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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
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•
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•
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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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