Genetic Variation

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• Genetic Variation
• Provides raw material for natural selection
• Homogeneous population little opportunity for
  differential fitness
• Sources
• Mutation
• Crossing over
• Independent assortment (Meiosis)
• Random fertilization

2

• Genetic Variation
• Within Populations
• Variation in
• Discrete characters
• Ex Color in some flowers (pink or white)
• Quantitative characters
• Ex Skin color in humans
• Polymorphism
• Two or more alleles at a single locus
• Extensive in most populations
• Phenotypic Different morphs (body forms)
• Genotypic May not produce discrete phenotypes
• Measurement
• Drosophila 14 heterozygosity, 1 nucleotide
  variability
• Homo sapiens 0.1 nucleotide variability

3

• Genetic Variation
• Between Populations
• Geographic Variation
• Differences among genetically distinct
  populations within a species
• Differences may be due to random variation
• Differences may occur over a geographic range
• Cline Graded variation in phenotype and
  genotype over a geographic range
• Common among species with continuous ranges over
  large areas
• Higher latitudes Smaller individuals (plants)
• Higher latitudes Larger individuals (animals)
• Why?

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• Genetic Variation
• Natural Selection
• Can alter frequency distribution of heritable
  traits
• Directional selection
• Environmental change over time favors phenotypes
  at one extreme
• Possible only if population contains multiple
  alleles, at least one of which is favored
• Ex Black bears in Europe larger during glacial
  periods, smaller during interglacials
• Disruptive selection
• Favors extremes at expense of mean
• Also called diversifying selection
• Ex During a drought, Galápagos finches with long
  beaks able to open cactus fruits, birds with wide
  beaks stripped off tree bark to expose insects,
  intermediate beaks less useful
• Stabilizing selection
• Favors mean at expense of extremes
• Reduces variation
• Ex Birth weight in humans

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Fig. 23.13
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• Genetic Variation
• Natural Selection
• Can alter frequency distribution of heritable
  traits
• Directional selection
• Environmental change over time favors phenotypes
  at one extreme
• Possible only if population contains multiple
  alleles, at least one of which is favored
• Ex Black bears in Europe larger during glacial
  periods, smaller during interglacials
• Disruptive selection
• Favors extremes at expense of mean
• Also called diversifying selection
• Ex During a drought, Galápagos finches with long
  beaks able to open cactus fruits, birds with wide
  beaks stripped off tree bark to expose insects,
  intermediate beaks less useful
• Stabilizing selection
• Favors mean at expense of extremes
• Reduces variation
• Ex Birth weight in humans

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Fig. 23.13
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• Genetic Variation
• Natural Selection
• Can alter frequency distribution of heritable
  traits
• Directional selection
• Environmental change over time favors phenotypes
  at one extreme
• Possible only if population contains multiple
  alleles, at least one of which is favored
• Ex Black bears in Europe larger during glacial
  periods, smaller during interglacials
• Disruptive selection
• Favors extremes at expense of mean
• Also called diversifying selection
• Ex During a drought, Galápagos finches with long
  beaks able to open cactus fruits, birds with wide
  beaks stripped off tree bark to expose insects,
  intermediate beaks less useful
• Stabilizing selection
• Favors mean at expense of extremes
• Reduces variation
• Ex Birth weight in humans

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Fig. 23.13
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Stabilizing Selection
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Fig. 23.13
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• Genetic Variation
• Preservation of Variation
• Why arent we all homozygous for the most
  favorable alleles?
• Balancing selection occurs when natural selection
  maintains two or more phenotypes in a population
  balanced polymorphism
• Heterozygote advantage
• Heterozygotes more fit than homozygotes
• Ex Sickle-cell disease
• Frequency-dependent selection
• Phenotypic fitness depends on rarity in
  population
• Ex Non-selective predation

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Fig. 23.17
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• Genetic Variation
• Preservation of Variation
• Why arent we all homozygous for the most
  favorable alleles?
• Balancing selection occurs when natural selection
  maintains two or more phenotypes in a population
  balanced polymorphism
• Heterozygote advantage
• Heterozygotes more fit than homozygotes
• Ex Sickle-cell disease
• Frequency-dependent selection
• Phenotypic fitness depends on rarity in
  population
• Ex Non-selective predation