any copies, because they contract milder cases of malaria and thus are more likely to survive it.

Even though homozygotes rarely pass on their genes, because of their low likelihood of surviving to reproduce, the advantage of having one copy is high enough that natural selection continues to favor presence of the gene in these populations. Thus a malarial environment can keep the gene frequency high. However, in temperate regions where malaria is absent (such as North America), there is no heterozygote advantage to the sickle cell gene. Because heterozygotes still suffer from the disease, they are less likely to survive and reproduce. Thus, selection is gradually depleting the gene from the African American population that harbors it.

Artificial Selection

One of the first uses of genetic knowledge to improve yields and the quality of plant products was applied to hybrid seed production at the start of the twentieth century by George Shull. Artificial selection today is still done by hobbyists who garden or raise domestic animals. It is done on a more professional level in agriculture and animal breeding. The benefits are enormous. Virtually all commercial animal and plant breeding uses selection to isolate new combinations of traits to meet consumer needs. In these organisms, most of the variation is preexisting in the population or in related populations in the wild. The breeder's task is to combine (hybridize) the right organisms and select offspring with the desired traits.

In the antibiotic industry selection is used to identify new antibiotics. Usually, microorganisms are intentionally mutated to produce variation. Mutations can be induced with a variety of physical and chemical agents called mutagens, which randomly alter genes. Some early strains of penicillin-producing molds were x-rayed and their mutations selected for higher yields.

Biologists also make use of selection in the process called molecular cloning. Here, a new gene is inserted into a host along with a marker gene. The marker is typically a gene for antibiotic resistance. To determine if the host has taken up the new genes, it is exposed to antibiotics. The ones who survive are those that took up the resistance gene, and so also have the gene of interest. This selection process allows the researcher to quickly isolate only those organisms with the new gene.

Selection in Humans

Both natural and artificial selection occur in human beings. If a trait is lethal and kills before reproductive maturity, then that gene mutation is gradually depleted from the population. Mutations with milder effects persist longer and are more common than very severe mutations, and recessive mutations persist for much longer than dominant ones. With a recessive trait, such as albinism, the parents are usually both carriers of a single copy of the gene and may not know that they carry it. If a child receives a copy of this gene from both of the carrier parents, the albino child may die young, may find it difficult to find a partner, or may end up marrying much later in life. This is usually considered a form of natural selection.

Considerable abuse of genetic knowledge in the first half of the twentieth century led to the eugenics movement. Advocates of eugenics claimed some people were more fit and others less fit (or unfit), and argued that the least fit should be persuaded or forced not to reproduce. Eugenicists typically defined as unfit those who were "feeble-minded, criminal, socially deviant, or otherwise undesirable." Coerced sterilization, a form of artificial selection, was practiced on some of these individuals.

SEEALSO CLONING GENES; EUGENICS; HARDY-WEINBURG EQUILIBRIUM; MULLER, HERMANN; MUTAGENESIS; MUTATION.

Bibliography

Huxley, Julian. "Adaptation and Selection." In Evolution: The Modern Synthesis. New York: Harper and Brothers, 1942.

Pianka, Eric. Evolutionary Biology, 6th ed. San Francisco: Addison-Wesley-Longman, 2000.

—Elof Carlson

The practice by which growers select seed from only the best plants, continuing the process over the years to produce a superior strain.

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1. choosing the individual units to be included in a sample. See also random selection.
2. choosing the animals to be retained for breeding purposes; genetic selection.

• artificial s. — selection based on human decisions.
• s. coefficient — proportionate reduction in the average genetic contribution made by a specific genotype, relative to the contribution made by another genotype. Denoted by s.
• s. criteria — the animal characteristic which is used in a selection program.
• s. differential — a measure of the gain achieved by selection; the phenotypic superiority of selected individuals, compared to the population from which they were selected.
• s. index — a single overall estimate of the patient's true breeding value obtained from as many sources of information as are available.
• individual s. — selection on the results of performance testing of the subject.
• s. intensity — the superiority of the individuals selected for breeding, relative to the population from which they were selected.
• s. limit — the situation in which the entire population is homozygous for the same set of favorable genes; called also selection plateau.
• s./mutation balance — when the rate of removal of a gene from the population by selection equals the rate at which mutations occur.
• s. plateau — see selection limit.
• s. program — the method used to select individuals from a population to be used for breeding. Usually includes nomination of the characters to be selected, the optimum size of the population in which the program is to operate, the intensity of selection available, the accuracy of the selection procedures, lengths of the generations in the species, the target rate of response.

1. the act of choosing between or among a variety of options or alternatives. 2. the process by which various factors or mechanisms determine and modify the reproductive ability of a genotype within a specific population. Also referred to as natural selection.

• Genetics, Heredity, and Evolution - selection: natural selection

For other uses, see Selection (disambiguation).

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In the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the succeeding generation than others do. When these traits have a genetic basis, selection can increase the prevalence of those traits, because offspring will inherit those traits from their parents. When selection is intense and persistent, adaptive traits become universal to the population or species, which may then be said to have evolved. Scientists who do experimental genetics employ artificial selection experiments that permit the survival of organisms with user-defined phenotypes. Artificial selection is widely used in the field of microbial genetics, especially molecular cloning.

Contents 1 Overview 2 Types and subtypes 2.1 Patterns of selection 2.2 Mechanisms of selection 3 See also 4 Further reading

Overview

Whether or not selection takes place depends on the conditions in which the individuals of a species find themselves. Adults, juveniles, embryos, and even eggs and sperm may undergo selection. Factors fostering selection include limits on resources (nourishment, habitat space, mates) and the existence of threats (predators, disease, adverse weather). Biologists often refer to such factors as selective pressures.

Natural selection is the most familiar type of selection by name. The breeding of dogs, cows and horses, however, represents "artificial selection". Subcategories of natural selection are also sometimes distinguished. These include sexual selection, ecological selection, stabilizing selection, disruptive selection and directional selection (more on these below).

Selection occurs only when the individuals of a population are diverse in their characteristics—or more specifically when the traits of individuals differ with respect to how well they equip them to survive or exploit a particular pressure. In the absence of individual variation, or when variations are selectively neutral, selection does not occur.

Meanwhile, selection does not guarantee that advantageous traits or alleles will become prevalent within a population. Through genetic drift, such traits may become less common or disappear. In the face of selection even a so-called deleterious allele may become universal to the members of a species. This is a risk primarily in the case of "weak" selection (e.g. an infectious disease with only a low mortality rate) or small populations.

Though deleterious alleles may sometimes become established, selection may act "negatively" as well as "positively." Negative selection decreases the prevalence of traits that diminish individuals' capacity to succeed reproductively (i.e. their fitness), while positive selection increases the prevalence of adaptive traits.

In biological discussions, traits subject to negative selection are sometimes said to be "selected against," while those under positive selection are said to be "selected for," as in the sentence Desert conditions select for drought tolerance in plants and select against shallow root architectures.

Types and subtypes

Selection is hierarchically classified into natural and artificial selection. Natural selection is further subclassified into ecological and sexual selection

Patterns of selection

Aspects of selection may be divided into effects on a phenotype and their causes. The effects are called patterns of selection, and do not necessarily result from particular causes (mechanisms); in fact each pattern can arise from a number of different mechanisms. Stabilizing selection favors individuals with intermediate characteristics while its opposite, disruptive selection, favors those with extreme characteristics; directional selection occurs when characteristics lie along a phenotypic spectrum and the individuals at one end are more successful; and balancing selection is a pattern in which multiple characteristics may be favored.

Mechanisms of selection

Distinct from patterns of selection are mechanisms of selection; for example, disruptive selection often is the result of disassortative sexual selection, and balancing selection may result from frequency-dependent selection and overdominance.

See also

• Natural selection
• Sexual selection
• Artificial selection
• Evolution

Further reading

• Bell, Graham (1997). Selection: The Mechanism of Evolution. New York: Chapman & Hall. pp. 699 p. ISBN 041205521X.  (2nd edition published in 2008 by Oxford University Press, 553 p., ISBN 0-19-856972-6)

v · d · eTopics in population genetics Key concepts Hardy-Weinberg law · Genetic linkage · Linkage disequilibrium · Fisher's fundamental theorem · Neutral theory · Price equation Selection Natural · Sexual · Artificial · Ecological Effects of selection on genomic variation Genetic hitchhiking · Background selection Genetic drift Small population size · Population bottleneck · Founder effect · Coalescence · Balding–Nichols model Founders R. A. Fisher · J. B. S. Haldane · Sewall Wright Related topics Evolution · Microevolution · Evolutionary game theory · Fitness landscape · Genetic genealogy List of evolutionary biology topics

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Dansk (Danish)
n. - udvalg, udvælgelse, kåring

Nederlands (Dutch)
selectie, sortiment, sortering

Français (French)
n. - sélection, assortiment, choix, (Sport) sélection, choix de

Deutsch (German)
n. - Wahl, Auswahl, Auslese

Ελληνική (Greek)
n. - εκλογή, επιλογή, διαλογή, συλλογή, ανθολογία

Italiano (Italian)
selezione, scelta, assortimento

Português (Portuguese)
n. - seleção (f)

Русский (Russian)
выбор, кто или то, что отобрано, подборка, избранное произведение, селекция

Español (Spanish)
n. - elección, selección, surtido

Svenska (Swedish)
n. - urval, val, uttagning, selektion

中文（简体）(Chinese (Simplified))
选择, 被挑选出的人, 选拔, 精选品

中文（繁體）(Chinese (Traditional))
n. - 選擇, 被挑選出的人, 選拔, 精選品

한국어 (Korean)
n. - 선택, 도태, (같은 종류의) 구색

日本語 (Japanese)
n. - 選択, 選ばれたもの, 淘汰

العربيه (Arabic)
‏(الاسم) انتقاء, أختيار‏

עברית (Hebrew)
n. - ‮בחירה, מבחר, אדם/דבר שנבחר, ברירה טבעית (ביולוגיה)‬

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