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Evolution as Genetic Change
16.2 Evolution as Genetic Change
Natural selection can affect
phenotypes in a population in 3 ways
Directional
Selection
Stabilizing
Selection
Disruptive
Selection
Directional Selection
Higher fitness at ONE END of curve than at the other
# of Individuals
in the population
All phenotypes in population shift toward HIGHER FITNESS
Traits of Population
DIRECTIONAL SELECTION
Selection Pressure (Against Phenotype)
Low Fitness
High
Fitness
New graph shifts in the DIRECTION of Higher Fitness
Directional Selection
Example: Darwin’s Finches
# Birds in Population
# Birds in Population
Beak Size
Beak Size
Small seeds become scarce. Only large seeds are available.
irds with LONGER beaks gather food, survive and reproduce
Average Beak Size INCREASES
B
Stabilizing Selection
Higher fitness at the CENTER of the curve
Middle Stays THE SAME
# of Individuals
in the population
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
Traits of Population
STABILIZING SELECTION
Selection Pressure (Against Phenotype)
Low Fitness
High
Fitness
Low Fitness
New graph is STABILIZED in the middle
Stabilizing Selection
Example: Human Birth Weight
# Babies in Population
# Babies in Population
Birth Weight
Birth Weight
Smaller babies are LESS healthy. Larger babies are LESS healthy.
verage Sized Babies become Most Common
A
Disruptive Selection
Higher fitness at TWO ENDS of the curve
Middle phenotype DECREASES in frequency
Can result in 2 subgroups
# of Individuals
in the population
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
Traits of Population
DISRUPTIVE SELECTION
Selection Pressure (Against Phenotype)
High
Fitness
Low Fitness
High
Fitness
New graph is DISRUPTED in the middle.
Disruptive Selection
Example: Darwin’s Finches
Middle-sized seeds disappear. Only very large and very small seeds are left.
# Birds in Population
# Birds in Population
Beak Size
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.
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 frequency
Happens by CHANCE EVENTS
Happens in SMALL POPULATIONS
NOT 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 Effects
Northern Elephant Seals
Bottleneck Event =
HUMAN HUNTING (1890s)
Population decreased to
20 Seals
Now…
have have 30,000 seals
With 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 Effect
Ellis-von Creveld Syndrome
A recessive disorder
Founders?
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
States that allele frequencies in a population will remain constant as long as 5 things are true…
Random Mating
Everyone gets an EQUAL chance to pass on alleles
NO mate selecting
Hardy-Weinberg Principle
States that allele frequencies in a population will remain constant as long as 5 things are true…
2. Large Population
Less effect of GENETIC DRIFT
Hardy-Weinberg Principle
States that allele frequencies in a population will remain constant as long as 5 things are true…
3. No Movement Into or Out of the Population
No MIGRATION
Keep GENE POOL separate
Hardy-Weinberg Principle
States that allele frequencies in a population will remain constant as long as 5 things are true…
4. No Mutations
No NEW alleles in the population
Hardy-Weinberg Principle
States that allele frequencies in a population will remain constant as long as 5 things are true…
5. No Natural Selection
All genotypes have equal FITNESS
No ADVANTAGES for anyone