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Selection

Selection

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.

see also Cloning Genes; Eugenics; Hardy-Weinburg Equilibrium; Muller, Hermann; Mutagenesis; Mutation.

Elof Carlson

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.

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Selection

Selection

Evolutionary selection pressures act on all living organisms, regardless whether they are prokaryotic or higher eukaryotes . Selection refers to an evolutionary pressure that is the result of a combination of environmental and genetic pressures that affect the ability of an organism to live and, equally importantly, to produce reproductively successful offspring (including prokaryotic strains of cells).

As implied, natural selection involves the natural (but often complex) pressures present in an organism's environment. Artificial selection is the conscious manipulation of mating, manipulation, and fusion of genetic material to produce a desired result.

Evolution requires genetic variation, and these variations or changes (mutations ) are usually deleterious because environmental factors already support the extent genetic distribution within a population.

Natural selection is based upon expressed differences in the ability of organisms to thrive and produce biologically successful offspring. Importantly, selection can only act to exert influence (drive) on those differences in genotype that appear as phenotypic differences. In a very real sense, evolutionary pressures act blindly.

There are three basic types of natural selection: directional selection favoring an extreme phenotype ; stabilizing selection favoring a phenotype with characteristics intermediate to an extreme phenotype (i.e., normalizing selection); and disruptive selection that favors extreme phenotypes over intermediate genotypes.

The evolution of pesticide resistance provides a vivid example of directional selection, wherein the selective agent (in this case DDT) creates an apparent force in one direction, producing a corresponding change (improved resistance) in the affected organisms. Directional selection is also evident in the efforts of human beings to produce desired traits in many organisms ranging from bacteria to plants and animals.

Not all selective effects are directional, however. Selection can also produce results that are stabilizing or disruptive. Stabilizing selection occurs when significant changes in the traits of organisms are selected against. An example of this is birth weight in humans. Babies that are much heavier or lighter than average do not survive as well as those that are nearer the mean (average) weight.

On the other hand, selection is said to be disruptive if the extremes of some trait become favored over the intermediate values. Although not a factor for microorganisms , sexual selection and sexual dimorphism can influence the immunologic traits and capacity of a population.

Sometimes the fitness of a phenotype in some environment depends on how common (or rare) it is; this is known as frequency-dependent selection. Perhaps an animal enjoys an increased advantage if it conforms to the majority phenotype in the population. Conversely, a phenotype could be favored if it is rare, and its alternatives are in the majority. Frequency-dependent selection provides an interesting case in which the gene frequency itself alters the selective environment in which the genotype exists.

Many people attribute the phrase "survival of the fittest" to Darwin, but in fact, it originated from another naturalist/philosopher, Herbert Spencer (18201903). Recently, many recent evolutionary biologists have asked: Survival of the fittest what? At what organismal level is selection most powerful? What is the biological unit of natural selection-the species, the individual, or even the gene?

Selection can provide interesting consequences for bacteria and viruses . For example, reduced virulence in parasites , who depend on the survival of their hosts for their own survival may increase the reproductive success of the invading parasite. The myxoma virus, introduced in Australia to control imported European rabbits (Oryctolagus cuniculus ), at first caused the deaths of many individuals. However, within a few years, the mortality rate was much lower, partly because the rabbits became resistant to the pathogen, but also partly because the virus had evolved a lower virulence. The reduction in the virulence is thought to have been aided because the virus is transmitted by a mosquito, from one living rabbit to another. The less deadly viral strain is maintained in the rabbit host population because rabbits afflicted with the more virulent strain would die before passing on the virus. Thus, the viral genes for reduced virulence could spread by group selection. Of course, reduced virulence is also in the interest of every individual virus, if it is to persist in its host. Scientists argue that one would not expect to observe evolution by group selection when individual selection is acting strongly in an opposing direction.

Some biologists, most notably Richard Dawkins (1941), have argued that the gene itself is the true unit of selection. If one genetic alternative, or allele, provides its bearer with an adaptive advantage over some other individual who carries a different allele then the more beneficial allele will be replicated more times, as its bearer enjoys greater fitness. In his book The Selfish Gene, Dawkins argues that genes help to build the bodies that aid in their transmission; individual organisms are merely the "survival machines" that genes require to make more copies of themselves.

This argument has been criticized because natural selection cannot "see" the individual genes that reside in an organism's genome, but rather selects among phenotypes, the outward manifestation of all the genes that organisms possess. Some genetic combinations may confer very high fitness, but they may reside with genes having negative effects in the same individual. When an individual reproduces, its "bad" genes are replicated along with its "good" genes; if it fails to do so, even its most advantageous genes will not be transmitted into the next generation. Although the focus among most evolutionary biologists has been on selection at the level of the individual, this example raises the possibility that individual genes in genomes are under a kind of group selection. The success of single genes in being transmitted to subsequent generations will depend on their functioning well together, collectively building the best possible organism in a given environment.

When selective change is brought about by human effort, it is known as artificial selection. By allowing only a selected minority of individuals or specimen to reproduce, breeders can produce new generations of organisms (e.g. a particular virus or bacterium) that feature desired traits.

See also Epidemiology; Evolution and evolutionary mechanisms; Evolutionary origin of bacteria and viruses; Rare genotype advantage

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selection

selection: In Darwinism, the mechanism of natural selection is considered of major importance in the process of evolution. Popular formulations sometimes envisage a struggle for existence in which direct competition for mates or for various factors in the environment (e.g., food, water, and suitable space) counteracts the tendency toward overproduction of plants and animals resulting from the process of reproduction. But there are diverse ways other than direct struggle through which those organisms better adapted to the environment can survive and reproduce more successfully than those less fitted. A special form of natural selection, sexual selection, is also stressed in Darwinism. It attempts to account for secondary sexual characteristics that are not necessarily valuable in the struggle for existence. It assumes that the female selects as a mate one having the most highly developed of such characteristics, e.g., elaborate plumage or superior song, thereby perpetuating those characteristics. However, this interpretation is now questioned by many scientists. Artificial selection, the selection by humans of individuals best suited for their purposes, is common in plant and animal breeding.

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selection

se·lec·tion / səˈlekshən/ • n. 1. the action or fact of carefully choosing someone or something as being the best or most suitable: such men decided the selection of candidates they objected to his selection. ∎  a number of carefully chosen things: the publication of a selection of his poems. ∎  a range of things from which a choice may be made: the restaurant offers a wide selection of hot and cold dishes. ∎  a horse or horses tipped as worth bets in a race or meeting. 2. Comput. data highlighted on a computer screen that is a target for various manipulations: your selection may not contain two different data types. ∎  the action or capability of selecting data in this way. 3. Biol. a process in which environmental or genetic influences determine which types of organism thrive better than others, regarded as a factor in evolution.See also natural selection.

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selection

selection A process that results from the differential reproduction of one phenotype as compared with other phenotypes in the same population. This determines the relative share of different genotypes which individuals possess and propagate in a population. The relative probability of survival and reproduction of a phenotype is termed ‘fitness’ or ‘Darwinian fitness’.

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selection

selection A process that results from the differential reproduction of one phenotype as compared with other phenotypes in the same population. This determines the relative share of different genotypes which individuals possess and propagate in a population. The relative probability of survival and reproduction of a phenotype is termed ‘fitness’ or ‘Darwinian fitness’.

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selection

selection The process that determines the relative share of different genotypes which individuals possess and propagate in a population. The relative probability of survival and reproduction of a genotype is termed the adaptive value. Selection may be natural (i.e. by nature) or artificial (i.e. by human action, e.g. plant or animal breeders).

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selection

selection The process by which one or more factors acting on a population produce differential mortality and favour the transmission of specific characteristics to subsequent generations. See artificial selection; directional selection; disruptive selection; natural selection; sexual selection; stabilizing selection.

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Selection

Selection

collection of things selected.

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selection

selectionashen, fashion, passion, ration •abstraction, action, attraction, benefaction, compaction, contraction, counteraction, diffraction, enaction, exaction, extraction, faction, fraction, interaction, liquefaction, malefaction, petrifaction, proaction, protraction, putrefaction, redaction, retroaction, satisfaction, stupefaction, subtraction, traction, transaction, tumefaction, vitrifaction •expansion, mansion, scansion, stanchion •sanction •caption, contraption •harshen, Martian •cession, discretion, freshen, session •abjection, affection, circumspection, collection, complexion, confection, connection, convection, correction, defection, deflection, dejection, detection, direction, ejection, election, erection, genuflection, imperfection, infection, inflection, injection, inspection, insurrection, interconnection, interjection, intersection, introspection, lection, misdirection, objection, perfection, predilection, projection, protection, refection, reflection, rejection, resurrection, retrospection, section, selection, subjection, transection, vivisection •exemption, pre-emption, redemption •abstention, apprehension, ascension, attention, circumvention, comprehension, condescension, contention, contravention, convention, declension, detention, dimension, dissension, extension, gentian, hypertension, hypotension, intention, intervention, invention, mention, misapprehension, obtention, pension, prehension, prevention, recension, retention, subvention, supervention, suspension, tension •conception, contraception, deception, exception, inception, interception, misconception, perception, reception •Übermenschen • subsection •ablation, aeration, agnation, Alsatian, Amerasian, Asian, aviation, cetacean, citation, conation, creation, Croatian, crustacean, curation, Dalmatian, delation, dilation, donation, duration, elation, fixation, Galatian, gyration, Haitian, halation, Horatian, ideation, illation, lavation, legation, libation, location, lunation, mutation, natation, nation, negation, notation, nutation, oblation, oration, ovation, potation, relation, rogation, rotation, Sarmatian, sedation, Serbo-Croatian, station, taxation, Thracian, vacation, vexation, vocation, zonation •accretion, Capetian, completion, concretion, deletion, depletion, Diocletian, excretion, Grecian, Helvetian, repletion, Rhodesian, secretion, suppletion, Tahitian, venetian •academician, addition, aesthetician (US esthetician), ambition, audition, beautician, clinician, coition, cosmetician, diagnostician, dialectician, dietitian, Domitian, edition, electrician, emission, fission, fruition, Hermitian, ignition, linguistician, logician, magician, mathematician, Mauritian, mechanician, metaphysician, mission, monition, mortician, munition, musician, obstetrician, omission, optician, paediatrician (US pediatrician), patrician, petition, Phoenician, physician, politician, position, rhetorician, sedition, statistician, suspicion, tactician, technician, theoretician, Titian, tuition, volition •addiction, affliction, benediction, constriction, conviction, crucifixion, depiction, dereliction, diction, eviction, fiction, friction, infliction, interdiction, jurisdiction, malediction, restriction, transfixion, valediction •distinction, extinction, intinction •ascription, circumscription, conscription, decryption, description, Egyptian, encryption, inscription, misdescription, prescription, subscription, superscription, transcription •proscription •concoction, decoction •adoption, option •abortion, apportion, caution, contortion, distortion, extortion, portion, proportion, retortion, torsion •auction •absorption, sorption •commotion, devotion, emotion, groschen, Laotian, locomotion, lotion, motion, notion, Nova Scotian, ocean, potion, promotion •ablution, absolution, allocution, attribution, circumlocution, circumvolution, Confucian, constitution, contribution, convolution, counter-revolution, destitution, dilution, diminution, distribution, electrocution, elocution, evolution, execution, institution, interlocution, irresolution, Lilliputian, locution, perlocution, persecution, pollution, prosecution, prostitution, restitution, retribution, Rosicrucian, solution, substitution, volution •cushion • resumption • München •pincushion •Belorussian, Prussian, Russian •abduction, conduction, construction, deduction, destruction, eduction, effluxion, induction, instruction, introduction, misconstruction, obstruction, production, reduction, ruction, seduction, suction, underproduction •avulsion, compulsion, convulsion, emulsion, expulsion, impulsion, propulsion, repulsion, revulsion •assumption, consumption, gumption, presumption •luncheon, scuncheon, truncheon •compunction, conjunction, dysfunction, expunction, function, junction, malfunction, multifunction, unction •abruption, corruption, disruption, eruption, interruption •T-junction • liposuction •animadversion, aspersion, assertion, aversion, Cistercian, coercion, conversion, desertion, disconcertion, dispersion, diversion, emersion, excursion, exertion, extroversion, immersion, incursion, insertion, interspersion, introversion, Persian, perversion, submersion, subversion, tertian, version •excerption

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