1. Evolution as Genetic Change

3. 16.2 Evolution as Genetic Change
Natural selection can affect
phenotypes in a population in 3 ways
Directional
Selection
Stabilizing
Disruptive

4. Directional Selection
Higher fitness at ONE END of curve than at the other
All phenotypes in population shift toward HIGHER FITNESS
# of Individuals
in the population
Traits of Population

5. DIRECTIONAL SELECTION
Selection Pressure (Against Phenotype)
Low Fitness
High Fitness
New graph shifts in the DIRECTION of Higher Fitness

6. Average Beak Size INCREASES
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.
Birds with LONGER beaks gather food, survive and reproduce
Average Beak Size INCREASES

7. Stabilizing Selection
Higher fitness at the CENTER of the curve
Middle Stays THE SAME
Ends get NARROWER
# of Individuals
in the population
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

8. STABILIZING SELECTION
Selection Pressure (Against Phenotype)
Low Fitness
High Fitness
Low Fitness
New graph is STABILIZED in the middle

9. 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.
Average Sized Babies become Most Common

10. Disruptive Selection EXAMPLE- large seeds and small seeds become more
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
# of Individuals
in the population
Traits of Population

12. DISRUPTIVE SELECTION New graph is DISRUPTED in the middle.
Selection Pressure (Against Phenotype)
High Fitness
Low Fitness
High Fitness
New graph is DISRUPTED in the middle.

13. Example: Darwin’s Finches
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.

15. Types Of Selection With Bird Beaks

16. Which Type of Selection Is It?

17. Genetic Drift Coin Flip 1,000 times How many Heads? 10 times
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

18. Bottleneck Effect
A large percentage of a population IS KILLED or prevented from REPRODUCING
INCREASES
genetic drift

19. Northern Elephant Seals
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

21. The Founder Effect

22. 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!

23. Now let’s get reproducing!
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…

24. Founder Effect From Your Articles: Amish Communities in Pennsylvania
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

25. Ellis-von Creveld Syndrome
The Founder Effect
Ellis-von Creveld Syndrome
A recessive disorder
Founders?
SAMUEL KING AND HIS WIFE

26. Chance Events and Genetic Drift
RANDOM DISASTERS
Rock Slide
Tsunami
Volcano Eruption
Meteor Impact
Nuclear War
Etc.

27. GENETIC EQUILIBRIUM DEFINITION: Hardy-Weinberg Principle
When allele frequencies in a population DON’T CHANGE
NO EVOLUTION HAPPENS
States that allele frequencies in a population will remain CONSTANT as long as 5 things are true…

29. Hardy-Weinberg Principle
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…

30. Hardy-Weinberg Principle
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…

31. Hardy-Weinberg Principle
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…

32. Hardy-Weinberg Principle
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…

33. Hardy-Weinberg Principle
5. No Natural Selection
All genotypes have equal FITNESS
No ADVANTAGES for anyone
Hardy-Weinberg Principle
States that allele frequencies in a population will remain constant as long as 5 things are true…