DO NOW

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DO NOW

• Why does natural selection act on the phenotype
  rather than the genotype of the organism?
• How does natural selection determine the
  different survival of different organisms?
• What is genetic drift??????

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AIM

• Notes on genotype phenotype
• Genetic drift
• Trait wheel
• Homework
• Notes Chapter 15
• Friday Evolution Project oral presentation and
  written due

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Generic Bell Curve for Polygenic Trait
Section 16-1
Frequency of Phenotype
Phenotype (height)
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Figure 162 Relative Frequencies of Alleles
Section 16-1
Gene Pools
Frequency of Alleles
allele for brown fur
allele for black fur
48 heterozygous black
16 homozygous black
36 homozygous brown
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Figure 163 Phenotypes for Single-Gene Trait
Section 16-1
Widows peak allele is dominant
100 80 60 40 20 0
Frequency of Phenotype ()
Widows peak
No widows peak
Phenotype
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Genetic Drift
Section 16-2
Random change in allele frequency
Sample of Original Population
Descendants
Founding Population A
Founding Population B
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Genetic Drift
Section 16-2
In small populations an allele may become
popular and common in population
Sample of Original Population
Descendants
Founding Population A
Founding Population B
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Genetic Drift
Section 16-2
Sample of Original Population
Descendants
Founding Population A
Founding Population B
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DO NOW
Section 16-2

• . . . All the Help I Can Get
• Natural selection operates on traits in different
  ways. You might be ableto predict which traits
  natural selection would favor if you think about
  the demands of an organisms environment.

1. Choose an animal that you know something
about, such as a deer, and write its name at the
top of a sheet of paper. Then, divide your paper
into two columns, and write the heading Trait in
one column and Advantage in the other. 2. Under
Trait, write in several of the animals traits.
3. Under Advantage, write in how you think the
trait would be helpful to the animal.
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Section Outline
Section 16-2

• 162 Evolution as Genetic Change
• A. Natural Selection on Single-Gene Traits
• B. Natural Selection on Polygenic Traits
• 1. Directional Selection
• 2. Stabilizing Selection
• 3. Disruptive Selection
• C. Genetic Drift
• D. Evolution Versus Genetic Equilibrium
• 1. Random Mating
• 2. Large Population
• 3. No Movement Into or Out of the Population
• 4. No Mutations
• 5. No Natural Selection

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Natural Selection on polygenic traits

• There are three ways that natural selection
  affects the phenotype of an organism

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Figure 166 Graph of Directional Selection
Section 16-2
Key
Directional Selection
Low mortality, high fitness
High mortality, low fitness
Food becomes scarce.
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Figure 167 Graph of Stabilizing Selection
Section 16-2
Stabilizing Selection
Key
Low mortality, high fitness
Selection against both extremes keep curve narrow
and in same place.
High mortality, low fitness
Percentage of Population
Birth Weight
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Figure 168 Graph of Disruptive Selection
Section 16-2
Disruptive Selection
Largest and smallest seeds become more common.
Key
Population splits into two subgroups specializing
in different seeds.
Low mortality, high fitness
Number of Birdsin Population
Number of Birdsin Population
High mortality, low fitness
Beak Size
Beak Size
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Section Outline
Section 16-3

• 163 The Process of Speciation
• A. Isolating Mechanisms
• 1. Behavioral Isolation
• 2. Geographic Isolation
• 3. Temporal Isolation
• B. Testing Natural Selection in Nature
• 1. Variation
• 2. Natural Selection
• 3. Rapid Evolution
• C. Speciation in Darwins Finches
• 1. Founders Arrive
• 2. Separation of Populations
• 3. Changes in the Gene Pool
• 4. Reproductive Isolation
• 5. Ecological Competition
• 6. Continued Evolution
• D. Studying Evolution Since Darwin

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Internet
Go Online

• Links from the authors on restricting use of
  antibiotics
• Interactive test
• For links on population genetics, go to
  www.SciLinks.org and enter the Web Code as
  follows cbn-5161.
• For links on speciation, go to www.SciLinks.org
  and enter the Web Code as follows cbn-5163.

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Interest Grabber
Section 16-1

• Yes, No, or Maybe
• Some traits, such as a widows peak, fall into
  neat categories You either have a widows peak
  or you dont. Other traits, such as height,
  arent so easy to categorize.

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Interest Grabber continued
Section 16-1

• 1. Make a list of physical traits that you think
  are influenced by genes.Then, write next to each
  trait whether you have the trait or not(e.g., a
  widows peak) or whether there are many
  variations of thetrait (e.g., hair color).
• 2. Are most of the traits you listed clear-cut or
  are they mostly traitsthat have many variations?
  Which traits in your list are difficult
  tocategorize?
• 3. Compare your list with that of another
  student. Did he or she think ofany traits that
  you missed? Why do you think some traits are
  clear-cut,while others are not?