The Three Types of Natural Selection:

1
The Three Types of Natural Selection

• Directional Selection, Stabilizing Selection,
  Disruptive Selection

2
Three effects of selection on a characteristic

• 1. Directional Selection
• 2. Stabilizing Selection
• 3. Disruptive Selection

3
Figure 23.12 Modes of selection
4
Directional Selection

• When individuals at one end of the population
  curve have higher fitness than individuals in the
  middle or at either end of the population curve.
• Causes entire curve to move as character trait
  changes
• Ex ?beak size of Galapagos finches
• ?peppered moths
• ? antibiotic resistance

5
Figure 23.13 Directional selection for beak size
in a Galápagos population of the medium ground
finch
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Directional Selection
Section 16-2
Key
Directional Selection
Low mortality, high fitness
High mortality, low fitness
Food becomes scarce.
7
Stabilizing Selection

• When individuals near the center of the curve
  have higher fitness than individuals at either
  end of the curve
• Intermediate forms of a trait are favored and
  alleles that specify extreme forms are eliminated
  from a pop.
• Counteracts the effects of mutation, gene flow,
  and genetic drift preserves the most common
  phenotypes.
• Ex. Weight of human babies at birth

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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
9
Disruptive Selection

• When individuals at the upper and lower ends of
  the curve have higher fitness than individuals
  near the middle.
• Forms at both ends of the range of variation are
  favored and intermediate forms are selected
  against selection creates two, distinct
  phenotypes
• Ex. Bird beak size no middle sized seeds,
  only large seeds and small seeds thus, small and
  large beaks are favored

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Figure 23.14 Diversifying selection in a finch
population
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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