evolution as genetic change
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Evolution as Genetic Change
16.2 Evolution as Genetic Change
Natural selection can affect phenotypes in a population in 3
waysA. Directional
SelectionB. Stabilizing
SelectionC. Disruptive
Selection
Directional Selection• Higher fitness at ONE END of
curve than at the other
• All phenotypes in population shift toward HIGHER FITNESS
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Traits of Population
Selection Pressure (Against Phenotype)
Low Fitness
High Fitness
DIRECTIONAL SELECTION
New graph shifts in the DIRECTION of Higher Fitness
Directional SelectionExample: Darwin’s Finches
Beak Size
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Beak Size
Small seeds become scarce. Only large seeds are available.Birds with LONGER beaks gather food, survive and reproduce
Average Beak Size INCREASES
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
Low Fitness
Low Fitness
STABILIZING SELECTION
New graph is STABILIZED in the middle
High Fitness
Selection Pressure (Against Phenotype)
Stabilizing SelectionExample: Human Birth Weight
Birth Weight
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Birth Weight
Smaller babies are LESS healthy. Larger babies are LESS healthy.
Average Sized Babies become Most Common
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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
High Fitness
High Fitness
DISRUPTIVE SELECTION
New graph is DISRUPTED in the middle.
Low Fitness
Selection Pressure (Against Phenotype)
Disruptive SelectionExample: Darwin’s Finches
Beak Size
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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.
Types Of Selection With Bird Beaks
http://player.discoveryeducation.com/index.cfm?guidAssetId=7016E4B0-0335-42A9-BF3E-BB21D2A4D9C8
Which Type of Selection Is It?
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?
Bottleneck Effect• A large percentage of a population IS KILLED or
prevented from REPRODUCING
• INCREASES genetic drift
Bottleneck EffectsNorthern Elephant Seals
Bottleneck Event = HUMAN HUNTING (1890s)
Population decreased to 20 Seals
Now…have have 30,000 sealsWith Reduced VARIATION from
Bottleneck
The Founder Effect
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!
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…
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
The Founder EffectEllis-von Creveld Syndrome
A recessive disorderFounders?SAMUEL KING AND HIS WIFE - 1744
Chance Events and Genetic Drift
RANDOM DISASTERS
• Rock Slide• Tsunami• Volcano Eruption• Meteor Impact• Nuclear War• Etc.
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…
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…
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…
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…
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…
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…
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