Evolution as Genetic Change. 16.2 Evolution as Genetic Change Natural selection can affect...
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![Page 1: Evolution as Genetic Change. 16.2 Evolution as Genetic Change Natural selection can affect phenotypes in a population in 3 ways A.Directional Selection.](https://reader036.fdocuments.net/reader036/viewer/2022062620/551b21b4550346cf5a8b58ab/html5/thumbnails/1.jpg)
Evolution as Genetic Change
![Page 2: Evolution as Genetic Change. 16.2 Evolution as Genetic Change Natural selection can affect phenotypes in a population in 3 ways A.Directional Selection.](https://reader036.fdocuments.net/reader036/viewer/2022062620/551b21b4550346cf5a8b58ab/html5/thumbnails/2.jpg)
16.2 Evolution as Genetic Change
Natural selection can affect phenotypes in a population in 3
waysA. Directional
SelectionB. Stabilizing
SelectionC. Disruptive
Selection
![Page 3: Evolution as Genetic Change. 16.2 Evolution as Genetic Change Natural selection can affect phenotypes in a population in 3 ways A.Directional Selection.](https://reader036.fdocuments.net/reader036/viewer/2022062620/551b21b4550346cf5a8b58ab/html5/thumbnails/3.jpg)
Directional Selection
• Higher fitness at ONE END of curve than at the other
• All phenotypes in population shift toward HIGHER FITNESS
# of
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Traits of Population
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Selection Pressure (Against Phenotype)
Low Fitness
High Fitness
DIRECTIONAL SELECTION
New graph shifts in the DIRECTION of Higher Fitness
![Page 5: Evolution as Genetic Change. 16.2 Evolution as Genetic Change Natural selection can affect phenotypes in a population in 3 ways A.Directional Selection.](https://reader036.fdocuments.net/reader036/viewer/2022062620/551b21b4550346cf5a8b58ab/html5/thumbnails/5.jpg)
Directional SelectionExample: Darwin’s Finches
Beak Size
# Bi
rds
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# Bi
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Beak Size
Small seeds become scarce. Only large seeds are available.
Birds with LONGER beaks gather food, survive and reproduceAverage Beak Size INCREASES
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Stabilizing Selection
• Higher fitness at the CENTER of the curve
• Middle Stays THE SAME
• Ends get NARROWER
EXAMPLE- birth weight of human babies• Smaller than average babies = less likely to be healthy• Larger than average babies = less likely to be healthy
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Traits of Population
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Low Fitness
Low Fitness
STABILIZING SELECTION
New graph is STABILIZED in the middle
High Fitness
Selection Pressure (Against Phenotype)
![Page 8: Evolution as Genetic Change. 16.2 Evolution as Genetic Change Natural selection can affect phenotypes in a population in 3 ways A.Directional Selection.](https://reader036.fdocuments.net/reader036/viewer/2022062620/551b21b4550346cf5a8b58ab/html5/thumbnails/8.jpg)
Stabilizing SelectionExample: Human Birth Weight
Birth Weight
# Ba
bies
in P
opul
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Birth Weight
Smaller babies are LESS healthy. Larger babies are LESS healthy.
Average Sized Babies become Most Common
# Ba
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in P
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Disruptive Selection
• Higher fitness at TWO ENDS of the curve
• Middle phenotype DECREASES in frequency
EXAMPLE- large seeds and small seeds become more common and there are few medium seeds
• Both birds with small beaks and large beaks are best adapted to
eat those seeds• Can result in 2 subgroups
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Traits of Population
![Page 10: Evolution as Genetic Change. 16.2 Evolution as Genetic Change Natural selection can affect phenotypes in a population in 3 ways A.Directional Selection.](https://reader036.fdocuments.net/reader036/viewer/2022062620/551b21b4550346cf5a8b58ab/html5/thumbnails/10.jpg)
High Fitness
High Fitness
DISRUPTIVE SELECTION
New graph is DISRUPTED in the middle.
Low Fitness
Selection Pressure (Against Phenotype)
![Page 11: Evolution as Genetic Change. 16.2 Evolution as Genetic Change Natural selection can affect phenotypes in a population in 3 ways A.Directional Selection.](https://reader036.fdocuments.net/reader036/viewer/2022062620/551b21b4550346cf5a8b58ab/html5/thumbnails/11.jpg)
Disruptive SelectionExample: Darwin’s Finches
Beak Size
# Bi
rds
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latio
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# Bi
rds
in
Popu
latio
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Beak Size
Average-sized beaks are least common. Birds with VERY LARGE beaks and VERY SMALL beaks are best adapted.
This can result in 2 subgroups.
Middle-sized seeds disappear. Only very large and very small seeds are left.
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Types Of Selection With Bird Beaks
http://player.discoveryeducation.com/index.cfm?guidAssetId=7016E4B0-0335-42A9-BF3E-BB21D2A4D9C8
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Which Type of Selection Is It?
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Genetic Drift
• RANDOM change in allele frequencyHappens by CHANCE
EVENTSHappens in SMALL
POPULATIONSNOT NATURAL SELECTION(Not related to fitness)
• Coin Flip– 1,000 times• How many Heads?
– 10 times• How many Heads?
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Bottleneck Effect
• A large percentage of a population IS KILLED or prevented from REPRODUCING
• INCREASES genetic drift
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Bottleneck EffectsNorthern Elephant Seals
Bottleneck Event = HUMAN HUNTING (1890s)
Population decreased to 20 Seals
Now…have have 30,000 sealsWith Reduced VARIATION from
Bottleneck
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The Founder Effect
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The Founder Effect
• Example: The Cocklebur– Main population with LOTS OF
VARIATION (many different colors)
• A FEW hitch a ride to an area where there are no cockleburs.
• …and start a NEW POPULATION
Let’s get out of here!
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The Founder Effect• They are the FOUNDERS. Their
VARIATION gives rise to the variation in the entire NEW POPULATION
We made it!
Woo!
Now let’s get reproducing!
I miss yellow…
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Founder Effect
• A type of Genetic Drift after a SUBGROUP breaks away to form a new population
• From Your Articles:Amish Communities in
Pennsylvania– Ellis-van Creveld syndrome
• EXTRA fingers + toes• Abnormal TEETH + nails• A hole in the HEART
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The Founder EffectEllis-von Creveld Syndrome
A recessive disorderFounders?SAMUEL KING AND HIS WIFE - 1744
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Chance Events and Genetic Drift
RANDOM DISASTERS
• Rock Slide• Tsunami• Volcano Eruption• Meteor Impact• Nuclear War• Etc.
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GENETIC EQUILIBRIUM
DEFINITION:
When allele frequencies in a population DON’T CHANGE
NO EVOLUTION HAPPENS
Hardy-Weinberg Principle
States that allele frequencies in a population will remain CONSTANT as long as 5 things are true…
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Hardy-Weinberg Principle
1. Random Mating– Everyone gets an EQUAL
chance to pass on alleles– NO mate selectingStates that allele
frequencies in a population will remain constant as long as 5 things are true…
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Hardy-Weinberg Principle
2. Large Population– Less effect of GENETIC
DRIFT
States that allele frequencies in a population will remain constant as long as 5 things are true…
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Hardy-Weinberg Principle
3. No Movement Into or Out of the Population– No MIGRATION– Keep GENE POOL separateStates that allele
frequencies in a population will remain constant as long as 5 things are true…
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Hardy-Weinberg Principle
4. No Mutations– No NEW alleles in the
population
States that allele frequencies in a population will remain constant as long as 5 things are true…
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Hardy-Weinberg Principle
5. No Natural Selection– All genotypes have equal
FITNESS– No ADVANTAGES for
anyoneStates that allele frequencies in a population will remain constant as long as 5 things are true…