AP Biology – Chapter 23
The Evolution of Populations
Population Genetics
AP Biology
2007-2008
Smallest Unit of Evolution
  Natural selection acts on individuals
  BUT…..the evolutionary impact of natural
selection is only apparent in the changes in a
population over time
  It is always a population that evolves NOT
an individual
  Microevolution – change in allele
frequencies in a population over generations
  Natural selection
  Genetic drift
  Gene flow
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Forces of evolutionary change
  Natural selection

traits that improve survival
or reproduction will accumulate
in the population
  adaptive change
  Genetic drift

frequency of traits can change
in a population due to
random chance events
  random change
AP Biology
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AP Biology – Chapter 23
Causes of MicroEvolution
1.  Genetic Variation – great amount of
genetic variation can only be determined
with biochemical methods (Darwin unaware
of much variation)




Genetic variation can only be observed at the molecular
level. Ex: cannot identify blood type from his or her
appearance
Characters that vary within a population may be discrete or
quantitative
Discrete – ex: color of Mendel’s pea plants (purple or white
flowers) classified on an either-or basis
Quantitative – influence of two or more genes on a single
phenotypic character. Vary along a continuum within a
population
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Average Heterozygosity
  The average percent of loci that are
heterozygous
Two different alleles for a single locus
Fruit fly example
  Nucleotide variability
  Gene variability (average
heterozygosity)


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Variation Between Populations
  Geographic Variation – differences in the
genetic composition of separate
populations
mice population – evolved in isolation
Karyotype differences
  Variation resulted from chance events


  Cline


Other types of variation occur as a cline
Graded change in a character along a geographic
axis
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AP Biology – Chapter 23
Mutations
  Ultimate source of new alleles
Change in the nucleotide sequence of an
organism’s DNA
  Only mutations in cell lines that produce gametes
can be passed to offspring

  Point Mutations


Change in one base in a gene
Unlikely that a mutation will improve the
phenotype
  Increases variability
  HIV viral mutations
  Sexual reproduction
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Measuring
Evolution of Populations
AP Biology
2010-2011
5 Agents of evolutionary change
Mutation
Gene Flow
Chemical
Changes to DNA
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Migration
Non-random mating
Sexual Selection
Genetic Drift
Selection
Small population
Natural Selection
Differential Survival
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AP Biology – Chapter 23
Gene Pools and Allele Frequencies
  Population – group of individuals of the same
species that live in the same area, interbreed,
producing fertile offspring
  Gene Pool – consists of all the alleles for all the loci
in all individuals of the population
  If only one allele exists for a particular locus in a
population that allele is fixed in the gene pool
  Each allele has a frequency (proportion) in the
population
  p and q allele frequency representations
  Sum of all allele frequencies must equal 1 (100%)
AP Biology
Hardy-Weinberg Theorem
  Mathematical principle that attempts to determine the
current allele frequencies in a population
  The frequencies of alleles and genotypes in a
population will remain constant from generation to
generation

Provided that only Mendelian segregation and
recombination of alleles are at work
  Sets up the necessary conditions for a population to
NOT evolve
  Focus is on the combination of alleles in ALL of the
genetic crosses in a population
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Hardy-Weinberg equilibrium
  Hypothetical situation


serves as null hypothesis
non-evolving population
REMOVE all agents of evolutionary change
1.  no genetic drift (very large population size )
2.  no gene flow (no migration in or out)
3.  no mutation (no chemical change to DNA)
4.  random mating (no sexual selection)
5.  no natural selection (equal survival)
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AP Biology – Chapter 23
HW Conditions
  No mutations
  Random mating
  No natural selection
  Extremely large population size
  No gene flow
Departure from any of these conditions
usually results in evolutionary change!
  It is also common for natural populations
to be in HW equilibrium for specific genes

AP Biology
Hardy-Weinberg Equation
p2 + 2pq + q2 = 1.0
and p + q = 1.0
p2 = frequency of individuals homozygous dominant
2pq = frequency of individuals heterozygous
q2 = frequency of individuals homozygous recessive
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Example of strong selection pressure
  Tay Sachs
primarily in Ashkenazi Jews & Cajuns
recessive disease = aa
  lysosomal storage disease


  lack of one functional digestive enzyme in
lysosome
  build up undigested fat in
brain cells
  children die before they
are 5 years old
So where do new cases of
come from?
APTay-Sachs
Biology
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AP Biology – Chapter 23
Example of heterozygote advantage
  Sickle cell anemia

inherit a mutation in gene coding for one
of the subunits in hemoglobin
  oxygen-carrying blood protein
  normal allele = Hb
  mutant allele = Hs
  recessive trait = HsHs

low oxygen levels causes
RBC to sickle
  clogging small blood vessels
  damage to organs

AP Biology
often lethal
Sickle cell frequency
  High frequency of heterozygotes
1 in 5 in Central Africans = HbHs
  unusual for allele with severe
detrimental effects in homozygotes

  1 in 100 = HsHs
  usually die before reproductive age
Why is the Hs allele maintained at such high
levels in African populations?
Suggests some selective advantage of
being heterozygous… HbHs
AP Biology
Single-celled eukaryote parasite
(Plasmodium) spends part of its
life cycle in red blood cells
Malaria
1
liver
2
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AP Biology – Chapter 23
Heterozygote Advantage
  In tropical Africa, where malaria is common:

homozygous dominant (normal)

homozygous recessive

heterozygote carriers
  reduced survival or reproduction from malaria: HbHb
  reduced survival & reproduction from sickle cell anemia: HsHs
  survival & reproductive advantage: HbHs
Hypothesis:
In malaria-infected
cells, the O2 level is
lowered enough to
cause sickling which
kills the cell & destroys
the
parasite.
AP Biology
Frequency of sickle cell allele &
distribution of malaria
Any Questions??
AP Biology
2010-2011
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