ap biology evolution of populations doonesbury - sunday february 8, 2004
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AP Biology
Evolution of Populations
Doonesbury - Sunday February 8, 2004Doonesbury - Sunday February 8, 2004
AP Biology
Populations evolve Natural selection acts on individuals
differential survival “survival of the fittest”
differential reproductive success who bears more offspring
Populations evolve genetic makeup of
population changes over time
favorable traits (greater fitness) become more common
Presence of lactate dehydrogenase
Mummichog
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Changes in populationsBent Grass on toxic mine site Pocket Mice in desert lava flows
Pesticidemolecule
Insect cellmembrane
Target site
Resistanttarget site
Insecticide resistance
Target site
Decreased number of target sites
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Individuals DON’T evolve!!!
AP Biology
Individuals DON’T evolve…Individuals survive or don’t survive…Populations evolve
Individuals are selected
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Fitness Survival & Reproductive
success individuals with one
phenotype leave more surviving offspring
Body size & egg laying in water striders
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Natural selection Natural selection adapts a population to
its environment a changing environment
climate change food source availability new predators or diseases
combinations of alleles that provide “fitness” increase in the population
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Variation impacts natural selection Natural selection requires a source of
variation within the population there have to be differences some individuals must be more fit than
others
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5 Agents of evolutionary changeMutation Gene Flow
Genetic Drift Selection
Non-random mating
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1. Mutation & Variation Mutation creates variation
new mutations are constantly appearing
Mutation changes DNA sequence changes amino acid sequence? changes protein?
changes structure? changes function?
changes in protein may change phenotype & therefore change fitness
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2. Gene Flow Movement of individuals &
alleles in & out of populations seed & pollen distribution by
wind & insect migration of animals
sub-populations may have different allele frequencies
causes genetic mixing across regions
reduce differences between populations
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Human evolution today Gene flow in human
populations is increasing today transferring alleles
between populations
Are we moving towards a blended world?Are we moving towards a blended world?
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3. Non-random mating Sexual selection
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Sex & Variation Sex spreads variation
one ancestor can have many descendants
sex causes recombination offspring have new combinations
of traits = new phenotypes
Sexual reproduction recombines alleles into new arrangements in every offspring
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Warbler
finch
Tree
finc
hes
Ground finches
4. Genetic drift Effect of chance events
founder effect small group splinters off & starts a new colony
bottleneck some factor (disaster) reduces population to
small number & then population recovers & expands again
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Founder effect When a new population is started
by only a few individuals some rare alleles may be at high
frequency; others may be missing
skew the gene pool of new population human populations that
started from small group of colonists
example: colonization of New World
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Distribution of blood types Distribution of the O type blood allele in native
populations of the world reflects original settlement
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Distribution of blood types Distribution of the B type blood allele in native
populations of the world reflects original migration
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Out of AfricaLikely migration paths of humans out of AfricaLikely migration paths of humans out of Africa
Many patterns of human traits reflect this migrationMany patterns of human traits reflect this migration
50,000ya
10-20,000ya
10-20,000ya
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Bottleneck effect When large population is drastically
reduced by a disaster famine, natural disaster, loss of habitat… loss of variation by chance event
alleles lost from gene pool not due to fitness
narrows the gene pool
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Cheetahs All cheetahs share a small number of alleles
less than 1% diversity as if all cheetahs are
identical twins
2 bottlenecks 10,000 years ago
Ice Age last 100 years
poaching & loss of habitat
AP Biology
Conservation issues Bottlenecking is an important
concept in conservation biology of endangered species loss of alleles from gene pool reduces variation reduces adaptability
Breeding programs must consciously outcrossBreeding programs must consciously outcross
Peregrine Falcon
Golden Lion Tamarin
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5. Natural selection Differential survival & reproduction due
to changing environmental conditions climate change food source availability predators, parasites, diseases toxins
combinations of alleles that provide “fitness” increase in the population adaptive evolutionary change
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5 Agents of evolutionary changeMutation Gene Flow
Genetic Drift Selection
Non-random mating
AP Biology
MeasuringEvolution of Populations
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AP Biology
Populations & gene pools Concepts
a population is a localized group of interbreeding individuals
gene pool is collection of alleles in the population remember difference between alleles & genes!
allele frequency is how common is that allele in the population how many A vs. a in whole population
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AP Biology
Evolution of populations Evolution = change in allele frequencies
in a population hypothetical: what would it be like if
allele frequencies didn’t change? non-evolving population
1. very large population size (no genetic drift)
2. no migration (movement in or out)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (no selection)
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AP Biology
Hardy-Weinberg equilibrium Hypothetical, non-evolving population
preserves allele frequencies
Serves as a model natural populations rarely in H-W equilibrium useful model to measure if forces are acting
on a population measuring evolutionary change
W. Weinbergphysician
G.H. Hardymathematician 28
AP Biology
Hardy-Weinberg theorem Alleles
assume 2 alleles = B, b frequency of dominant allele (B) = p frequency of recessive allele (b) = q
frequencies must add to 100%, so:
p + q = 1
bbBbBB
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AP Biology
Hardy-Weinberg theorem Individuals
frequency of homozygous dominant: p x p = p2 frequency of homozygous recessive: q x q = q2 frequency of heterozygotes: (p x q) + (q x p) = 2pq
frequencies of all individuals must add to 100%, so:
p2 + 2pq + q2 = 1
bbBbBB
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AP Biology
Using Hardy-Weinberg equation
q2 (bb): 16/100 = .16
q (b): √.16 = 0.40.4
p (B): 1 - 0.4 = 0.60.6
q2 (bb): 16/100 = .16
q (b): √.16 = 0.40.4
p (B): 1 - 0.4 = 0.60.6
population: 100 cats84 black, 16 whiteHow many of each genotype?
population: 100 cats84 black, 16 whiteHow many of each genotype?
bbBbBB
p2=.36p2=.36 2pq=.482pq=.48 q2=.16q2=.16
Must assume population is in H-W equilibrium!
Must assume population is in H-W equilibrium!
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AP Biology
Using Hardy-Weinberg equation
bbBbBB
p2=.36p2=.36 2pq=.482pq=.48 q2=.16q2=.16
Assuming H-W equilibriumAssuming H-W equilibrium
Sampled data Sampled data bbBbBB
p2=.74p2=.74 2pq=.102pq=.10 q2=.16q2=.16
How do you explain the data? How do you explain the data?
p2=.20p2=.20 2pq=.642pq=.64 q2=.16q2=.16
How do you explain the data? How do you explain the data?
Null hypothesis Null hypothesis
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How do allele frequencies change?
Think of all the factors that would keep a population out of H-W equilibrium!
Think of all the factors that would keep a population out of H-W equilibrium!
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Real world application of H-W Frequency of allele in human population
Example: What % of human population carries allele for
PKU (phenylketonuria) Should you screen prospective parents?
~ 1 in 10,000 babies born in the US is born with PKU results in mental retardation, if untreated
disease is caused by a recessive allele
PKU = homozygous recessive (aa)
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AP Biology
H-W & PKU disease frequency of homozygous recessive individuals
q2 (aa) = 1 in 10,000 = 0.0001 frequency of recessive allele (q):
q = √0.0001 = 0.010.01 frequency of dominant allele (p):
p (A) = 1 – 0.01 = 0.99 frequency of carriers, heterozygotes:
2pq = 2 x (0.99 x 0.01) = 0.0198 = ~2% ~2% of the US population carries the PKU allele
300,000,000 x .02 = 6,000,000 people
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AP Biology
Hardy-WeinbergLab data
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AP Biology
Hardy Weinberg Lab: No Selection
total alleles = 48
p (A): (6+6+18)/48 = .6.6
q (a): 18/48 = .4.4
total alleles = 48
p (A): (6+6+18)/48 = .6.6
q (a): 18/48 = .4.4
24 individuals
48 alleles
A: 0.50.5
a: 0.50.5
24 individuals
48 alleles
A: 0.50.5
a: 0.50.5
Original populationOriginal population
AA66
Aa1818
aa00
How do you explain these data? How do you explain these data?
Case #1Case #1
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AP Biology
Hardy Weinberg Lab: Selection
total alleles = 48
p (A): (19+19+5)/48 = .9.9
q (a): 5/48 = .1.1
total alleles = 48
p (A): (19+19+5)/48 = .9.9
q (a): 5/48 = .1.1
24 individuals
48 alleles
A: 0.50.5
a: 0.50.5
24 individuals
48 alleles
A: 0.50.5
a: 0.50.5
Original populationOriginal population
AA1919
Aa55
aa00
How do you explain these data? How do you explain these data?
Case #2Case #2
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AP Biology
Hardy Weinberg Lab:
total alleles = 48
p (A): (9+9+15)/48 = .7.7
q (a): 15/48 = .3.3
total alleles = 48
p (A): (9+9+15)/48 = .7.7
q (a): 15/48 = .3.3
24 individuals
48 alleles
A: 0.50.5
a: 0.50.5
24 individuals
48 alleles
A: 0.50.5
a: 0.50.5
Original populationOriginal population
AA99
Aa1515
aa00
How do you explain these data? How do you explain these data?
Case #3Case #3
Heterozygote Advantage
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AP Biology
Hardy Weinberg Lab: Genetic Drift
8 individuals
16 alleles
A: 0.50.5
a: 0.50.5
8 individuals
16 alleles
A: 0.50.5
a: 0.50.5
Original populationOriginal population
How do you explain these data? How do you explain these data?
AA331100
Aa556633
aa001155
p q
Case #4Case #4
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AP Biology
Hardy Weinberg Lab: Genetic Drift
total alleles = 16
p (A): (3+3+5)/16 = .7.7
q (a): 5/16 = .3.3
8 individuals
16 alleles
A: 0.50.5
a: 0.50.5
8 individuals
16 alleles
A: 0.50.5
a: 0.50.5
Original populationOriginal population
How do you explain these data? How do you explain these data?
AA331100
Aa555544
aa001155
p q
Case #4Case #4
.7.7 .3.3
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AP Biology
Hardy Weinberg Lab: Genetic Drift
total alleles = 16
p (A): (3+3+5)/16 = .7.7
q (a): 5/16 = .3.3
total alleles = 16
p (A): (3+3+5)/16 = .7.7
q (a): 5/16 = .3.3
8 individuals
16 alleles
A: 0.50.5
a: 0.50.5
8 individuals
16 alleles
A: 0.50.5
a: 0.50.5
Original populationOriginal population
How do you explain these data? How do you explain these data?
total alleles = 14
p (A): (1+1+5)/14 = .5.5
q (a): (5+1+1)/14 = .5.5
total alleles = 14
p (A): (1+1+5)/14 = .5.5
q (a): (5+1+1)/14 = .5.5
AA331100
Aa555544
aa001155
p.7.7.5.5
q.3.3.5.5
Case #4Case #4
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AP Biology
Hardy Weinberg Lab: Genetic Drift
total alleles = 16
p (A): (3+3+5)/16 = .7.7
q (a): 5/16 = .3.3
total alleles = 16
p (A): (3+3+5)/16 = .7.7
q (a): 5/16 = .3.3
8 individuals
16 alleles
A: 0.50.5
a: 0.50.5
8 individuals
16 alleles
A: 0.50.5
a: 0.50.5
Original populationOriginal population
How do you explain these data? How do you explain these data?
total alleles = 14
p (A): (1+1+5)/14 = .5.5
q (a): (5+1+1)/14 = .5.5
total alleles = 14
p (A): (1+1+5)/14 = .5.5
q (a): (5+1+1)/14 = .5.5
total alleles = 18
p (A): 4/18 = .2.2
q (a): (4+5+5)/18 = .8.8
total alleles = 18
p (A): 4/18 = .2.2
q (a): (4+5+5)/18 = .8.8
AA331100
Aa555544
aa001155
p.7.7.5.5.2.2
q.3.3.5.5.8.8
Case #4Case #4
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AP Biology
Essential Questions How do populations change over time? What factors can cause changes in
populations over time? How did modern understandings of
genetics impact evolutionary thought?
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