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1
Evolutionary Mechanisms I
22.122.2
2
Populations evolve,
individuals do not
Simpson video
3
The Modern Synthesis
• Established in the 1930s and 1940s
• Connected the units of evolution (genes) with the
mechanism of evolution (selection on populations)
• Integrated Darwinian evolution with Mendelian
genetics
+
4
Architects of the modern synthesis
JBS Haldane
G Ledyard Stebbins
Theodosius Dobzhansky
Sewall Wright
5
Hardy Weinberg principle
• Given a series of assumptions, allele
frequencies and genotype frequencies
will not change between generations
• The assumptions are:
–
No selection
–
No mutation
–
No migration
–
Infinitely large population
–
Random mating
AA
Aa
aa
AA
Aa
aa
aa
Generation 1, “Gene pool” Generation 2,
allele freq p and q
allele freq p and q
AA
Aa
aa
AA
Aa
aa
aa
A
a
a
A
A
a
a
A
a
If these assumptions are met, allele and genotype
frequencies will not change between generations
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7
Some terminology
• Imagine a populations made up of 100
diploid individuals: 40AA, 40Aa and
20aa
• The genotype frequencies are?……….
• The allele frequencies are?…….
• The genotype numbers are?…….
• The allele numbers are….
.
8
Remember the multiplication
and addition rules?
•
If you just roll one die, what’s
the chance of getting a 4?
•
What’s the chance of getting
4 twice in a row?
•
The combined probability of
independent events
occurring together is the
product
of their individual
probabilities
9
•
If you just roll one die,
what’s the probability of
getting
either
a 4 or a 6?
•
The combined probability of
2 mutually exclusive events
is the
sum
of their individual
probabilities
10
A
A
a
a
0.5
0.5
Eggs
Sperm
Under HardyWeinberg equilibrium, allele
frequencies predict genotype frequencies
Here’s a numerical example using Punnett squares:
0.5
0.5
0.25 AA
0.50 Aa
0.25 aa
Freq A = p = 0.5
Freq a = q = 0.5
.25 AA
.25 aA
.25 Aa
.25 aa
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A
A
a
a
p
q
Eggs
Sperm
Here’s the same thing using symbols.
..
p
q
p
2
AA
2pq Aa
q
2
aa
Always check your work: p + q = 1
p
2
+ 2pq + q
2
= 1
p
2
AA
pq aA
pq Aa
q
2
aa
12
The same principle applies when there
are more than 2 alleles (although we’ll
need to use more than p and q)
Alleles: A
1
A
2
A
3
A
4
A
5
A
6
...
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 Fall '06
 11:0001:50pm
 Evolution

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