Transcript 04Molles5e

Population Genetics and Natural Selection
Chapter 4
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Outline
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Darwin
Gregor Mendel
Variation Within Populations
Hardy Weinberg
Natural Selection
Change Due to Chance
Evolution and Agriculture
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Darwin
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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
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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
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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
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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
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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
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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
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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
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SS (81%)
SA (18%)
AA (1%)
 Frequency of S allele ?
 SS + 1/2SA = .81 + ½(.18) = .90
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 (.90) + 2(.9x.1) + (.10) = 1.0
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Conditions Necessary for Hardy Weinberg
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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
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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
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Stabilizing selection acts to impede changes
in a population by acting against extreme
phenotypes and favoring average
phenotypes.
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Directional Selection
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Directional selection leads to changes in
phenotypes by favoring an extreme
phenotype over other phenotypes in the
population.
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Disruptive Selection
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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
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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
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VG : Genetic variance
VP: Phenotypic variance
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Adaptive Change in Colonizing Lizards
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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
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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
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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
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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
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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
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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
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“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
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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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Review
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Darwin
Gregor Mendel
Variation Within Populations
Hardy Weinberg
Natural Selection
Change Due to Chance
Evolution and Agriculture
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