II.) Types of Natural Selection

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II.) Types of Natural Selection

• A.) Most traits are polygenic controlled by
  many genes.
• 1.) These traits therefore have a normal
  distribution and a characteristic shape when
  phenotypes are graphed.

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Ex Height in Humans

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• B.) Natural Selection can change this normal
  distribution in 3 ways
• 1.) Directional Selection individuals at one
  end of the curve have higher fitness than
  individuals at the middle or other end.

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Ex Beak Size of finches
Key
Directional Selection
Low mortality, high fitness
High mortality, low fitness
Food becomes scarce.
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• 2.) Stabilizing Selection individuals at the
  center of the curve have higher fitness.

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Ex Normal weight human babies vs. high or low
birth weight.
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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• 3.) Disruptive Selection individuals at both
  ends of the curve have higher fitness than
  individuals in the middle.

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Ex Food goes from medium size seeds to large and
small seeds. Birds with small large beaks are
now favored.
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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• http//wps.prenhall.com/wps/media/objects/1110/113
  6802/17_3.html

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III. Other Mechanisms of Evolution

• (No, natural selection is not the only one!) The
  following are events that can bring about a
  change in allele frequencies in a population

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• A.) Genetic Drift - random changes in allele
  frequency in small populations.
• 1.) Individuals that carry a particular allele
  may leave more offspring by chance. Think
  probability over time, this can cause an allele
  to become more common in a population.

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• 2.) This can occur due to
• a.) Founders effect when a small group of
  organisms founds a new population, they many have
  different relative frequencies of alleles than
  the larger population they came from.
• i.) If so, the population they start can be quite
  different from their original population.

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Section 16-2
Sample of Original Population
Descendants
Founding Population A
Founding Population B
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• b.) Bottleneck effect occurs when a small
  population is left behind after a
  disaster/disturbance and has different relative
  frequencies than original population.

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• B.) Nonrandom mating which can be termed sexual
  selection.
• 1.) Individuals in population choose mates based
  on certain traits.
• http//www.pbs.org/wgbh/evolution/library/01/6/l_0
  16_09.html

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• C.) Gene flow the movement of alleles from one
  population to another due to immigration/emigrat
  ion.