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Population
Genetics
Genetic structure of a population.
Population Bell Curve
 In
any given gene pool:
Extreme
Extreme
MOST
Individuals
Variation
 Environment
– Food
– Shelter
– Pollutants
 Heredity
– Mutation
– Recombination
– Random gametes
Population genetics
• genetic structure of a population
group of individuals
of the same species
that can interbreed
Describing genetic structure
• phenotype frequencies
• allele frequencies
rr = white
Rr = pink
RR = red
Describing genetic structure
• phenotype frequencies
• allele frequencies
200 white
500 pink
300 red
phenotype
frequencies:
200/1000 = 0.2 white
500/1000 = 0.5 pink
300/1000 = 0.3 red
total = 1000 flowers
Describing genetic structure
• genotype frequencies
• allele frequencies
200 rr = 400 r
500 Rr = 500 r
= 500 R
300 RR = 600 R
allele
frequencies:
900/2000 = 0.45 r
1100/2000 = 0.55 R
total = 2000 alleles
for a population
with genotypes:
100 GG
160 Gg
140 gg
calculate:
Phenotype frequencies
Allele frequencies
for a population
with genotypes:
100 GG
160 Gg
calculate:
Phenotype frequencies
260/400 = 0.65 green
140/400 = 0.35 brown
Allele frequencies
140 gg
360/800 = 0.45 G
440/800 = 0.55 g
Population genetics – Outline
 What is population genetics?
 Calculate
- genotype frequencies
- allele frequencies
Why is genetic variation important?
How does genetic structure change?
Why is genetic variation important?
variation
global
warming
survival
EXTINCTION!!
no variation
Why is genetic variation important?
variation
no variation
Why is genetic variation important?
divergence
variation
no variation
NO DIVERGENCE!!
Hardy-Weinberg Equilibrium
A population is at equilibrium ( does not
evolve) IF:
1. There are NO mutations
2. NO migration in or out
3. Population is LARGE
4. Individuals mate RANDOMLY
5. Natural Selection does NOT occur
p2 + 2pq + q2 = 1
How does genetic structure change?
changes in allele frequencies and/or
genotype frequencies through time
• mutation
• migration
• natural selection
• genetic drift
• non-random mating
How does genetic structure change?
• mutation
spontaneous change in DNA
• migration
• natural selection
• creates new alleles
• ultimate source of all
genetic variation
• genetic drift
• non-random mating
How does genetic structure change?
• mutation
• migration
• natural
individuals move in to or out of
a population
•
introduces
new
alleles
selection
• genetic drift
• non-random mating
“gene flow”
How does genetic structure change?
• mutation
• migration
certain genotypes produce
more offspring
• natural selection
• genetic drift
• differences in survival
or reproduction
differences in“fitness”
• leads to adaptation
• non-random mating
Types of Natural Selection
1. Average is
most fit
2. Two
extremes
are most fit
3. One of two
extremes is
most fit
How does genetic structure change?
• mutation
• migration
genetic change by chance alone
• natural selection
• genetic drift
• non-random mating
• sampling error
• misrepresentation
• small populations
Genetic drift
Before:
8 RR
8 rr
0.50 R
0.50 r
After:
2 RR
6 rr
0.25 R
0.75 r
How does genetic structure change?
• mutation
• migration
• natural selection
mating combines alleles
into genotypes
• genetic drift
• non-random mating
• non-random mating
non-random
allele combinations
Types of Natural Selection
4. Sexual Selection – Mates choose their
partner based on particular favored
traits.
What is a SPECIES??
 Morphological
Concept
– Species classified based on external and
internal STRUCTURES
 Biological
Concept
– Population of organisms that can
SUCCESSFULLY INTERBREED to create
fertile offspring
Causes of Speciation
 Geographic
Isolation
– Separated by rivers, mountain ranges,
canyons, etc.
 Reproductive
Isolation
– Organisms fail to reproduce successfully
– Fertile during different periods
– Incompatible morphology and/or
behavior
Rate of Speciation
 Gradual
Change
– Species are constantly changing over
time (very slow)
 Punctuated
Equilibrium
– Species go through periods of very little
change, then rapid change
– What could cause this???
Natural selection
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
Natural selection
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
Natural selection
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
Generation 2: 0.96 not resistant
0.04 resistant
mutation!
Natural selection
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
Generation 2: 0.96 not resistant
0.04 resistant
Generation 3: 0.76 not resistant
0.24 resistant
Natural selection
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
Generation 2: 0.96 not resistant
0.04 resistant
Generation 3: 0.76 not resistant
0.24 resistant
Generation 4: 0.12 not resistant
0.88 resistant
Natural selection can cause
populations to diverge
divergence
Selection on sickle-cell allele
aa – abnormal ß hemoglobin very low
fitness
sickle-cell anemia
AA – normal ß hemoglobin
vulnerable to malaria
Aa – both ß hemoglobins
resistant to malaria
intermed.
fitness
high
fitness
Selection favors heterozygotes (Aa).
Both alleles maintained in population (a at low level).
A A A
A A a
A
A
a
A
A
0.8
A
0.8
a
0.2
AA
0.8 x 0.8
aA
0.2 x 0.8
aa x AA
aa x AA
a
0.2
Aa
0.8 x 0.2
aa
0.2 x 0.2
aa AA
allele frequencies:
A = 0.8
A = 0.2
genotype frequencies:
AA = 0.8 x 0.8 = 0.64
Aa = 2(0.8 x0.2) = 0.32
aa = 0.2 x 0.2 = 0.04