Negative Selection (natural Selection)
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Negative Selection (natural Selection)
In natural selection, negative selection or purifying selection is the selective removal of alleles that are deleterious. This can result in stabilising selection through the purging of deleterious genetic polymorphisms that arise through random mutations. Purging of deleterious alleles can be achieved on the population genetics level, with as little as a single point mutation being the unit of selection. In such a case, carriers of the harmful point mutation have fewer offspring each generation, reducing the frequency of the mutation in the gene pool. In the case of strong negative selection on a locus, the purging of deleterious variants will result in the occasional removal of linked variation, producing a decrease in the level of variation surrounding the locus under selection. The incidental purging of non-deleterious alleles due to such spatial proximity to deleterious alleles is called background selection. This effect increases with lower mutation rate but decreases wit ...
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Natural Selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generations. Charles Darwin popularised the term "natural selection", contrasting it with selective breeding, artificial selection, which is intentional, whereas natural selection is not. Genetic diversity, Variation of traits, both Genotype, genotypic and phenotypic, exists within all populations of organisms. However, some traits are more likely to facilitate survival and reproductive success. Thus, these traits are passed the next generation. These traits can also become more Allele frequency, common within a population if the environment that favours these traits remains fixed. If new traits become more favoured due to changes in a specific Ecological niche, niche, microevolution occurs. If new traits become more favoured due to changes in the ...
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Pinus Sylvestris
''Pinus sylvestris'', the Scots pine (UK), Scotch pine (US), Baltic pine, or European red pine is a species of tree in the pine family Pinaceae that is native plant, native to Eurasia. It can readily be identified by its combination of fairly short, blue-green leaves and orange-red bark. Description ''Pinus sylvestris'' is an evergreen coniferous tree growing up to in height and in trunk diameter when mature, exceptionally over tall and in trunk diameter on very productive sites. The tallest on record is a tree over 210 years old growing in Estonia which stands at . The lifespan is normally 150–300 years, with the oldest recorded specimens in Lapland (Finland), Lapland, Northern Finland over 760 years. The Bark (botany), bark is thick, flaky and orange-red when young to scaly and gray-brown in maturity, sometimes retaining the former on the upper portion. The habit of the mature tree is distinctive due to its long, bare and straight trunk topped by a rounded or flat- ...
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Selection
Selection may refer to: Science * Selection (biology), also called natural selection, selection in evolution ** Sex selection, in genetics ** Mate selection, in mating ** Sexual selection in humans, in human sexuality ** Human mating strategies, in human sexuality * Social selection, within social groups * Selection (linguistics), the ability of predicates to determine the semantic content of their arguments * Selection in schools, the admission of students on the basis of selective criteria * Selection effect, a distortion of data arising from the way that the data are collected * A selection, or choice function, a function that selects an element from a set Religion * Divine selection, selection by God * Papal selection A conclave is a gathering of the College of Cardinals convened to appoint the pope of the Catholic Church. Catholics consider the pope to be the apostolic successor of Saint Peter and the earthly head of the Catholic Church. Concerns around po ...
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Stabilizing Selection
Stabilizing selection (not to be confused with negative or purifying selection) is a type of natural selection in which the population mean stabilizes on a particular non-extreme trait value. This is thought to be the most common mechanism of action for natural selection because most traits do not appear to change drastically over time. Stabilizing selection commonly uses negative selection (a.k.a. purifying selection) to select against extreme values of the character. Stabilizing selection is the opposite of disruptive selection. Instead of favoring individuals with extreme phenotypes, it favors the intermediate variants. Stabilizing selection tends to remove the more severe phenotypes, resulting in the reproductive success of the norm or average phenotypes. This means that most common phenotype in the population is selected for and continues to dominate in future generations. History The Ukrainian and later Soviet evolutionary biologist Ivan Schmalhausen founded the theo ...
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Mutation–selection Balance
Mutation–selection balance is an equilibrium in the number of deleterious alleles in a population that occurs when the rate at which deleterious alleles are created by mutation equals the rate at which deleterious alleles are eliminated by selection.Herron, JC and S Freeman. 2014. Evolutionary Analysis, 5th Edition. Pearson. The majority of genetic mutations are neutral or deleterious; beneficial mutations are relatively rare. The resulting influx of deleterious mutations into a population over time is counteracted by negative selection, which acts to purge deleterious mutations. Setting aside other factors (e.g., balancing selection, and genetic drift), the equilibrium number of deleterious alleles is then determined by a balance between the deleterious mutation rate and the rate at which selection purges those mutations. Mutation–selection balance was originally proposed to explain how genetic variation is maintained in populations, although several other ways for deleterious ...
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Koinophilia
Koinophilia is an evolutionary hypothesis proposing that during sexual selection, animals preferentially seek mates with a minimum of unusual or mutant features, including functionality, appearance and behavior. Koinophilia intends to explain the clustering of sexual organisms into species and other issues described by Darwin's dilemma. The term derives from the Greek word ''koinos'' meaning "common" or "that which is shared", and ''philia'', meaning "fondness". Natural selection causes beneficial inherited features to become more common at the expense of their disadvantageous counterparts. The koinophilia hypothesis proposes that a sexually-reproducing animal would therefore be expected to avoid individuals with rare or unusual features, and to prefer to mate with individuals displaying a predominance of common or average features. Mutants with peculiar features would be avoided because most mutations that manifest themselves as changes in appearance, functionality or behavi ...
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Genetic Purging
Genetic purging is the increased pressure of natural selection against deleterious alleles prompted by inbreeding. Purging occurs because deleterious alleles tend to be recessive, which means that they only express all their harmful effects when they are present in the two copies of the individual (i.e., in homozygosis). During inbreeding, as related individuals mate, they produce offspring that are more likely to be homozygous so that deleterious alleles express all their harmful effects more often, making individuals less fit. Purging reduces both the overall number of recessive deleterious alleles and the decline of mean fitness caused by inbreeding (the inbreeding depression for fitness). The term "purge" is sometimes used for selection against deleterious alleles in a general way. It would avoid ambiguity to use "purifying selection" in that general context, and to reserve "purging" to its more strict meaning defined above. The mechanism Deleterious alleles segregating in ...
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Fluctuating Selection
Fluctuating selection is a mode of natural selection characterized by the fluctuation of the directional selection, direction of selection on a given phenotype over a relatively brief period of evolutionary time. For example, a species of plant may come in two varieties: one which prefers wetter soil and one which prefers dryer soil. During a period of wet years, the wet variety will be more fitness (biology), fit and produce more offspring, and thereby increase the frequency of wet-preferring plants. If this wet period is followed by drought, the dry variety will be selected for and its numbers will increase. As periods of dryness and wetness fluctuate, so too does selection on dry-preferring and wet-preferring plants. Fluctuating selection is also manifest at the genic level. Consider two alleles, A and B, which are found at the same locus (genetics), locus. Fluctuating selection dynamics are at play when selection favors A at time t0, B at t1 and A again at t2. Fluctuating select ...
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Dysgenics
Dysgenics refers to any decrease in the prevalence of traits deemed to be either socially desirable or generally adaptive to their environment due to selective pressure disfavouring their reproduction. In 1915 the term was used by David Starr Jordan to describe the supposed deleterious effects of modern warfare on group-level genetic fitness because of its tendency to kill physically healthy men while preserving the disabled at home. Similar concerns had been raised by early eugenicists and social Darwinists during the 19th century, and continued to play a role in scientific and public policy debates throughout the 20th century. More recent concerns about supposed dysgenic effects in human populations were advanced by the controversial psychologist and self-described " scientific racist" Richard Lynn, notably in his 1996 book '' Dysgenics: Genetic Deterioration in Modern Populations'', which argued that changes in selection pressures and decreased infant mortality since the I ...
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Disruptive Selection
In evolutionary biology, disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups. In this more individuals acquire peripheral character value at both ends of the distribution curve. Overview Natural selection is known to be one of the most important biological processes behind evolution . There are many variations of traits, and some cause greater or lesser reproductive success of the individual. The effect of selection is to promote certain alleles, traits, and individuals that have a higher chance to survive and reproduce in their specific environment. Since the environment has a carrying capacity, nature acts on this mode of selection on individuals to let only the most fit offspring survive and reproduce to their full potential. The more advantageou ...
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Directional Selection
In population genetics, directional selection is a type of natural selection in which one extreme phenotype is favored over both the other extreme and moderate phenotypes. This genetic selection causes the allele frequency to shift toward the chosen extreme over time as allele ratios change from generation to generation. The advantageous extreme allele will increase in frequency among the population as a consequence of survival and reproduction differences among the different present phenotypes in the population. The allele fluctuations as a result of directional selection can be independent of the dominance of the allele, and in some cases if the allele is recessive, it can eventually become fixed in the population. Directional selection was first identified and described by naturalist Charles Darwin in his book ''On the Origin of Species'' published in 1859. He identified it as a type of natural selection along with stabilizing selection and disruptive selection. These types ...
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Balancing Selection
Balancing selection refers to a number of selective processes by which multiple alleles (different versions of a gene) are actively maintained in the gene pool of a population at frequencies larger than expected from genetic drift alone. Balancing selection is rare compared to purifying selection. It can occur by various mechanisms, in particular, when the heterozygotes for the alleles under consideration have a higher fitness than the homozygote. In this way genetic polymorphism is conserved. Evidence for balancing selection can be found in the number of alleles in a population which are maintained above mutation rate frequencies. All modern research has shown that this significant genetic variation is ubiquitous in panmictic populations. There are several mechanisms (which are not exclusive within any given population) by which balancing selection works to maintain polymorphism. The two major and most studied are heterozygote advantage and frequency-dependent selection. Me ...
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