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Population Genetics and Natural Selection

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Darwin

• 1835 Charles Darwin visited the Galapagos Islands and became convinced various populations evolved from ancestral form.

• 1838 After reading an essay by Thomas Malthus, he theorized some individuals would have a competitive advantage conferred by favorable characteristics.

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Darwin’s Theory of Natural Selection

• Organisms begat like organisms.• Chance variation between individuals.– Some are heritable.

• More offspring are produced each generation than can survive.

• Some individuals, because of physical or behavioral traits, have a higher chance of surviving than others in the same population.

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Gregor Mendel

• Augustinian Monk– Studied garden pea (Pisum sativum).– Discovered characteristics pass from parent to

offspring in form of discrete packets called genes.• Exist in alternate forms - alleles.• Some prevent expression of others.

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Variation Within Populations

• Variation in Plant Populations– Many plant species differ dramatically in form

from one elevation to another.• Clausen et al. found evidence of adaptation by

ecotypes to local environmental conditions in Potentilla glandulosa.– Distinctive ecotypes.

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Variation Within Populations

• Variation in Plant Populations– Phenotypic differences (growth and flower

production) within clones grown at the 3 elevations are the result of environmental differences• Phenotypic plasticity

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Variation in Animal Populations

• Genetic Variation in Alpine Fish– Movement of cold adapted aquatic species into

the headwaters of glacial valleys that lace the Alps created clusters of geographically isolated populations.• Douglas and Brunner used microsatellite DNA to

conclude Coregonus populations are highly diverse and exhibit a high level of differentiation.

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Hardy Weinberg

• Hardy Weinberg principle states that in a population mating at random in the absence of evolutionary forces, allele frequencies will remain constant.

p2+2pq+q2 = 1.0

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Calculating Gene Frequencies

• SS (81%) SA (18%) AA (1%)– Frequency of S allele ?• SS + 1/2SA = .81 + ½(.18) = .90

– (.90)2 + 2(.9x.1) + (.10)2 = 1.0

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Conditions Necessary for Hardy Weinberg

• Random Mating• No Mutations• Large Population Size• No Immigration• Equitable Fitness Between All Genotypes– Likely, at least one of these will not be met and

allele frequencies will change.• Potential for evolutionary change in natural

populations is very great.

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Natural Selection

• Some individuals in a population, because of their phenotypic characteristics, will have higher survival and produce more offspring.– Fitness is the measure of an individual’s

contribution of offspring, or genes, to future generations.

– Natural selection can favor, disfavor, or conserve the genetic make-up of a population.

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Stabilizing Selection• Stabilizing selection acts to impede changes

in a population by acting against extreme phenotypes and favoring average phenotypes.

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Directional Selection• Directional selection leads to changes in

phenotypes by favoring an extreme phenotype over other phenotypes in the population.

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Disruptive Selection• Disruptive selection creates bimodal distributions

by favoring two or more extreme phenotypes over the average phenotype in a population.

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Evolution by Natural Selection

• Natural selection, which changes genotypic and phenotypic frequencies in populations, can result in adaptation to the environment.– Depends on heritability of trait.

h2 = VG / VP

• VG : Genetic variance

• VP: Phenotypic variance

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Adaptive Change in Colonizing Lizards

• Losos et al.– Genus Anolis• Great diversity includes large amount of variation in

size and body proportions.– Length of hind limbs appears to reflect selection for effective

use of vegetation.» Diameter of perching surfaces.

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Rapid Adaptation by Soapberry Bugs

• Carroll and Boyd– Soapberry Bug (Jadera haematoloma) feeds on

seeds from family Sapindaceae.• Slender beaks to pierce fruit walls.

– Distance from outside fruit wall to seeds varies widely - beak length should be under selection.» Found close relationship between fruit radius and beak

length.

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Change Due To Chance• Random processes such as genetic drift can

change gene frequencies in populations, especially in small populations.

• Major concern of habitat fragmentation is reducing habitat availability to the point where genetic drift will reduce genetic diversity within natural populations.

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Evidence of Genetic Drift in Chihuahua Spruce

• Picea chihuahuana now restricted to peaks of Sierra Madre Occidental in N. Mexico.– Ledig et al. examined populations to determine if

the species has lost genetic diversity as a consequence of reduced population size.• Found significant positive correlation between

population size and genetic diversity of study populations.

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Genetic Variation In Island Populations

• In general, genetic variation is lower in isolated and generally smaller, island populations.– Reduced genetic variation indicates a lower

potential for a population to evolve.

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Genetic Diversity and Butterfly Extinctions

• Frankham and Ralls point out inbreeding may be a contributor to higher extinction rates in small populations.– Reduced fecundity, depressed juvenile survival,

shortened life-span.• Saccheri et al. conducted genetic studies on

populations of Glanville fritillary butterflies (Melitacea cinxia).– Populations with highest levels of inbreeding had

highest probabilities of extinction.

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Evolution and Agriculture

• “Artificial selection” is used to describe selective breeding of domesticated organisms to produce or maintain desirable traits.

• “Genetic engineering” is the introduction or deletion of genes in domesticated organisms.– These organisms are termed “genetically modified

organisms” or GMOs.

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Unintended Evolutionary Consequences

• The use of chemicals in agriculture can have evolutionary consequences– Plant and animal pests may evolve resistance to

the chemicals used to control them• Resistance among pests have been shown to be quick

and widespread– Vila-Aiub et al. showed how Johnsongrass quickly evolved

resistance to herbicides in Argentina

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