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Topic: Balancing selection

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	Wikinfo \| Selection (Site not responding. Last check: 2007-10-14)
		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.
		Directional selection occurs when characteristic lie along a phenotypic spectrum and the individuals at one end are more successful, and balancing selection is a form of selection in which multiple characteristics may be favored.
		A mechanisms of selection should be distinguished from their phenotypic effect, for example, disruptive selection often is the result of disassortative sexual selection and balancing selection may result from frequency-dependent selection and overdominance.
	www.wikinfo.org /wiki.php?title=Selection (667 words)

	Balancing selection - Wikipedia, the free encyclopedia
		Balancing selection is in contrast to directional selection which favor a single allele.
		A balanced polymorphism is a situation in which balancing selection within a population is able to maintain stable frequencies of two or more phenotypic forms.
		Note that balancing selection will not always result in an observable phenotypic difference because the genotype may not be one-to-one with the phenotype.
	en.wikipedia.org /wiki/Balancing_selection (526 words)

	Heredity - The evolutionary ecology of the major histocompatibility complex (Site not responding. Last check: 2007-10-14)
		Selection acting in contemporary populations is expected to have a detectable effect on genotypic frequencies within that population (Hedrick et al, 2000).
		Selection is expected to both increase the apparent rate of nucleotide substitution relative to neutral expectations, and also retain mutations longer than would normally be expected under a neutral model.
		Balancing selection can act to retain alleles among species for considerably longer periods of time (Takahata and Nei, 1990), increasing the time over which there is incomplete lineage sorting and delaying the time to monophyly.
	www.nature.com /uidfinder/10.1038/sj.hdy.6800724 (10154 words)

	evolgen: Detecting Natural Selection (Part 7)
		Because we are detecting the recovery from the selective sweep and not the sweep itself, it will take some time after the sweep for the pattern to appear, and it will eventually fade away once the polymorphism has recovered to normal levels.
		Balancing selection is expected to maintain excess polymorphism (relative to the neutral expectation) around the site under selection.
		Unlike detecting positive selection, the signature of balancing selection is expected to persist for as long as the polymorphism is maintained by selection.
	scienceblogs.com /evolgen/2006/01/detecting_natural_selection_pa_1.php (1479 words)

	Rediscovering Biology - Online Textbook: Unit 9 Human Evolution
		This situation, where selection actively maintains two or more alleles at a locus, is called balancing selection.
		Balancing selection can arise by the heterozygotes having a selective advantage, as in the case of sickle cell anemia.
		This balancing selection is so powerful that alleles are maintained that predate the human/chimp split.
	www.learner.org /channel/courses/biology/textbook/humev/humev_8.html (549 words)

	ITB Load Balancing
		Select elements for migration by "peeling" slices from the interprocessor boundary.
		Select elements for migration by choosing elements with the greatest difference in the distances from the element to the centroids of the sending and receiving processors.
		Select elements for migration by choosing elements with the greatest projected difference in the distances from the element to the centroids of the sending and receiving processors.
	www.cs.williams.edu /~terescoj/research/pmdb_manual/node11.html (609 words)

	Selection - Wikipedia, the free encyclopedia
		Aspects of selection may be divided into effects on a phenotype and their causes.
		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.
		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.
	en.wikipedia.org /wiki/Selection (689 words)

	[No title] (Site not responding. Last check: 2007-10-14)
		Balancing selection is known to maintain genetic polymorphisms in natural populations.
		However, the extent to which balancing selection can be responsible for genetic variability depends on the specific selection mechanism considered.
		More to the point, supergene selection is another balancing selection force that can contribute to the maintenance of inversion polymorphisms.
	www.lcb.uu.se /smnr.php?object=52 (272 words)

	Frequency Dependent Selection [MT Dorak]
		Natural selection would favor those parasite genotypes that can best evade host resistance mechanisms of the most common host genotypes, because they would be capable of persisting in a large proportion of the host population.
		Frequency dependent selection is a dynamic process while the other form of balancing selection -heterozygous advantage- is a stable one (see below).
		Another form of selection, overdominance or heterozygous advantage where heterozygous genotype is selected over either of the homozygous genotypes, also has a (negative) frequency dependent component because rare alleles are disproportionately found in heterozygous genotypes, whereas common alleles are disproportionately found in homozygous genotypes.
	dorakmt.tripod.com /evolution/fselect.html (631 words)

	Research
		Mhc genes are under such selection and are so diverse presumably because the parasites that are targets of Mhc genes are constantly changing, trying to escape detection by the host immune system -- a true molecular coevolutionary arms race.
		Since obvious phenotypic targets of sexual selection have been difficult to find in red-winged flbirds (the red epaulettes are more important in male-male competition for territories), maybe sexual selection is directed at "good genes" such as Mhc genes, or the immunological phenotypes conferred by such genes.
		The intron sequences reveal a pattern of polymorphism which is characteristic of directional selection, rather than a pattern expected by linkage to a balanced polymorphism.
	www.oeb.harvard.edu /faculty/edwards/research/projects/rwmhcproj.html (718 words)

	McDonald et al. 1996 (Site not responding. Last check: 2007-10-14)
		This conclusion is startling; not only does it require that there is balancing selection on all of the 14 allozyme polymorphisms, it also requires that for every allozyme polymorphism, selection favors similar allele frequencies in two environments, the Gulf of Mexico and the Atlantic Ocean, which differ in temperature and perhaps many other environmental variables.
		The only explanation other than balancing selection on allozyme polymorphisms would be that the markers used in Karl and Avise (1992) did not accurately represent the amount of geographic variation of neutral markers in oysters.
		Balancing selection at allozyme loci in oysters: implications from nuclear RFLPs.
	udel.edu /~mcdonald/oysters.html (3741 words)

	gender selection, family balancing, choosing your baby's sex - Vermesh.com (Site not responding. Last check: 2007-10-14)
		Sex selection is a very difficult procedure to obtain in the United States and world wide.
		"preconceptional sex selection can be justified on social grounds in certain cases for the objective of allowing children of the two sexes to enjoy the love and care of parents.
		"parents have traditionally had great discretion in their procreative decisions and that sex selection might provide perceived individual and social goods such as gender balance or distribution in a family with more than one child, parental companionship with a child of one's own gender, and a preferred gender order among one's children".
	www.vermesh.com /family_balancing.html (749 words)

	[No title]
		Test, adjust and balance all air and hydronic systems so that each room, piece of equipment or terminal device is using the quantities indicated on the drawings and in the specifications.
		Accomplish testing, adjusting and balancing work in a timely manner that allows partial occupancy of major buildings, occupancy of one building when the project involves many buildings, and completion of the entire project in the time stated in the Instruction to Bidders and in accordance with the completion schedule established for this project.
		The reports to be certified proof that the systems have been tested, adjusted and balanced in accordance with the referenced standards; are an accurate representation of how the systems have been installed and are operating; and are an accurate record of all final quantities measured to establish normal operating values of the systems.
	www.doa.state.wi.us /dsf/documents/tbpp98r1.doc (4029 words)

	The Scientist : No cannibalism signature in human gene (Site not responding. Last check: 2007-10-14)
		Too many common haplotypes is the mark of balancing selection, Kreitman said, because it suggests that multiple gene variants have been selected for simultaneously.
		The values of their statistical results suggest that positive selection may have acted upon the gene, Bertranpetit said, although complex scenarios, including episodes of both positive and purifying selection, may also explain the data.
		Mead et al., "Balancing selection at the prion protein gene consistent with prehistoric kurulike epidemics," Science 300:640-643, 25 Apr 2003.
	www.the-scientist.com /news/display/22927 (777 words)

	Balancing interclonal selection (Site not responding. Last check: 2007-10-14)
		Balancing interclonal selection and the maintenance of more than one hemiclone in populations of the hybrid frog Rana esculenta.
		We hypothesize that its widespread occurrence may be the result of either natural selection for a single hemiclone acting as a broadly adapted "general-purpose" genotype, or of balancing interclonal selection that maintains multiple hemiclones that each perform differently across environments that differ in space or time.
		They also indicate that fitness advantages change depending on the environment, and that in temporally and spatially variable habitats like ponds, balancing selection among hemiclones may promote coexistence among multiple hemiclones.
	evolution.genetics.washington.edu /PBhtmls/abstracts/selection.html (641 words)

	sex selection, genetic screening, gender selection - Vermesh.com (Site not responding. Last check: 2007-10-14)
		Sex selection allows determination of the sex of the child to be born.
		Couples may be interested in selecting the sex of the child because a specific genetic mutation is known to be present in one of the parents, which can manifest itself with disease in male or female offspring only.
		If an intended mother carries the mutation on one of her X chromosomes (although she is not affected with the disease), there is a 50% likelihood of having an affected male offspring, while no female offspring would be affected.
	www.vermesh.com /sex_selection.html (470 words)

	General Information on Alfalfa Leafcutting Bees (Site not responding. Last check: 2007-10-14)
		The third type of selection that maintains both genotypic and phenotypic variation in populations is frequency-dependent selection.
		Frequency-dependent selection occurs when the fitnesses associated with different phenotypes depends on the relative frequency of each phenotype.
		Frequency-dependent selection is most commonly negatively frequency-dependent, meaning that the relative fitness of a phenotype declines as it becomes more common in a population, whereas the relative fitness of the alternative phenotype (or phenotypes) increases as they become rarer.
	scarab.msu.montana.edu /biol403/BIOL_403_Selection-Freq-Dependent.html (997 words)

	[No title]
		This document proposes guidelines for implementation of load balancing such that path test mechanisms are not impacted.
		The mechanisms to select which path are currently unspecified and frequently have payload and/or label stack dependencies.
		The object of this draft is to provide guidelines such that operators may balance the need for testability and operational friendliness with the need for smooth randomization in load balancing.
	www3.ietf.org /proceedings/03jul/I-D/draft-allan-mpls-loadbal-04.txt (1948 words)

	[Frontiers in Bioscience 3, d509-516, May 26, 1998]
		This selection has several consequences for MHC biology that make these genes different from neutrally evolving genes: an enhanced rate of nonsynonymous nucleotide substitution in codons encoding the peptide-binding region; long-lasting ("trans-species") polymorphism; and homogenization of introns relative to exons as a result of recombination and subsequent genetic drift.
		Overdominant selection is expected to produce such a pattern of nucleotide substitution because this type of selection accelerates the rate of amino acid replacement (18); and certain other types of balancing selection may have a similar effect.
		Because selectively neutral polymorphisms are not expected to be maintained for long periods of time, the long persistence of MHC polymorphisms is evidence that they are selectively maintained (22).
	www.bioscience.org /1998/V3/d/hughes/d510-516.htm (5091 words)

	General Information on Alfalfa Leafcutting Bees (Site not responding. Last check: 2007-10-14)
		Thus far, when discussing selection on a single locus with two alleles, we have restricted ourselves to examples in which one of the homozygotes has the highest fitness (W = 1.0.
		The resulting form of natural selection is referred to as balancing selection.
		Second, balancing selection maintains both alleles in the population indefinitely.
	scarab.msu.montana.edu /Biol403/BIOL_403_Selection-Balancing.html (391 words)

	GenSelect's Good News Gender Selection Blog : GenSelect.com
		The argument for permitting preconception gender selection is that it serves the desires of couples who have strong preferences about the gender of their offspring, some of whom might use abortion or embryo selection to realize their goal or be unhappy Ã—
		The concept of family balancing is a measured approach to the use of preconception gender selection.
		Family balancing provides married couples having at least one child the opportunity to use various methods to increase the chance of having another child of the less represented sex in the family.
	www.genselect.com /goodnews (462 words)

	McDonald 1996 (Site not responding. Last check: 2007-10-14)
		Natural selection, in the form of balancing selection or selective sweeps, can result in a decoupling of the amounts of molecular polymorphism and divergence.
		Near a balanced polymorphism that is much older than the average coalescence time, the within-species branches of the phylogeny will be longer.
		If they are under balancing selection, these amino acid polymorphisms may have arisen too recently to have accumulated excess silent polymorphism at linked sites, as suggested by the lack of silent DNA variation within one allozyme allele of Sod (Hudson et al.
	udel.edu /~mcdonald/runs.html (4723 words)

	Molecular Population Genetics and the Search for Adaptive Evolution in Plants -- Wright and Gaut 22 (3): 506 -- ... (Site not responding. Last check: 2007-10-14)
		balancing selection elevates diversity (Kaplan, Darden, and Hudson 1988).
		to a selective sweep or balancing selection in A.
		Long-range patterns of diversity and linkage disequilibrium surrounding the maize Y1 gene are indicative of an asymmetric selective sweep.
	mbe.oxfordjournals.org /cgi/content/full/22/3/506 (7867 words)

	BIO 304. Ecology & Evolution: Population Genetics
		Selection for small or large body size in the guppies (Poecilia reticulata) in Trinidad.
		Maintenance of high frequencies of harmful or deleterious alleles is usually due to balancing selection through heterozygote advantage (i.e.
		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)

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