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

# of

Indi

vidu

als

in th

e po

pula

tion

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

# Bi

rds i

n Po

pula

tion

# Bi

rds i

n Po

pula

tion

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

# of

Indi

vidu

als

in th

e po

pula

tion

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

# Ba

bies

in P

opul

ation

Birth Weight

Smaller babies are LESS healthy. Larger babies are LESS healthy.

Average Sized Babies become Most Common

# Ba

bies

in P

opul

ation

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

# of

Indi

vidu

als

in th

e po

pula

tion

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

# Bi

rds i

n Po

pula

tion

# Bi

rds i

n Po

pula

tion

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…