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Topic: Directional selection

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	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 shift in one direction.
		It is in contradistinction to balancing selection where selection may favor multiple alleles, or background selection which removes deleterious mutations from a population.
		Directional selection is a particular mode or mechanism of natural selection.
	en.wikipedia.org /wiki/Positive_selection (281 words)

	Darwin was right, IU scientists show
		Directional natural selection, in which certain traits such as a giraffe's long neck are favored over others, causes most of the differences between species.
		Directional selection is a specific kind of natural selection that favors less or more of a particular trait, such as smaller fingers or darker fur giving an individual advantages over other members of its species.
		Article citation: "Directional selection is the primary cause of phenotypic diversification," Proceedings of the National Academy of Sciences of the United States of America, Vol.
	newsinfo.iu.edu /news/page/normal/552.html (513 words)

	Natural selection - Open Encyclopedia (Site not responding. Last check: 2007-10-09)
		The basic concept of natural selection is that environmental conditions (or "nature") determine (or "select") how well particular traits of organisms can serve the survival and reproduction of the organism; organisms lacking these traits might die before reproducing, or be less prolific.
		Natural selection does not distinguish between ecological selection and sexual selection, as it is concerned with traits, e.g.
		In addition, many theories of Artificial selection have been proposed to suggest that economic or social fitness factors assessed by other humans or their built environments are somehow biological or inevitable - Social Darwinism.
	open-encyclopedia.com /Natural_selection (1411 words)

	Introduction to Natural and Sexual Selection
		Directional selection is the natural analogue of artificial selection, which is practiced in the laboratory or cultivated field.
		Directional selection also has a unique effect on the population -- it leads to an evolutionary response to selection that changes the mean of the trait from one generation to the next if the trait has a heritable component.
		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)

	News & Features: (Abstract) Directional selection is the primary cause of phenotypic diversification
		Directional selection is the primary cause of phenotypic diversification
		Moreover, interspecific trait differences were more strongly or consistently selected than intraspecific differences, strengthening a growing consensus among students of speciation that directional selection is the primary cause of speciation.
		Contrary to studies of selection in contemporary populations, life history traits appear to be selected more strongly than morphological traits, but traits related to the timing of development are weakly selected relative to most other traits.
	www.iscid.org /boards/ubb-get_topic-f-1-t-000059.html (310 words)

	Selection Forces Causing the Evolution of Modularity
		Selection for adaptation rate is based on the fact that modularity can enhance the rate of evolution, because it avoids interference between different functional systems.
		The magnitude of the selection coefficients is primarily determined by stabilizing selection against the variance caused by pleiotropic effects.
		A study in which the causal components of the selection coefficient were measured shows that about 90 to 95% of the selection coefficient is caused by this direct stabilizing selection on the variance caused by pleiotropic effects.
	www.cbc.yale.edu /old/cce/papers/HomNat/node4.html (1372 words)

	Predictions of the Dynamics of a Polygenic Character Under Directional Selection -- from Mathematica Information Center
		A quantitative genetic model for the response of the distribution of a single metric trait to directional selection is investigated.
		Selection is imposed according to an exponentially increasing fitness function, populations mate at random, have discrete generations, and all genetic effects are supposed to be additive.
		It is concluded that a significant initial increase in variance is unlikely to be observed in selection experiments if the effective populations size is not larger than 500 and if response is caused by additive genes.
	library.wolfram.com /infocenter/Articles/3234 (281 words)

	MCDB 2150 -- Lecture 38
		Directional selection: When selection alters the balance of alleles consistently in one direction, it is called directional selection.
		Selection against the homozygous recessive phenotype: The textbook presents a series of formulas for calculating change in allelic frequency due to selection that is directed only against the homozygous recessive phenotype (one of the most common patterns of selection).
		The total frequency of recessive alles after selection is therefore one half of the frequency of the heterozygote Aa plus one half of two times the frequency of the homozygous recessive aa, divided by the total allelic frequency.
	www.colorado.edu /MCDB/MCDB2150Fall/notes99/99L38.html (2515 words)

	select (Site not responding. Last check: 2007-10-09)
		Directional selection occurs when the environment of a population is changing in some definitive way.
		This is a type of selection that tends to maintain rare alleles in a population due to the fact that other forms of selection tend to act on the organisms that are present in larger numbers.
		The type of selection operating on populations is subject to dynamic change and the specific type of selection is always relative to populations in some specific context.
	taggart.glg.msu.edu /isb200/select.htm (1028 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)

	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)

	Evolution - A-Z - Directional selection (Site not responding. Last check: 2007-10-09)
		Natural selection may be directional: it may favor, for example, smaller individuals and will, if the character is inherited, produce a decrease in average body size.
		Directional selection could, of course, also produce an evolutionary increase in body size if larger individuals had higher fitness.
		Directional selection has been studied using artificial selection experiments.
	www.blackwellpublishing.com /ridley/a-z/Directional_selection.asp (150 words)

	Biology524.Lectures4
		The rate at which w bar changes under directional selection is roughly equal to the additive genetic variance, and proportional to the frequency of heterozygotes in the population; this is
		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.
		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)

	Directional selection (from evolution) -- Encyclopædia Britannica
		More results on "Directional selection (from evolution)" when you join.
		The principle of random selection from among qualified voters has come to be common in the United States, England, Canada, and...
		Selections from the liturgy used by the Assyrian Church of the East.
	www.britannica.com /eb/article-49874?tocId=49874 (814 words)

	Untitled
		Abstract.--Heritabilities, evolutionary rates, and selection intensities were calculated for 34 morphometric measurements across 211 specimens representing 14-16 species and metaspecies of the Ordovician bryozoan genus Peronopora.
		Upper limit means for 86% of characters indicate mutation-drift; stabilizing selection is indicated for 10% of characters and directional selection for 4%.
		Finally, the intensity of selection required (Lande’s [1976] b) and minimum mortality rates necessary to explain observed species transitions solely by directional selection have means of 5.5 standard deviation units from the average phenotype, and 17 selective deaths per million individuals per generation, respectively.
	www.geology.iupui.edu /research/paleolab/Projects/Rates.htm (439 words)

	Evolution
		The forces of natural selection act on phenotypes, but only if there is a change in the genotypes of a population has evolution occurred.
		If the offspring of selected parents occupy the same range as the entire population, environmental factors are working alone.
		Natural selection often works to weed out individuals at both extremes of a range of phenotypes resulting in the reproductive success of those near the mean.
	users.rcn.com /jkimball.ma.ultranet/BiologyPages/E/Evolution.html (1757 words)

	SparkNotes: Natural Selection: Types of Natural Selection
		When selective pressures select against the two extremes of a trait, the population experiences stabilizing selection.
		In directional selection, one extreme of the trait distribution experiences selection against it.
		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)

	BIO 304. Ecology & Evolution: Population Genetics
		Selection for small or large body size in the guppies (Poecilia reticulata) in Trinidad.
		Since valine carries an apolar side group, this part of the molecule is less soluble in water, and stretches out making the red blood cells elongated and rigid.
		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)

	Natural selection (Site not responding. Last check: 2007-10-09)
		Directional selection occurs when an organism must adapt to changing conditions.
		Industrial melanism in the peppered moth (Biston betularia) during the industrial revolution in England is one of the best document examples of directional selection.
		Antibiotic resistance in bacteria is another example of directional selection.
	arnica.csustan.edu /biol3020/selection/selection.htm (995 words)

	Evolution Biol 4801 (Site not responding. Last check: 2007-10-09)
		Directional selection: One extreme has higher fitness than the other and the mean phenotype shifts after selection.
		a) The extremes of a distribution are selected against, and the mean is at a selective advantage.
		Intense natural selection in a population of Darwin's Finches (Geospizinae) in the Galapagos.
	www.d.umn.edu /~tcraig/04evol/Selection04.htm (704 words)

	EvoTutor: Selection Modes
		This typically results in a change in the mean value of the trait under selection.
		occurs when selection favors the extreme trait values over the intermediate trait values.
		Populations under this type of selection typically experience a decrease in the amount of additive genetic variation for the trait under selection.
	www.evotutor.org /Selection/Sl5A.html (97 words)

	Lab 6 - Gall Size
		While we have referred to natural selection as a "force" that causes populations to evolve, it is perhaps more properly considered as an outcome of an interaction between phenotypic variation in a population and the current environment that population experiences (where the environment is broadly construed to include abiotic and biotic factors).
		Thus, if you use the difference in mean values before and after selection (2 cm) as the "strength" of selection, you would conclude that selection acted in the same way in both populations.
		If this is the case, then attack by wasps may be a factor causing directional selection on the size of galls induced by the flies.
	web.grinnell.edu /individuals/brownj/edu/136_lab6.html (2793 words)

	Strength and tempo of directional selection in the wild -- Hoekstra et al. 98 (16): 9157 -- Proceedings of the National ...
		Strength and tempo of directional selection in the wild -- Hoekstra et al.
		Strength and tempo of directional selection in the wild
		Directional selection is a major force driving adaptation and evolutionary change.
	www.pnas.org /cgi/content/full/98/16/9157 (2533 words)

	Effective Size and Polymorphism of Linked Neutral Loci in Populations Under Directional Selection -- Santiago and ...
		under directional selection is extended to cover linkage.
		a neutral gene and selected genes on chromosomes.
		for all the selective loci in the genome is
	www.genetics.org /cgi/content/full/149/4/2105 (6144 words)

	Epistasis Blog: The role of epistatic gene interactions in the response to selection and the evolution of evolvability.
		Epistasis Blog: The role of epistatic gene interactions in the response to selection and the evolution of evolvability.
		The role of epistatic gene interactions in the response to selection and the evolution of evolvability.
		We argue that directional epistasis is likely to have major effects on evolutionary dynamics and should be the focus of empirical studies of epistasis.
	compgen.blogspot.com /2005/08/role-of-epistatic-gene-interactions-in.html (280 words)

	Directional selection is the primary cause of phenotypic diversification -- Rieseberg et al. 99 (19): 12242 -- ...
		Directional selection is the primary cause of phenotypic diversification -- Rieseberg et al.
		evaluate the importance of directional selection in phenotypic divergence.
		in the direction of selection (1), and stabilizing selection,
	www.pnas.org /cgi/content/full/99/19/12242 (2770 words)

	Directional selection -- Facts, Info, and Encyclopedia article (Site not responding. Last check: 2007-10-09)
		Directional selection -- Facts, Info, and Encyclopedia article
		It is in contradistinction to (additional info and facts about balancing selection) balancing selection where selection may favor multiple alleles, or background selection which removes deleterious mutations from a population.
		Gradually, the allele frequency shifted towards the dominant allele, as more and more dark-bodied moths survived to reproduce.
	www.absoluteastronomy.com /encyclopedia/d/di/directional_selection.htm (272 words)

	Directional Selection (Site not responding. Last check: 2007-10-09)
		The phenotypic distribution before selection is a bell-curve with a mean value of 6.
		The Fitness Function is a line with positive slope, so that phenotypes with a higher mean value have greater fitness.
		The result of selection is a shift in the mean phenotype value to 8.
	www.mun.ca /biology/scarr/2900_Directional_Selection.htm (48 words)

	BIOdotEDU (Site not responding. Last check: 2007-10-09)
		Over long periods of time, in the constant presence of hungry peccaries, the population of cacti will gradually shift in the direction of the more heavily spined cactus varieties.
		The later (newer) distribution curves will show a virtual absence of low-spine-numbered plants and a much greater preponderance of high-spine-numbered plants.
		Directional selection favors one extreme value for a particular trait in a distribution of these values.
	www.brooklyn.cuny.edu /bc/ahp/LAD/C21/C21_Directional.html (226 words)

	Evidence for a Period of Directional Selection Following Gene Duplication in a Neurally Expressed Locus of ...
		Evidence for a Period of Directional Selection Following Gene Duplication in a Neurally Expressed Locus of Triosephosphate Isomerase
		selection was involved in the functional divergence of the neural
		In the absence of selective pressure, the ratio of observed
	www.genetics.org /cgi/content/full/159/2/689 (5536 words)

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