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5/6/15
Ch 23: Evolution of Populations
Population
Species
Why is a population important?
relative fitness
lineage
genetic variation
gene variability
nucleotide variability
new alleles
sexual reproduction
Non-heritable variation: diet-based variation
on oak flowers
leaves
on oak
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Temperature-based cline (graded change): mummichog fish enzyme
Cold adaptation
Population genetics: study of the distributions and changes
of allele frequency in a population
Modern Evolutionary Synthesis
Sewall Wright
R.A. Fisher
J.B.S. Haldane
Also: J. Huxley, T. Dobzhansky, E.B. Ford, E. Mayr, B. Rensch, S. Chetverikov,
G.G. Simpson, G.L. Stebbins
Hardy-Weinberg equilibrium
If the population is not evolving,
5 conditions that need to be met:
1) No mutations
2) Random mating
3) No natural selection
G. H. Hardy
(Math)
Wilhelm Weinberg
(Ob-Gyn)
4) Extremely large population size
5) No gene flow
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Hardy-Weinberg equilibrium: Defined relationships
p+
=1
p = frequency of dominant allele
= frequency of recessive allele
p2 + 2p +
=1
p2 = expected freq. of homozyg dom. genotype
= expected freq. of homozyg rec. genotype
2p = expected freq. of heterozyg. genotype
Prediction if
population in
HW
equilibrium
How is this applied?
Example:
Recessive allele (q) Data
Gen 1
Gen1: 330/1100
Gen2: 573/1910
Is the above population in
H-W Equil?
Gen 2
Another example: 1/10,000 born with phenylketonuria (PKU) in US
Have to inherit this as 2 recessives -> = q2
-> q = square root of 1/10,000 = 0.01
-> p = freq. of dominant allele = 1 - q = 0.99
-> freq. of heterozygotes = 2pq
= 2 (0.99)(0.01) = 0.0198 or ~2 %
⇒ This is the predicted presence of PKU carriers in US population
Another example to think about:
Cystic fibrosis (CF) is found at a rate of 1/3500.
How many of us are carriers?
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How can the gene pool be changed?
mutation
non-random mating
inbreeding
Egyptian in-breeding
eg: Akhenaten
How can the gene pool be changed?
mutation
non-random mating
inbreeding
In-breeding depression
1.4"
Weekly survival"
1.2"
Non-inbred mice"
1.0"
0.8"
0.6"
Inbred mice"
0.4"
0.2"
1"
2"
3"
4" 5" 6" 7" 8"
Weeks after release"
9" 10" 11"
SBM Fig. 18.02!
How can the gene pool be changed?
mutation
non-random mating
inbreeding
assortative mating
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How can the gene pool be changed?
mutation
non-random mating
inbreeding
assortative mating
sexual selection
How can the gene pool be changed?
genetic drift
How can the gene pool be changed?
genetic drift
bottleneck effect
eg: N. elephant seals, cheetahs, humans
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How can the gene pool be changed?
genetic drift
founder effect
eg: more polydactyly than is seen
in other populations
How can the gene pool be changed?
gene flow
How can the gene pool be changed?
natural selection
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Natural selection moves the population toward better adaptation to the environment.
Directional: finch beak depth
Percent of infant death - log scale (red)"
Stabilizing: human infant weight
100"
15"
30"
10"
10"
Percent of infant population (blue)"
20"
5"
5"
2"
1" 2" 3" 4" 5" 6" 7" 8" 9"10"11"
Body weight in pounds"
http://www.hhmi.org/biointeractive/
origin-species-beak-finch
Preservation of genetic variation
diploidy
balancing selection
heterozygote advantage
Preservation of genetic variation
balancing selection
frequency-dependent selection
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Example
already
seen:
Rock pocket
mice
At the DNA-level, some neutral variation:
sometimes differences in DNA don t seem to matter (much).
Futurama!
Neutral President: I have no strong feelings
one way or the other."
Natural selection “tinkers”
It does not make perfect organisms but it does help
propagate those that are better than...
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