Evolution%20of%20Populations

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Evolution of Populations

• Evolution as Genetic Change

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Natural Selection on Single Gene Traits

• Natural selection never acts on genes, always on
  the whole organism
• But, if that organism produce a gene change that
  affects survival, natural selection will act
  differently on that organism because of that gene
  change.
• Ex Lizard color
• Brown vs. Black
• Change the survival, change the gene pool

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Distribution of Phenotypes in Polygenic Traits

• When many genes affect one trait, you get many
  phenotypes
• The graph to the right shows the number of
  individuals (area under the curve) for the
  different phenotypes

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Natural Selection on Polygenic Traits

• Directional Selection when individuals at one
  end of the curve have higher fitness than
  individuals in the middle or at the other
  end-Thus the curve will shift left or right
• ExBeak Shape

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Natural Selection on Polygenic Traits

• Stabilizing Selectionwhen individuals near the
  center of the curve have higher fitness than the
  others-Thus the curve will get narrow and tall
• -Survival favors the average
• Ex Birth weight in humans

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Natural Selection on Polygenic Traits

• Disruptive Selectionwhen individuals at opposite
  ends of the curve have higher fitness-Thus causes
  the curve to dip in the middle
• -Average struggle to
• survive
• Ex Bird Beaks

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Genetic Drift

• In small populations, individuals that carry a
  particular allele may leave more descendants than
  others, just by chance
• Over time, this can cause a change in phenotype
  from the original population
• Ex Founders Effect occurs when a small
  population settles in a location separate from
  the rest of the population.
• Ex Amish

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Ex Bottleneck
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Genetic EquilibriumNo Evolution/Change

• Hardy-Weinberg Principle allele frequencies
  remain the same if
• Random Mating Occurs
• Large Population
• No immigration or emigration
• No mutations
• No natural selection