Evolution of Populations

1
Evolution of Populations

• Evolution as Genetic Change
• 16-2

2
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 Pg 397-Lizard color
• Brown vs. Black
• Change the survival, change the gene pool

3
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

4
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

5
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

6
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

7
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- small subgroup of
  individuals populate an area, thus the
  descendants have their alleles only

8
(No Transcript)
9
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