Download Week 8, Class 2

Natural Selection and Drift
Drift can affect evolution even when selection is strong
Natural selection and drift
Drift has a larger effect in smaller populations
Selection has a larger effect in larger populations
Natural selection in the lab
Selection for E. coli to survive
better in artificial media
Two Views on the Relationship
Between Mutation & Selection
• Lamarkian: the origin of variation depends on
selection. Variation arises (through mutation)
in response to challenges.
• Darwinian: the origin of variation is
independent of selection. Variation exists in a
population and some traits are passed down
due to their higher relative fitness.
Replica Plating as a Test of the Relationship
Between Mutation & Selection
strpSix bacterial colonies
replica
strp+
Only one bacterial colony grows in the
presence of streptomycin, demonstrating
that it has streptomycin-resistance. Did
exposure to streptomycin cause the
resistance mutation to appear, or did
presence of streptomycin resistance
mutation allow a resistant colony to
grow?
Replica Plating as a Test of the Relationship
Between Mutation & Selection
strpstrp+
replica
strp+
streak
Streptomycin resistant
mutants existed prior to
exposure to
streptomycin
strp+
Based on your understanding of natural selection
and of mutation as a mechanism of generating
variation, explain the changes that occurred in the
tree and dinosaur populations over time.
These represent the
average heights for an entire
population
(AAAS 1999)
time
So...
• Mutation doesn’t occur in response to
selection.
• Selection acts on existing genetic variation.
We can use DNA sequences to infer
evolutionary history
DNA Sequence Analysis
1
1
1
1
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1
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1
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0
1
2
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A
G
G
C
T
A
C
T
G
C
C
A
T
A
T
T
Species 1
ind X
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C
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T
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A
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ind Y
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C
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A
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A
C
ind Z
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A
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C
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T
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A
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A
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Species 2
ind X
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C
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G
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ind Y
T
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G
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G
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ind Z
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G
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G
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G
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Species 3
ind X
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C
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G
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ind Y
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C
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G
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ind Z
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C
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Common
Ancestor
Map the mutations on the tree
Common Ancestor
1 1 1 1 1 1 1
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
A G G C T A C T G C C A T A T T
X
Y
Z
Species one
X
Y
Z
Species two
X
Y
Z
Species three
DNA Sequence Analysis
1
1
1
1
1
1
1
1
2
3
4
5
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7
8
9
0
1
2
3
4
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6
A
G
G
C
T
A
C
T
G
C
C
A
T
A
T
T
Species 1
ind X
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C
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T
T
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A
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ind Y
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C
T
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T
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A
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A
C
ind Z
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A
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C
T
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T
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A
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A
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Species 2
ind X
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C
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G
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G
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ind Y
T
C
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G
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G
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ind Z
T
C
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G
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G
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G
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Species 3
ind X
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C
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T
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G
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ind Y
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C
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T
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C
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G
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C
ind Z
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C
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T
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C
A
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G
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Common
Ancestor
Common Ancestor
1 1 1 1 1 1 1
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
A G G C T A C T G C C A T A T T
1: A->T
X
Y
Z
Species one
X
Y
Z
Species two
X
Y
Z
Species three
Variation vs. Substitution
• Variation – within-species or withinpopulation diversity at a particular locus. E.g,
a gene sequence in different individuals may
have a variable base of A or G.
• Substitution – loss of an ancestral allele
through fixation of a new allele in a
population. The sequence has changed and
can be observed as a difference in comparison
with other species.
Is this variation or a substitution?
Common Ancestor
1 1 1 1 1 1 1
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
A G G C T A C T G C C A T A T T
1: A->T
X
Y
Z
Species one
X
Y
Z
Species two
X
Y
Z
Species three
The Journey of Man
• Spencer Wells
• Uses Y chromosome in men as a
molecular marker.
• All men descended from a
single man living in Africa 60,000
years ago
Human history, mitochondria, and
the Y-chromosome
• Y-chromosome only paternally inherited
– Mutations passed down, no recombination b/c
only ever one copy of Y-chr. in any male
• Can use these as markers to track patterns of
ancestry
INDIA
CENTRAL ASIA
EUROPE
EAST ASIA
OUT OF AFRICA
AMERICAS
OCEANIA
EASTERN AFRICA
WESTERN/CENTRAL
AFRICA
SOUTHERN AFRICA