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Topic: Stabilizing selection

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	Stabilizing selection - Wikipedia, the free encyclopedia
		Stabilizing selection, also known as purifying selection or negative selection, is a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value.
		Stabilizing selection operates most of the time in most populations.
		Stabilizing selection can sometimes be detected by measuring the fitness of the range of different phenotypes by various direct measures, but it can also be detected by a variety of tests of molecular sequence data, such as Ka/Ks ratios, changes in allele frequency distributions and the McDonald Kreitman test.
	en.wikipedia.org /wiki/Stabilizing_selection (245 words)

	Directional selection - Wikipedia, the free encyclopedia
		In population genetics, directional selection (sometimes referred to as positive selection) occurs when natural selection favors a single allele and therefore allele frequency continuously shifts in one direction.
		It stands in contrast to balancing selection where selection may favor multiple alleles, or purifying selection which removes deleterious mutations from a population.
		Directional selection is a particular mode or mechanism of natural selection.
	en.wikipedia.org /wiki/Directional_selection (118 words)

	[No title]
		Selection coefficients for each of the 15 dimensions of the shape space were drawn each generation from a normal distribution with a mean of zero and a standard deviation of 0.426.
		Stabilizing selection, in which selection pushes the morphology back towards the ancestral form if it moves too far away, was modelled in the third simulation.
		Stabilizing selection was simulated here using a single-peaked adaptive landscape to adjust the distribution from which selection coefficients were drawn (Figure 8).
	palaeo-electronica.org /2004_2/evo/results.htm (1681 words)

	Introduction to Natural and Sexual Selection (Site not responding. Last check: 2007-10-12)
		Stabilizing selection eliminates the extremes in a distribution of phenotypes, and as such it leads to a refinement of the exisiting type.
		While selection on costs of reproduction that the female experiences in terms of reduced fecundity tends to favor the production of small clutches, the selection on offspring survival to recruitment and the number of offspring that offspring produce at maturity are both under significant stabilizing selection.
		Selection on maternal investment should have a genetic basis if the trait is to respond to natural selection and indeed egg size of the mother is positively correlated with egg size of daughter's.
	bio.research.ucsc.edu /~barrylab/classes/animal_behavior/SELECT.HTM (11775 words)

	Evolution - Natural selection and variation
		Studies of birth weight in humans have provided good examples of stabilizing selection: the graph illustrates a classic result for a sample in London, UK, in 1935 - 1946 and similar results have been found in New York, Italy, and Japan.
		Stabilizing selection has probably operated on birth weight in human populations from the time of the evolutionary expansion of our brains about 1 - 2 million years ago until the 20th century.
		Now, in the 1990s in wealthy countries, the stabilizing selection that had been operating on human birth weight for over a million years has all but disappeared.
	www.blackwellpublishing.com /ridley/tutorials/Natural_selection_and_variation10.asp (176 words)

	Natural Selection
		Directional Selection - This is selection against one of the extremes of the population - it causes the population mean to change
		Stabilizing Selection (or Balancing Selection) - This is selection against all extreme phenotypes - the mean of the population remains constant but the variance decreases
		Disruptive Selection - Selection against individuals with a intermediate phenotype - The population mean remains constant but the variance increases.
	www.as.wvu.edu /coll03/bio/users/kgarbutt/public_html/EvolutionPage/NaturalSelection.htm (471 words)

	[No title] (Site not responding. Last check: 2007-10-12)
		We show that in polymorphic populations many polygenic traits pleiotropically related to fitness are expected to be under apparent "stabilizing selection" independently of the real selection acting on the population.
		Stabilizing selection is also observed if the polygenic system is at an equilibrium determined by a balance between selection and mutation (or migration) when both additive and nonadditive contributions of the loci to the trait value are random and independent of those to fitness.
		In the overdominant model and in the symmetric viability model the strength of apparent stabilizing selection is approximately 1/(2n) that of total selection on the whole phenotype.
	www.tiem.utk.edu /~gavrila/PAPS/pleiot.html (298 words)

	Gene Expression: Stabilizing selection & the illusion of the fossils
		Stabilizing selection & the illusion of the fossils
		Stabilizing selection "acts" against change by favoring intermediate phenotypes.
		It is important to note that selection is still operative, it simply reduces the fitness of those at the tails of the distribution, canalizing the species toward a particular phenotypic configuration (think more metastable as opposed to static).
	www.gnxp.com /MT2/archives/003628.html (451 words)

	Lecture VIII
		Stabilizing selection does not affect the mean of the trait value distribution (assuming completely symmetrical stabilizing selection).
		Pure, symmetric stabilizing selection is non-linear with an intermediate optimum.
		Selection on one trait (survival in the new host) causes an indirect reponse in the other trait (survival in the old host).
	www.indiana.edu /~curtweb/S318/S318/lectureviii/lectureviii.html (567 words)

	Pun 1.4.htm (Site not responding. Last check: 2007-10-12)
		He divorced evolution from natural selection by maintaining that natural selection merely has the negative effect of "pruning" the genetic varieties that are unfit to survive and thus plays no role in the diversification of genic variation that is the essence of evolution.
		Thus, selection tends to maintain the color of the mice within a narrow range that is determined by the color of the field.
		Directional selection is the force that drives the population to undergo evolutionary changes in one direction with respect to certain adaptive characteristics.
	www.ibri.org /Books/Pun_Evolution/Chapter1/1.4.htm (1693 words)

	Dynamics of Genetic Variability in Two-Locus Models of Stabilizing Selection -- from Mathematica Information Center
		We study a two locus model, with additive contributions to the phenotype, to explore the dynamics of different phenotype characteristics under stabilizing selection and recombination.
		We argue that if selection is not extremely weak relative to recombination, linkage disequilbrium generated by stabilizing selection influences the dynamics significantly.
		We demonstrate that under these conditions, which are plausible in nature and certainly the case in artificial stabilizing selection experiments, the model can have a polymorphic equilibrium with positive linkage disequilibrium that is stable simultaneously with monomorphic equilibria.
	library.wolfram.com /infocenter/Articles/1954 (217 words)

	Biology524.Lectures4
		Stabilizing selection results when itermediate phenotypes have the highest fitness, that is, when the fitness curve is higher in the middle than at either end.
		While stabilizing selection in a single-locus case results from heterozygous advantage, quantitative polygenic traits are subject to stabilizing sleection without the necessary link to heterozygosity.
		In many cases, the strength of selection for or against a genotype is affected by the frequency of that genotype in the population.
	bioweb.wku.edu /faculty/McElroy/BIOL524/524lects4.htm (722 words)

	Quant.genetics.2.S318
		If the phenotypic distribution of the trait is due to a large number of polygenic loci, then response to selection is a function of both the heritability of a trait (e.g., additive genetic variance) and the strength of selection on the trait (e.g., selection differential).
		The selection differential, s, is the difference in mean of the selected parents versus to unselected parents (red vs red+blue distributions on the horizontal axis).
		Response to selection will always be less than the selection differential because, h2, the slope of the line is less than one, or the proportion of phenotypic variation explained by additive genetic causes should be less than one.
	bio.research.ucsc.edu /~barrylab/classes/evolution/NS_H2.HTM (3512 words)

	The Questionable Authority Archive: Lonnig's "Dynamic Genomes" paper: A quick critique. (Site not responding. Last check: 2007-10-12)
		In simple terms, stabilizing selection is nature's way of saying, "if it ain't broke, don't fix it." In slightly more technical terms, stabilizing selection is a type of natural selection that occurs when the version of a trait that is currently present in the population is the one that is associated with the highest fitness.
		Stabilizing selection is, at the absolute minimum, something that needs to be considered as a possible explanation for a lack of evolutionary change in a trait.
		Stabilizing selection is typically discussed in introductory biology classes, so it's difficult to believe that he didn't know about it.
	thequestionableauthority.blogspot.com /2006/01/lonnigs-dynamic-genomes-paper-quick.html (2707 words)

	BIO 304. Ecology & Evolution: Population Genetics
		Selection for small or large body size in the guppies (Poecilia reticulata) in Trinidad.
		sexual selection: differences in mating opportunity among individuals due to male contest and female choice.
		Fact: Sexual selection results in dimorphism (pronounced phenotypic differences) between the sexes in many instances.
	www.micro.utexas.edu /courses/levin/bio304/popgen/nat.selection.html (380 words)

	Zoology 510, Chapter 9 notes
		Therefore, the selection coefficient associated with a particular genotype is, at best, an average over the various genetic environments (haplotypes) and external environments (ecological circumstances) in which the genotype may find itself.
		In artificial selection, the selection differential for a trait is defined as the difference between the mean of the selected individuals (those which breed the next generation) and the mean of the entire population from which those individuals were selected.
		Similarly, the response to selection is defined as the difference between the offspring mean and the mean of the entire population from which their parents were selected.
	www.science.siu.edu /zoology/king/510/mr09.htm (1923 words)

	Predictions of Patterns of Response to Artificial Selection in Lines Derived From Natural Populations -- Zhang and Hill ...
		With intensity i of selection on a quantitative trait with phenotypic
		and the response of the trait to artificial selection.
		Base populations for artificial selection are either drawn immediately from the wild ("NS") or maintained captive for 32 generations at a constant size of 160 individuals ("NS-c") under two recombination rates: free recombination between loci (thin solid lines) and three chromosomes each of length 1 M (dashed lines).
	www.genetics.org /cgi/content/full/169/1/411 (7188 words)

	Evolutionary Quantitative Genetics
		The response of fitness to selection is equal to the additive genetic variance for fitness.
		Stabilizing selection acts to reduce the phenotypic variance of the trait.
		However, there are real examples of misleading selection patterns, such as the bristle example given by Kearsey and Barnes 1970.
	www.zoology.ubc.ca /~whitlock/QGPG/QG6/Lecture.html (883 words)

	100 Concepts of Biology
		Stabilizing selection could select for these intermediate forms if the two extremes are more likely to be prey.
		Destabilizing selection can result in such a condition, with the continued presence of the two extreme phenotypes at the expense of the intermediate.
		Apparently, the beneficial effect on the heterozygotes is enough to ensure the continued presence of the sickle-cell trait, despite the clearly harmful effect of the homozygote.
	mywebpages.comcast.net /biologycentury/pages/natsel3.html (372 words)

	Genetic architecture of fitness and nonfitness traits: empirical patterns and development of ideas
		selection balance view, there is a continuous and frequent input of new mutations of which most persist very briefly, and consequently, most of the variation important to evolution and adaptation does not originate from standing variation, but from newly arisen mutations.
		Sexually selected traits often are, or have been, subject to strong directional selection (Andersson, 1994), and one should therefore probably expect to see similarities between genetic architecture of sexually selected and life history traits.
		Pomiankowski and Møller (1995) compiled 38 heritability estimates of sexually selected characters, and compared these to estimates of heritability of homologous characters in females, and characters suggested to be under stabilizing selection in the same species.
	www.nature.com /hdy/journal/v83/n2/full/6885850a.html (5111 words)

	[No title]
		Directional selection occurs when there is a change in the environment such that the phenotype at one extreme is not adapted to the new conditions while those at the other extreme have increasing survival and reproduction.
		This type of selection results in the allele that is lowest in frequency at the onset of selection being lost and the allele in highest frequency being fixed.
		In selection against the heterozygote, the allele in lowest frequency at the onset of selection will eventually be removed from the population.
	www.gwu.edu /~darwin/BiSc150/PopGen/NS.html (1742 words)

	SparkNotes: Natural Selection: Types of Natural Selection
		For example, if we were talking about height as a trait, we would see that without any selection pressure on this trait, the heights of individuals in a population would vary, with most individuals being of an average height and fewer being extremely short or extremely tall.
		When selective pressures select against the two extremes of a trait, the population experiences stabilizing selection.
		In disruptive selection, selection pressures act against individuals in the middle of the trait distribution.
	www.sparknotes.com /biology/evolution/naturalselection/section1.html (466 words)

	Introduction
		For instance, measuring natural selection on a phenotypic character may overestimate the strength of stabilizing selection acting on the character directly.
		This is because selection on a correlated character leads to apparent selection on the character under study (Lande, 1980; Lande and Arnold, 1983).
		A main result from phenotypic evolution models is that selection on the second character should not influence the evolution of the character under directional selection, as long as the Malthusian fitness surface is at most polynomial of degree two (`well behaved').
	www.cbc.yale.edu /old/cce/ccepapers/Inert2/node1.html (1542 words)

	Detection of stabilizing selection in favor of the Santa Cruz homokaryotype in Drosophila pseudoobscura populations ...
		Detection of stabilizing selection in favor of the Santa Cruz homokaryotype in Drosophila pseudoobscura populations from the high plateau of the Colombian Andes
		It is possible that different selective and stochastic evolutionary factors could be affecting not only the abundance of this species but also chromosomal rearrangements in this isolated population, and a new study on these rearrangements may shed some light on the divergent data reported by the authors cited above.
		However, at the moment we prefer to think that the selective events that are taking place in Colombia are due to environmental changes, especially trophic changes, or changes associated with human action or climatic events such as El Niño, and not to the emergence of new and better co-adapted complexes or super-genes.
	www.funpecrp.com.br /GMR/year2002/vol1-1/gmr0017_full_text.htm (9670 words)

	THE SELECTION LIMIT DUE TO THE CONFLICT BETWEEN TRUNCATION AND STABILIZING SELECTION WITH MUTATION -- Web of Science ... (Site not responding. Last check: 2007-10-12)
		A model of conflict between truncation and stabilizing selection in infinite population size is analysed in terms of the reduction in selection differential.
		Under the assumption of a normal phenotypic distribution, the limit to selection is found to be a function of kappa, the intensity of truncation selection, omega 2, a measure of the intensity of stabilizing selection, and sigma 2, the phenotypic variance of the character.
		It is found that truncation selection can substantially reduce the equilibrium genetic variance below that when only stabilizing selection is acting, and the proportional reduction in variance is greatest when the selection is very weak.
	statgen.ncsu.edu /zeng/Genetics-86-ZH.html (221 words)

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