CH 16
EVOLUTION OF
POPULATIONS
Crash Course: Population Genetics
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https://www.youtube.com/watch?v=WhFKPaRnTdQ
16-1 Genetic equilibrium
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Population genetics: study of evolution from a
genetic point of view
Basically how populations of a species evolve
But what is a population?
Group of members of the same species living in the
same area
Sources of genetic variation

1.
Three main sources
Mutations: any change in sequence of DNA
 Replication
mistakes
 Radiation/environmental causes
2.
3.
recombination: reshuffling of genes
Random pairing of gametes
Bell curve
# of individuals with that trait

Many traits in nature show trends like this
Phenotype continuum

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Number of phenotypes produced depends on how
many genes control that trait
Single gene traits- have two alleles
Two distinct phenotypes

Polygenic traits- controlled by two or more genes
 Results
in multiple phenotypes

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Gene pool- all genes, including all different alleles,
that are present in a population
frequency (of an allele)- number of times alleles
occur in a gene pool
 Percentage

Genetic definition of evolution?
 Change
in relative frequency of alleles in a population
over time
Phenotype frequency

How often a specific phenotype is observed in a
population?
 Can

be written mathematically
Frequency = # indiv. w/a particular phenotype
total # of indiv. in population
Hardy-Weinberg equilibrium
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
https://www.youtube.com/watch?v=oG7ob-MtO8c
When evolution is not occurring
 Allele

frequencies remain the same
In order for evolution to not occur,
certain conditions must be met.
Evolution Versus Genetic Equilibrium
Hardy-Weinberg principle = Genetic Equilibrium
• Random Mating – Equal opportunity to produce offspring
• Large Population – Genetic Drift does not effect Allele Frequency
• No Movement into or out of Population – The gene pool must be
kept together (no new alleles)
• No Mutations – Mutations cause new forms of alleles changing the
frequency
• No Natural Selection – All genotypes must have equal probability
of surviving.
Hardy-Weinberg equilibrium
Allele frequency equation
p+q=1
 p = frequency of dominant allele
 q = frequency of recessive allele
Together, they make 100% of alleles for a gene in
that population
 If p = 34%, what is q? 0.66
 If q = 19%, what is p? 0.81
Hardy-Weinberg equilibrium

GENOTYPIC FREQUENCY EQUATION
p2 + 2pq + q2 = 1
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p2 = homozygous dominant frequency
2pq = heterozygous frequency
q2 = homozygous recessive frequency
If p = .46, what is p2? 0.2116
If p = .12, what is q2? 0.7744 = 77%
If q =.31, what is 2pq? 0.4278
Worksheet Answers
Practice Problems
q = 0.55
a.
q2 = 0.30
b.
p = 0.45 (1 – q = p)
c.
p2 = 0.20 (0.45)2
d.
2pq = 0.50
Worksheet Answers
Colorblindness
1.
2,500
2.
25
3.
0.01
4.
0.1 = 10% = q
5.
0.9 = 90% = p
6.
0.81 = 81% = p2
7.
0.18 = 18% = 2pq
8.
0.01 = 1% = q2
0.81 + 0.18 + 0.01 = 1
Worksheet Answers
No freckles
1.
3,420
2.
513
3.
q2 = 0.15
4.
0.39 = 39% = q
5.
0.61 = 61% = p
6.
0.37 = 37% = p2
7.
0.48 = 48% = 2pq
8.
0.15 = 15% = q2
Worksheet Answers
1.
RR 0.09
p2
rr 0.49
q2
Rr 0.42
2pq
R 0.3
p
r 0.7
q
How many of 7,000 carry free ear lobe allele?
2,100 (7,000 x 0.3)
Worksheet Answers
3.
B = 0.4
b = 0.6
a. BB = 0.16 (p2)
b. Bb = 0.48 (2pq)
c. Bb = 0.36 (q2)
Worksheet Answers
4.
10% of 2,000 natives have recessive allele for
psychic ability (witch doctor).
p = 0.90 = 90%
q = 0.10 = 10%
 How
many witch doctors? 200
(2,000 x 0.10)
 Allele frequency of dom. Non-psychic? 0.90
 Proportion of population:
a.
b.
c.
BB = 0.81 (p2)
Bb = 0.18 (2pq)
bb = 0.1
(q2)
16-2 Disruption of genetic equilibrium

Mutation
 Occur
at a relatively constant rate over time
 Can be sped up when exposed to mutagens
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Gene flow: process of genes moving from one
population to another
Immigration: moving into a population
Emigration: moving out of a population
Genetic Drift
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Alleles can become rare by chance
Over time a series of chance occurrences can cause
an alleles to become common in a population
Effects of genetic drift are more dramatic with small
population size
Founder effect: change in allele frequencies as a
result of migration of a small subgroup of a
population
Genetic
Drift
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic
Drift
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic
Drift
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Nonrandom mating
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Sexual selection: tendency of individuals to choose
a mate with certain traits.
Common in birds
Peacock display
Tropical birds of paradise - Papua New Guinea
The amazing Lyrebird - Australia
Natural selection


Natural selection on a single gene traits can lead to
changes in allele frequencies
Natural selection on polygenic traits
3
possible effects
Directional selection
2.
Stabilizing selection
3.
Disruptive selection
https://www.youtube.com/watch?v=vCHdT9MWIaA
1.
Directional selection

When individuals at one end of curve have higher
fitness than individuals in the middle or the other
end
Stabilizing selection

When individuals near the middle have higher
fitness than the individuals at either end
Disruptive selection

When individuals at upper and lower ends have
higher fitness than individuals near the middle
16-3 Formation of Species
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As new species evolve, populations become
reproductively isolated from each other
Reproductive isolation: when two members of
populations cannot interbreed and produce fertile
offspring
Separate gene pools
Isolation Mechanisms
Geographic Isolation:
- separation of animals in a specific
region
- formation of river, canyon, mountain
Isolation Mechanisms
Behavioral Isolation:
- differences in courtship or reproductive
behaviors
-meadowlark songs
Temporal isolation:
-two or more species reproduce at
different times
-orchids
Formation of species

Allopatric speciation:
when species arise from
geographic isolation
 Different
places
https://www.youtube.com/w
atch?v=cSgulsydsQU
Reproductive isolation


Prezygotic isolation: premating
isolation
Species may live
 in
different places
 Reproduce at different times
 Have different mating behaviors

Postzygotic isolation: postmating
isolation
 Hybrids
may be weak
 Hybrids may be sterile
Sympatric
speciation

Sympatric
speciation: when
two subpopulations
become isolated
while living in the
same area
Rates of speciation

Gradualism:
speciation at gradual
and regular rate

Punctuated
equilibrium: periods
of sudden, rapid
change followed by
periods of
littelchange