Chap. 23

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Chap. 23 Population Evolution
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How do Populations evolve?

• Some characteristics become
• More/Less common within the population
• Microevolution a change in the genetic makeup
  of a population from one generation to the next

Before Drought
After Drought
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Definitions Population group of individuals of
the same species living in a certain area Species
a group of populations whose members can
interbreed and produce fertile offspring
Galapagos Penguin!
Happy Feet Penguin!
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Gene pool ALL alleles of ALL genes of ALL the
individuals in a population A gene is FIXED if
all members of the population are homozygous for
the same allele. ex. all bb ? Can this change?
Yes! By mutation, you can change a fixed allele
(bb)
Mutations Natural Selection Sexual
recombination Evolution
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So, why and when does a population evolve?

• Backwards approach
• What will be the hallmarks of a NONEVOLVING
  population?

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What is the frequency of alleles in a population?
A pink a white 500 plants
AA
Aa
aa
20 plants
160 plants
320 plants
There are 1000 copies of the gene in the
population A 2(320) 160 800 80
frequency 0.8 for A a 2(20) 160 200 20
frequency 0.2 a for a
p
AA
Aa
q
aa
Aa
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AA
Aa
aa
160 plants
20 plants
320 plants
Frequency of allele A 0.8
Frequency of allele a 0.2
Genotype frequencies are AA (320/500) .64
.8 x .8 Aa (160/500) .32 2(.8 x .2) aa
(20/500) .04 .2 x .2
p2
2pq
q2
p2 2pq q2 1
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Will this genotype become extinct very soon?
0.5A
0.5 a
0.5 A
0.5 a
0.25AA
0.25Aa
0.64AA
0.16Aa
0.8A
0.5A
0.25Aa
0.25aa
0.16Aa
0.04aa
0.5a
0.2a
Aa X Aa A 50 a 50
Aa X Aa A 80 a 20
Aa
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Hardy-Weinberg Theorem Frequencies of alleles
and genotypes in a populations gene pool remain
constant over generations (unless, of course,
there are outside circumstances)
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p frequency of dominant allele (A) q
frequency of recessive allele (a) p q 1 p2
2pq q2 1 p2 frequency of AA 2pq
frequency of Aa q2 frequency of aa
Hardy-Weinberg Equation
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Hardy- Weinberg Equilibrium
p2 2pq q2 1
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Given how many people are homozygous recessive
for a certain gene Can you can figure out the
allele frequencies in the population? ex
1/10,000 babies has PKU (recessive) q2
_____ p2 _____ q ___ 2pq
_____ (carriers) p ___
0.0001
0.9801
0.01
0.0198
0.99
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Hardy-Weinberg equilibrium only occurs if a
population is NOT evolving. MICROEVOLUTION is a
change in allele frequency from one generation to
the next in a population.
WHITE HAMSTERS bb
GREY HAMSTERS BB or Bb
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This will keep populations in Hardy Weinberg
Equilibrium (NO EVOLUTION)

• 1. Very large population size
• 2. No Migrations
• 3. No net mutations
• 4. Random mating
• 5. No natural selection (NO reproductively fit
  allele)

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• Microevolution is caused by departures from the 5
  conditions needed for equilibrium
• small populations (genetic drift -important)
• gene flow (migration)
• mutation
• nonrandom mating
• natural selection (important)

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Population Genetics Darwin Natural
Selection Mendel genetic basis for variation
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1. Genetic Drift change in allele frequency due
to CHANCE. Ex Billy goat determines which
plants survives by randomly chewing off some
flowers. So the allele frequency may be not 0.5
R and 0.5r in each generation.
2 types of drift.
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The Bottleneck Effect (skewed representation of
alleles after disasters) can lead to genetic
drift. Bottle neck is the disaster! Alleles
left after disaster may not be 0.5R and 0.5r.Ex
cheetahs and hunting
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The Founder Effecta small number of individuals
colonize a new, isolated area this can lead to
genetic drift. Ex eye disease alleles have a
high frequency in the founders of a colony
                                                
                      ltgt
1814 - Tristan da Cunha colonized by 15 people!
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2. Gene Flow - Migration
If populations arent completely isolated,
individuals can migrate and introduce alleles
into another population. Ex wind
pollinators This may cause a change in
allele frequency in the next generation.
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3. Mutations
Mutations can change the frequency of alleles in
a population, but this is a very slow effect in
humans. Bacteria - this is fast! Mutation is one
of the sources of genetic variation that leads
to natural selection.
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4. Nonrandom Mating
Inbreeding and assortive mating cause an increase
in homozygotes. Allele frequencies will not
change, but genotype frequencies will.
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Nonrandom Mating
M/M
M/M
M/N
M/N
N/N
N/N
Eskimo
Egyptian
Chinese
Australian
OBSERVED
EXPECTED from Hardy-Weinberg Theorem
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5. Natural Selection
Differential success in reproduction If aa is
less reproductively fit, then the frequency of
the a allele will decrease. Ex Cow likes red
flower!
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Allele under case study The Asian getting a B
is failing gene -AA must have an A -Aa -
will be okay with an A -aa - what a shame even
an A- is just awlright! (tsk tsk tsk)
Initial frequency of these genes in a populations
is p 0.8 (A) q 0.2 (a)
What 5 ways can you influence this allele
frequency to change and cause microevolution?
Break Hardy Weinbergs equilibrium
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• Variation within populations
• quantitative characteristics are
• Polygenic (height)
• 2. discrete characteristics are
• determined by a single gene locus
• (freckles)
• --2 or more forms represented
• polymorphic

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Geographical variationthere are differences in
gene structure between populations in different
areas
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Genetic variation is created by Mutation and
sexual recombination (Crossing over during
Prophase 1, Independent assortment during Anahase
1, and Randomn fertilization). This variation
leads to natural selection if there is a
selective advantage in one allele type. Diploidy
prevents natural selection from quickly getting
rid of an unfavorable allele. (Aa a hides)
Aa
(aa albino AA/Aa normal)
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How are undesirable alleles maintained in a
population?
Sickle Cell Anemia
1) Diploidy -heterozygote carries the hidden
recessive gene
2) Heterozygote advantange also maintains
undesirable alleles.
(aa anemia Aa carrier malaria protection AA
normal)
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Another ex. for how natural selection maintains
variation or undesirable alleles
Host Parasite Interaction Red Queen
Frequency-dependent selection Reproductive
success of one morph declines when it becomes
too common in the population due to coevolution
of predators. Ex Brown/green Fox and mite..
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Balanced polymorphism maintains genetic diversity
in a population via natural selection
Frequency- dependent selection
Heterozygote advantage
Undesirable alleles/variation is preserved by
Variation is caused by
Sexual Recombination
Heterozygote advantage
Frequency- dependent selection
And Mutations
Diploidy
Variation is the Raw Material for Natural
Selection
Natural selection
TO SUMMARIZE
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• Types of SELECTION
• Directional
• Diversifying
• Stabilizing

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• Types of SELECTION
• 4) Sexual Selection - natural selection for
  traits that increases mating chance
• Intersexual Selection female selects
  reproductively fit male
• Intrasexual selection males combat for female

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ARE YOU A FIT INDIVIDUAL?
Darwinian Fitness
the relative contribution that an individual
makes to the gene pool of the next generation
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• So why arent we perfect?
• organisms are locked into historical constraints
• adaptations often are compromises
• not all evolution is adaptive
• selection only can edit variations that exist