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Chapter 16
Evolution of Populations
Ch. 16 Evolution of
Populations
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16–1 Genes and Variation
A. How Common is Genetic Variation?
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Most genes have two forms.
Many genes have multiple alleles.
Many traits are polygenic.
B. Variation and Gene Pools
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Gene pools have frequency of certain traits.
This frequency changes over time.
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This increases or decreases the variation in the
population
Ch. 16 Evolution of
Populations
C. Sources of Genetic Variation
1. Mutations
•
http://www.koshlandsciencemuseum.org/exhib
itdna/crops04activity.jsp
2. Gene Shuffling
• Independent assortment of
chromosomes during meiosis.
D. Single-Gene and Polygenic Traits
– 2 phenotypes vs. many phenotypes
Click here to go to next
outline
Ch. 16 Evolution of
Populations
Traits will
make a
bell curve
when
graphed
Frequency Of phenotype
Polygenic
Phenotype (height)
Click here to return to outline-
Ch. 16 Evolution of
Populations
• A single gene trait makes a bar
graph with two phenotypes
Click here to
return to
outline
Ch. 16 Evolution of
Populations
• 16–2 Evolution as Genetic Change
A. Natural Selection on Single-Gene
Traits
– Natural selection on single-gene
traits can lead to changes in allele
frequencies and thus to evolution.
• Run this simulation of the perrered
moth evolution.
http://www.biologycorner.com/worksh
eets/pepperedmoth.html
Ch. 16 Evolution of
Populations
B. Natural Selection on Polygenic Traits
Natural selection can affect the
distributions of phenotypes in any of
three ways
1. Directional Selection
2. Stabilizing Selection
3. Disruptive Selection
Ch. 16 Evolution of
Click here to continue with
Populations
outline
– C.
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Ch. 16 Evolution of
Populations
Directional Selection
Low mortality,
high fitness
High mortality,
low fitness
• Click to return to outline
Ch. 16 Evolution of
Populations
Stabilizing Selection
Low mortality,
high fitness
High mortality,
low fitness
Selection
against both
extremes keep
curve narrow
and in same
place.
• Click here to return to outline
Ch. 16 Evolution of
Populations
Disruptive Selection
Largest and smallest seeds become more common.
Low mortality,
high fitness
Population splits
into two subgroups
specializing in
different seeds.
High mortality,
low fitness
Beak Size
Beak Size
• Click here to continue with outline
Ch. 16 Evolution of
Populations
D. Evolution Versus Genetic Equilibrium
The Hardy-Weinberg principle states
that allele frequencies in a population
will remain constant unless one or
more factors cause those frequencies
to change.
In other words, as long as everything
stays the same, evolution will not
happen.
Ch. 16 Evolution of
Populations
Genetic Equilibrium
Five conditions are required to maintain
genetic equilibrium from generation to
generation
1. Random Mating
2. Large Population
3. No Movement Into or Out of the
Population
4. No Mutations
5. No Natural Selection
Ch. 16 Evolution of
Populations
16–3 The Process of Speciation-how do
species evolve?
A. Isolating Mechanisms-ALL types of isolation
result in Reproductive Isolation.
1.Behavioral Isolation-have differences in courtship
rituals
2.Geographic Isolation-are separated by geographic
barriers such as rivers, mountains, or bodies of
water.
3.Temporal Isolation-reproduce at different times.
Ch. 16 Evolution of
Populations
B. Testing Natural Selection in Nature
Examples of Evolution observed in Nature
• Peppered Moth
• Antibiotic resistance in bacteria
• Human adults who can digest milk
• Gypsy Moth
• Insects able to survive insecticides
• Plants able to survive herbicides
• Darwin’s finches-see next page
• Sickle cell trait in humans
Ch. 16 Evolution of
Populations
C. Speciation in Darwin’s Finches
1. Founders Arrive-birds arrive from S. America
perhaps blown there by a storm
2. Separation of Populations-The small population is
separated on different islands.
3. Changes in the Gene Pool-Natural variation due to
mutation occurs
4. Reproductive Isolation-Birds are isolated by
behavior and geography
5. Ecological Competition-Each island has specific
foods in limited amounts
6.Continued Evolution-all of these drive changes in
the population (evolution) which is documented
even today
Ch. 16 Evolution of
Populations
D. Studying Evolution Since Darwin
• Advances in genetics, molecular biology,
ecology, physics and chemistry increase the
evidence that species change over time and
continue to change.
E. Limitations on Research
• Evidence of change exists. But no one has yet
witnessed the formation of a brand new species.
• Many new discoveries have led to new
hypotheses that refine and expand Darwin's
original ideas. Even Darwin has evolved.
• There are still many unanswered questions.