Transcript Ch16 Population Evolution

Measuring
Evolution of Populations
AP Biology
2007-2008
Populations & Gene Pools
 Concepts

a population is a
localized group of
the same species
that can interbreed

gene pool is
collection of alleles
in the population
 Gene pool is all of
the different genes
in that population
 remember difference
between alleles &
genes!
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Variation & Gene Pool
 allele frequency is the number of times
that the allele occurs in the population
 how
many A vs a in whole population
 Expressed in percentage (%)

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i.e. 40% Black & 60% Brown
Evolution of populations
 Evolution = change in allele frequencies
in a population


hypothetical: what conditions would
cause allele frequencies to not change?
non-evolving population
REMOVE all agents of evolutionary change
1. very large population size (no genetic drift)
2. no migration (no gene flow in or out)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (everyone is equally fit)
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5 Agents of evolutionary change
Mutation
Gene Flow
Genetic Drift
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Non-random mating
Selection
Sources of Variation
 Mutations
Caused by error in replication, radiation,
chemicals in the environment
 Only some mutations change the
phenotype & affect fitness

 Gene Shuffling: results from sexual reproduction



23 pairs of chromosomes can make 8.4 million gene
combinations
Crossing over causes differences in genes
Gene shuffling doesn’t change the allele frequency
 Still have same # of alleles in population, but recombined
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Single Gene vs Polygenic Traits
 The number of phenotypes
produced for a given trait
depends on how many
genes control the trait
 Single Gene Trait:
controlled by a single gene
(2 alleles)
 Polygenic Trait: traits
controlled by two or more genes

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Offers more variation
Evolution as Genetic Change
 evolutionary fitness is an organism’s success
in passing genes to the next generation
 an evolutionary adaptation as any genetically
controlled physiological, anatomical, or
behavioral trait that increases an individuals
ability to pass along its genes
 Remember that evolution is any change over
time in the relative frequency of alleles in a
population.
 This reminds us that it is populations, not
individual organisms that can evolve
overtime
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Natural Selection on Single Gene Trait
 Natural selection on single gene traits
can lead to changes in allele frequencies
and thus to evolution

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i.e. Lizards
Natural Selection on Polygenic Traits
 Natural selection can affect the
distributions of phenotypes in any of
three ways
1.
2.
3.
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Stabilizing Selection
Disruptive Selection
Directional Selection
Stabilizing Selection
 When individuals near the center of the
curve have higher fitness than
individuals at either end of the curve
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Disruptive Selection
 When individuals at the upper and
lower ends of the curve have higher
fitness than individuals near the middle
 Can create 2 distinct phenotypes
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Directional Selection
 When individuals at one end of the curve
have higher fitness than individuals in
the middle or at the other end
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Genetic Drift
 Genetic Drift: the random change in allele frequency

Occurs in small populations that break away from larger
groups
 In small populations, an allele can become more or less common
by chance


Caused by individuals entering & leaving (migrating)
Ex. Founder’s Effect
 When the allele
frequency changes as
a result of migration
of a small group
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Hardy-Weinberg equilibrium
 Hypothetical, non-evolving population

preserves allele frequencies
 Serves as a model (null hypothesis)


natural populations rarely in H-W equilibrium
useful model to measure if forces are acting on
a population
 measuring evolutionary change
G.H. Hardy
AP mathematician
Biology
W. Weinberg
physician
Non-Evolving Population
 Needs 5 conditions for equilibrium:
1.
2.
3.
4.
5.
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Random Mating
Large Population (No Genetic Drift)
No Migration In or Out
No Mutation
No Natural Selection
Hardy-Weinberg theorem
 Counting Alleles
assume 2 alleles = B, b
 frequency of dominant allele (B) = p
 frequency of recessive allele (b) = q

 frequencies must add to 1 (100%), so:
p+q=1
BB
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Bb
bb
Hardy-Weinberg theorem
 Counting 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 1 (100%), so:
p2 + 2pq + q2 = 1
BB
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Bb
bb
H-W formulas
 Alleles:
p+q=1
B
 Individuals:
p2 + 2pq + q2 = 1
BB
BB
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b
Bb
Bb
bb
bb
Using Hardy-Weinberg equation
population:
100 cats
84 black, 16 white
How many of each
genotype?
p2=.36
BB
q2 (bb): 16/100 = .16
q (b): √.16 = 0.4
p (B): 1 - 0.4 = 0.6
2pq=.48
Bb
q2=.16
bb
Must
What
assume
are thepopulation
genotype frequencies?
is in H-W equilibrium!
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Using Hardy-Weinberg equation
p2=.36
Assuming
H-W equilibrium
2pq=.48
q2=.16
BB
Bb
bb
p2=.20
=.74
BB
2pq=.64
2pq=.10
Bb
q2=.16
bb
Null hypothesis
Sampled data
How do you
explain
the data?
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Biology
Speciation
 Speciation: formation of a new species
 Reproductive Isolation:
As new species evolve, populations
become more reproductively isolated
from each other.
 Isolation Mechanisms:

 Temporal Isolated
 Two species reproduce at different times
 Behaviorally Isolated
 Can breed, but have different courtship behaviors
 Geographically Isolated
 Barriers such as rivers, mountains, bodies of water
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Temporal Isolation
Rana aurora - breeds January March
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Rana boylii - breeds late March May
Behavioral Isolation
Eastern & Western Meadowlark
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Geographic Isolation
Albert & Kaibab Squirrels
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