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Topic: Disruptive selection

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Fitness (biology)

Ecological selection

Natural selection

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Charles Darwin

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	Natural Selection Theme @ CreatedByGod.com (Created by God) (Site not responding. Last check: 2007-10-13)
		The 'unit of selection' is not limited to the level of individuals, but includes other levels within the hierarchy of biological organisation, such as genes, cells and relatives.
		A strong selective sweep results in a region of the genome where the positively selected haplotype (the allele and its neighbours) are essentially the only ones that exist in the population, resulting in a large reduction of the total genetic variation in that locus.
		Selective mating can be the result of, for example, a change in the physical environment (physical isolation by an extrinsic barrier), or by sexual selection resulting in assortative mating.
	www.createdbygod.com /encyclopedia/Natural_selection (4347 words)

	EvolutionLab Assignment 1 Rubric
		The concepts of directional, stabilizing and disruptive selection are clearly understood and are used appropriately to describe the results of the selection experiments.
		Average responses and selection coefficients are calculated correctly and are properly interpreted as to their significance to evolution.
		The concepts of directional, stabilizing and disruptive selection are not understood and are used inappropriately to describe the results of the selection experiments.
	www.csuchico.edu /~jbell/Biol207/evolveit1R.html (495 words)

	EvoTutor: Selection Modes
		This typically results in a change in the mean value of the trait under selection.
		Disruptive selection plays an important role in speciation.
		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)

	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 (291 words)

	Quant.genetics.2.S318
		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.
		Disruptive selection acts against individuals in the middle of the phenotype distribution and tends to favor individuals in the extremes.
		While the pattern of disruptive selection is common, it is not found for both the upper and lower bills or for all age and sex classes.
	bio.research.ucsc.edu /~barrylab/classes/evolution/NS_H2.HTM (3512 words)

	Introduction to Natural and Sexual 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.
		Disruptive selection acts against the individuals in the middle of the range of phenotypes and tends to favor individuals in the extremes.
		A simple form of disruptive selection on a single locus with two alleles where the heterozygous individuals are at a disadvantage relative to the two homozygous classes (see Side Box 2.1).
	bio.research.ucsc.edu /~barrylab/classes/animal_behavior/SELECT.HTM (11775 words)

	Disruptive selection: a tail color polymorphism in Acris tadpoles in response to differential predation.
		Disruptive selection: a tail color polymorphism in Acris tadpoles in response to differential predation.
		Disruptive selection is cost likely the mechanism responsible for maintenance of this polumorphism.
		Caldwell, J.P. Disruptive selection: a tail color polymorphism in Acris tadpoles in response to differential predation.
	www.uga.edu /srel/Reprint/0817.htm (163 words)

	Stabilizing selection - Wikipedia, the free encyclopedia
		Stabilizing selection, also known as purifying selection, is a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value.
		This type of selection acts to prevent divergence of form and function.
		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 (246 words)

	Evolution - Natural selection and variation (Site not responding. Last check: 2007-10-13)
		In nature, sexual dimorphism is probably a common example of disruptive selection; but here we shall use an experiment by Thoday and Gibson on the Drosophila melangaster fruitfly as an example.
		As the graph shows, after 12 generations of this disruptive selection, the population had noticeably diverged.
		Disruptive selection is of particular theoretical interest, both because it can increase the genetic diversity of a population and promote speciation.
	www.blackwellpublishing.com /ridley/tutorials/Natural_selection_and_variation11.asp (149 words)

	Genetic and Statistical Analyses of Strong Selection on Polygenetic Traits: What, Me Normal? -- from Mathematica ...
		We develop a general population genetic framework for analyzing selection on many loci, and apply it to strong truncation and disruptive selection on an additive polygenetic trait.
		The population is represented by multilocus cumulants describing the distribution of haploid genotypes, and selection is described by the relation between mean fitness and these cumulants.
		For arbitrary epistasis and linkage, we describe a consistent infinitesimal limit in which the short-term selection response is dominated by infinitesimal allele frequency changes and linkage disequilibria.
	library.wolfram.com /infocenter/Articles/1775 (361 words)

	Evolutionary Significance of Phenotypic Plasticity in Plants
		The influence that selection may have on the stability or plasticity of a character is perhaps most elegantly demonstrated in the investigations on genetic assimilation by Waddington and others (Waddington, 1961).
		Disruptive selection can be due to recurrent variation in selection in either time or space.
		Where selection is acting in a stabilizing manner it can permit the population to assume a uniform phenotype, while retaining a considerable degree of genetic variation.
	www.bulbnrose.com /Heredity/Bradshaw/bradshaw2_files/bradshaw2.html (14742 words)

	Pun 1.4.htm (Site not responding. Last check: 2007-10-13)
		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)

	Untitled Document
		Under stabilizing selection, even though over generations, the frequency distribution may look the same, the extreme data from both ends of the frequency distribution are eliminated.
		Under directional selection, one extreme trait of the frequency distribution is favored, so the distribution in the subsequent generation is shifted from where it was in the parental generation, towards that of the favored trait.
		Under disruptive (diversifying) selection, both extreme traits are favored at the expense of intermediate varieties.
	biology.unm.edu /ccouncil/Biology_112/Summaries/Natural_Selection.html (721 words)

	Evolution - A-Z - Disruptive selection (Site not responding. Last check: 2007-10-13)
		Thoday and Gibson bred from fruitflies with high, or low, numbers of bristles on a certain region of the body; individuals with intermediate numbers of bristles were prevented from breeding.
		After 12 generations of this disruptive selection, the population had noticeably diverged (see graph).
		Disruptive selection is of particular theoretical interest, both because it can increase the genetic diversity of a population (by frequency-dependent selection) and promote speciation.
	www.blackwellpublishing.com /ridley/a-z/Disruptive_selection.asp (144 words)

	Disruptive Selection (Site not responding. Last check: 2007-10-13)
		The phenotypic distribution before selection is a relatively broad bell-curve.
		The result of selection is reduction of the middle of the distribution, so the trait variance is increased while the mean remains the same.
		Where the emphasis is on preservation of the tails of the distribution, this may also be called diversifying selection.
	www.mun.ca /biology/scarr/2900_Disruptive_Selection.htm (69 words)

	BIO 304. Ecology & Evolution: Population Genetics
		Selection for small or large body size in the guppies (Poecilia reticulata) in Trinidad.
		Fact: Sexual selection results in dimorphism (pronounced phenotypic differences) between the sexes in many instances.
		3./), etc. The theory of sexual selection is a prime example of the 'scientific method' in action, demonstrating how alternative hypotheses are generated, tested and refined.
	www.micro.utexas.edu /courses/levin/bio304/popgen/nat.selection.html (380 words)

	Evolutionary Quantitative Genetics (Site not responding. Last check: 2007-10-13)
		The response of fitness to selection is equal to the additive genetic variance for fitness.
		Unfortunately, while this type of analysis can be quite good at predicting short term response to selection, it is very poor at visualizing the actual relationship of the character and fitness.
		Stabilizing selection acts to reduce the phenotypic variance of the trait.
	www.zoology.ubc.ca /~whitlock/QGPG/QG6/Lecture.html (883 words)

	BIOL 1673 Study Guide Unit 2 Section 5
		Natural selection is the only thing that causes organisms to be adapted to their environment: "acting on phenotypes, NS indirectly adapts a population to its environment by increasing or maintaining favorable genotypes in the gene pool"
		Natural selection acts on the whole organism; the unit of selection is the individual.
		There are three general modes in which natural selection acts: stabilizing, directional, or diversifying (diversifying selection is also called disruptive selection; you don't have to know the term "disruptive" selection in this course but you may run into it in other biology courses).
	www.austincc.edu /akeddy/eeb_sg_u2s5 (1731 words)

	BIOdotEDU (Site not responding. Last check: 2007-10-13)
		A population of spiny cacti is in genetic equilibrium, with no forces of selection acting on it, the distribution curve of number of cacti showing a particular number of spines is broad and symmetrical.
		A road is built quite close to the study site, which keeps away the peccaries and the parasitic insects, but with the road comes the tourists.
		Disruptive selection favors both extreme values for a particular trait in a distribution curve.
	www.brooklyn.cuny.edu /bc/ahp/LAD/C21/C21_Disruptive.html (207 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)

	Disruptive selection: Facts and details from Encyclopedia Topic (Site not responding. Last check: 2007-10-13)
		Disruptive selection is a type of natural selection natural selection quick summary:
		Natural selection is the primary mechanism within the scientific theory of evolution, in that it alters the frequency of alleles within a population....
		When disruptive selection operates, EHandler: no quick summary.
	www.absoluteastronomy.com /encyclopedia/d/di/disruptive_selection.htm (386 words)

	Biology 3A - Student Resources
		Directional Selection - favors individuals possessing extreme values of a trait (like long necks in giraffes, coloration in peppered moths), which causes the population to move in a particular direction.
		Disruptive Selection - adapts individuals in a population to different habitats.
		Disruptive selection may occur in an area that provides different resources.
	www.biologycorner.com /bio3/notes-chap17.html (386 words)

	BioMed Central \| Full text \| Analysis of disruptive selection in subdivided populations
		Otherwise, there is disruptive selection, and "branching" of the distribution of phenotypes in the population may occur [3,6].
		Although disruptive selection has been found in other dispersal models (e.g., [22-24]), it is not expected to occur in this model, which serves mainly to compare the present approach with a previous computation of stable strategies.
		In the jargon of population genetics, the scramble competition model is a soft selection model while the interference competition model is a hard selection model (e.g., [39]).
	www.biomedcentral.com /1471-2148/3/22 (6066 words)

	[No title]
		Evolution - refers to temporal changes, whereas natural selection specifies one particular way in which these changes are brought about - more specifically refers to a change in gene frequencies or descent with modification.
		Some Examples Directional Selection - Body size of the medium ground finch, Geospiza fortis (Gibbs and Grant 1987) Frequency-dependent Selection - Handedness of a scale-eating cichlid (Hori 1993) Disruptive Selection - Alternative life histories in salmon (Gross 1985) Stabilizing Selection - Flower color in Delphinium nelsonii (Waser and Price 1981) F.
		Smith, T.B. Disruptive selection and the genetic basis of bill size polymorphism in the African finch Pyrenestes.
	www.life.uiuc.edu /ib/443/lectures/EvolDef_.doc (379 words)

	ImmInst.org -> Evolution (Site not responding. Last check: 2007-10-13)
		The point is that disruptive selection is not in respect to an environmental shift as much as probably overpopulation for example.
		arranged marriages, where parents rather than the young select the mate based on economic or even astrology factors, and where the sexual desires of the mated pair are often subordinated to these factors, are artificial unless wholly based on an ecological factor such as control of land which is held by their own force.
		In cases where ecological and sexual selection factors are strongly at odds, simultaneously encouraging and discouraging the same traits, it may also be important to distinguish them as sub-processes within natural selection.
	www.imminst.org /forum/index.php?act=ST&f=3&t=4628&s= (6577 words)

	Speciation and Population Genetics (Site not responding. Last check: 2007-10-13)
		Disruptive selection—selection for two alternate expressions of a trait, resulting in two peaks in values for trait expression and an increase in overall population variation
		However, variability that is exclusively due to environmental effects cannot be passed on to the offspring.
		Natural selection can act on variation only when it is expressed in the phenotype.
	acad.udallas.edu /biology/GenBio/Speciation.html (1064 words)

	untitled (Site not responding. Last check: 2007-10-13)
		Selection can occur if all members of a population are the same.
		A selection event that eliminates flies in small galls will reduce the variability of the population.
		Disruptive selection increases the variance in a population.
	biology.kenyon.edu /courses/biol09/goldenrod/evolquest.htm (172 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.
		To test whether there is significant stabilizing or disruptive selection, calculate the ratio of pre- and post-selection variances (larger variance over smaller variance) and compare that ratio to the appropriate critical value in the table below.
	web.grinnell.edu /individuals/brownj/edu/136_lab6.html (2793 words)

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