Transcript Slide 1

Population Genetics
Review:
1859: Darwin and the birth of modern biology
(explaining why living things are as they are) –
Heritable Traits and Environment  Evolution
Review:
1859: Darwin and the birth of modern biology
(explaining why living things are as they are) –
Heritable Traits and Environment  Evolution
Mendel: Heredity works by the
transmission of particles (genes)
that influence the expression of
traits
Review:
1859: Darwin and the birth of modern biology
(explaining why living things are as they are) –
Heritable Traits and Environment  Evolution
Mendel: Heredity works by the
transmission of particles (genes)
that influence the expression of
traits
Avery, McCarty, and MacLeod:
Genes are DNA
Review:
1859: Darwin and the birth of modern biology
(explaining why living things are as they are) –
Heritable Traits and Environment  Evolution
Mendel: Heredity works by the
transmission of particles (genes)
that influence the expression of
traits
Avery, McCarty, and MacLeod:
Genes are DNA
Watson and Crick: Here’s the
structure of DNA
Review:
1859: Darwin and the birth of modern biology
(explaining why living things are as they are) –
Heritable Traits and Environment  Evolution
Mendel: Heredity works by the
transmission of particles (genes)
that influence the expression of
traits
Avery, McCarty, and MacLeod:
Genes are DNA
Watson and Crick: Here’s the
structure of DNA
Modern Genetics: Here’s how DNA
influences the expression of traits
from molecule to phenotype
throughout development
Review:
1859: Darwin and the birth of modern biology
(explaining why living things are as they are) –
Heritable Traits and Environment  Evolution
Mendel: Heredity works by the
transmission of particles (genes)
that influence the expression of
traits
Avery, McCarty, and MacLeod:
Genes are DNA
Watson and Crick: Here’s the
structure of DNA
Modern Genetics: Here’s how DNA
influences the expression of traits
from molecule to phenotype
throughout development
How does evolution work at a
genetic level? Population Genetics
and the Modern Synthesis
Review:
1859: Darwin and the birth of modern biology
(explaining why living things are as they are) –
Heritable Traits and Environment  Evolution
Mendel: Heredity works by the
transmission of particles (genes)
that influence the expression of
traits
How does evolution work at a
genetic level? Population Genetics
and the Modern Synthesis
Avery, McCarty, and MacLeod:
Genes are DNA
Watson and Crick: Here’s the
structure of DNA
Modern Genetics: Here’s how DNA
influences the expression of traits
from molecule to phenotype
throughout development
How can we describe the patterns of
evolutionary change through DNA
analyses? Evolutionary Genetics
The Modern Synthesis
The Darwinian
Naturalists
The
Mutationists
Ernst Mayr
T. H. Morgan
R. Goldschmidt
The discontinuous variation between
species can only be explained by the
discontinuous variation we see expressed
as a function of new mutations; the
probabilistic nature of selection is too weak
to cause the evolutionary change we see
in the fossil record
Selection is the only
mechanism that can explain
adaptations; mutations are
random and cannot explain the
non-random ‘fit’ of organisms to
their environment
The Modern Synthesis
Sewall Wright
Random chance was an
important source of change in
small populations
J. B. S. Haldane
Developed mathematical
models of population genetics
with Fisher and Wright
R. A. Fisher
Multiple genes can produce
continuous variation, and
selection can act on this
variation and cause change
in a population
Theodosius Dobzhansky
Described genetic differences
between natural populations;
described evolution as a change
in allele frequencies.
Population Genetics
I. Basic Principles
Population Genetics
I. Basic Principles
A. Definitions:
- Population: a group of interbreeding organisms that share a
common gene pool; spatiotemporally and genetically defined
- Gene Pool: sum total of alleles held by individuals in a population
- Gene/Allele Frequency: % of genes at a locus of a particular allele
- Gene Array: % of all alleles at a locus: must sum to 1.
- Genotypic Frequency: % of individuals with a particular genotype
- Genotypic Array: % of all genotypes for loci considered = 1.
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
1. Determining the Gene and Genotypic Array:
Individuals
AA
Aa
aa
60
80
60
(200)
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
1. Determining the Gene and Genotypic Array:
AA
Aa
aa
Individuals
60
80
60
(200)
Genotypic
Array
60/200 =
0.30
80/200 = .40
60/200 =
0.30
=1
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
1. Determining the Gene and Genotypic Array:
AA
Aa
aa
Individuals
60
80
60
(200)
Genotypic
Array
60/200 =
0.30
80/200 = .40
60/200 =
0.30
=1
''A' alleles
120
80
0
200/400 =
0.5
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
1. Determining the Gene and Genotypic Array:
AA
Aa
aa
Individuals
60
80
60
(200)
Genotypic
Array
60/200 =
0.30
80/200 = .40
60/200 =
0.30
=1
''A' alleles
120
80
0
200/400 =
0.5
'a' alleles
0
80
120
200/400 =
0.5
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
1. Determining the Gene and Genotypic Array
2. Short Cut Method:
- Determining the Gene Array from the Genotypic Array
a. f(A) = f(AA) + f(Aa)/2 = .30 + .4/2 = .30 + .2 = .50
b. f(a) = f(aa) + f(Aa)/2 = .30 + .4/2 = .30 + .2 = .50
KEY: The Gene Array CAN ALWAYS be computed from the genotypic array; the
process just counts alleles instead of genotypes. No assumptions are made when you do
this.
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
C. Hardy-Weinberg Equilibrium:
1. If a population acts in a completely probabilistic manner, then:
- we could calculate genotypic arrays from gene arrays
- the gene and genotypic arrays would equilibrate in one generation
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
C. Hardy-Weinberg Equilibrium:
1. If a population acts in a completely probabilistic manner, then:
- we could calculate genotypic arrays from gene arrays
- the gene and genotypic arrays would equilibrate in one generation
2. But for a population to do this, then the following assumptions must be met
(Collectively called Panmixia = total mixing)
- Infinitely large (no deviation due to sampling error)
- Random mating (to meet the basic tenet of random mixing)
- No selection, migration, or mutation (gene frequencies must not change)
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
C. Hardy-Weinberg Equilibrium:
Agents of Change
Mutation
N.S.
Recombination
- crossing over
VARIATION
Sources of Variation
- independent assortment
Drift
Migration
Mutation
Non-random Mating
So, if NO AGENTS are acting on a
population, then it will be in
equilibrium and WON'T change.
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
C. Hardy-Weinberg Equilibrium:
3. PROOF:
- Given a population with p + q = 1.
- If mating is random, then the AA, Aa and aa zygotes will be formed at p2 + 2pq + q2
- They will grow up and contribute genes to the next generation:
- All of the gametes produced by AA individuals will be A, and they will be produced at a frequency
of p2
- 1/2 of the gametes of Aa will be A, and thus this would be 1/2 (2pq) = pq
- So, the frequency of A gametes in the “gamete/gene pool” will be p2 + pq = p(p + q) = p(1) = p
- Likewise for the 'a' allele (remains at frequency of q).
- Not matter what the gene frequencies, if panmixia occurs then the population will reach an
equilibrium after one generation of random mating...and will NOT change (no evolution)
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
C. Hardy-Weinberg Equilibrium:
Initial
genotypic freq.
Gene freq.
Genotypes, F1
Gene Freq's
Genotypes, F2
AA
Aa
aa
0.4
0.4
0.2
1.0
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
C. Hardy-Weinberg Equilibrium:
Initial
genotypic freq.
Gene freq.
Genotypes, F1
Gene Freq's
Genotypes, F2
AA
Aa
aa
0.4
0.4
0.2
f(A) = p = .4 + .4/2 = 0.6
1.0
f(a) = q = .2 + .4/2 = 0.4
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
C. Hardy-Weinberg Equilibrium:
Initial
genotypic freq.
Gene freq.
Genotypes, F1
Gene Freq's
Genotypes, F2
AA
Aa
aa
0.4
0.4
0.2
1.0
f(A) = p = .4 + .4/2 = 0.6
f(a) = q = .2 + .4/2 = 0.4
p2 = .36
q2 = .16
2pq = .48
= 1.00
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
C. Hardy-Weinberg Equilibrium:
Initial
genotypic freq.
Gene freq.
Genotypes, F1
Gene Freq's
Genotypes, F2
AA
Aa
aa
0.4
0.4
0.2
1.0
f(A) = p = .4 + .4/2 = 0.6
f(a) = q = .2 + .4/2 = 0.4
p2 = .36
q2 = .16
2pq = .48
f(A) = p = .36 + .48/2 = 0.6
= 1.00
f(a) = q = .16 + .48/2 = 0.4
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
C. Hardy-Weinberg Equilibrium:
Initial
genotypic freq.
Gene freq.
Genotypes, F1
Gene Freq's
Genotypes, F2
AA
Aa
aa
0.4
0.4
0.2
1.0
f(A) = p = .4 + .4/2 = 0.6
f(a) = q = .2 + .4/2 = 0.4
p2 = .36
q2 = .16
2pq = .48
f(A) = p = .36 + .48/2 = 0.6
.36
.48
= 1.00
f(a) = q = .16 + .48/2 = 0.4
.16
1.00
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
C. Hardy-Weinberg Equilibrium:
D. Utility
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
C. Hardy-Weinberg Equilibrium:
D. Utility
1. If no real populations can explicitly meet these assumptions, how can the
model be useful?
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
C. Hardy-Weinberg Equilibrium:
D. Utility
1. If no real populations can explicitly meet these assumptions, how can the
model be useful? It is useful for creating an expected model that real populations can be
compared against to see which assumption is most likely being violated.
Example:
CCR5 – a binding protein on the surface of white
blood cells, involved in the immune response.
CCR5-1 = functional allele
CCR5 – D32 = mutant allele – 32 base deletion
Curiously, homozygotes for D32 are resistant to
HIV, and heterozygotes show slower progression
to AIDS.
Mutant allele interrupts virus’s
ability to infect cells.
Example:
CCR5 – a binding protein on the surface of white
blood cells, involved in the immune response.
CCR5-1 = functional allele
CCR5 – D32 = mutant allele – 32 base deletion
Curiously, homozygotes for D32 are resistant to
HIV, and heterozygotes show slower progression
to AIDS.
GENOTYPES
32 base-pair deletion,
shortening one of the
fragments digested
with a restriction
enzyme
GENOTYPE OBSERVED
EXPECTED
O-E
(O – E)2
(O – E)2/E
1/1
223
224.2
-1.2
1.44
0.006
32/1
57
55.4
1.6
2.56
0.046
32/32
3
3.4
-0.4
0.16
0.047
X2 =
0.099
283
1/1 = 223/283 = 0.788
32/1 = 57/283 = 0.201
32/32 = 3/283 = 0.011
p = 0.788 + 0.201/2 = 0.89
q = 0.011 + 0.201/2 = 0.11
Expected 1/1 = p2 x 283 = (0.792) x 283 = 224.2
Expected 1/32 = 2pq x 283 = (0.196) x 283 = 55.4
Expected 32/32 = q2 x 283 = (0.0121) x 283 = 3.4
So this population is in HWE at this locus. HIV is still rare, and is
exerting too small a selective pressure on the whole population to
change gene frequencies significantly.
This is the percentage of CCR5 delta 32 in different ethnic populations:
European Descent: 16%
African Americans: 2%
Ashkenazi Jews: 13%
Middle Eastern: 2-6%
Why does the frequency differ in different populations? Drift or Selection?
Allelic frequency of CCR5-d32 in Europe
Galvani, Alison P. , and John Novembre. 2005. The evolutionary history of the CCR5-D32 HIVresistance mutation. Microbes and Infection 7 (2005) 302–309
Why Europe?
- the allele is a new mutation
- was it selected for in the past?
Spread of the Bubonic Plague
Why Europe?
- the allele is a new mutation
- was it selected for in the past?
Smallpox and CCR5
Smallpox in Europe
“In the 18th century in Europe, 400,000 people died
annually of smallpox, and one third of the survivors went
blind (4). The symptoms of smallpox, or the “speckled
monster” as it was known in 18th-century England,
appeared suddenly and the sequelae were devastating.
The case-fatality rate varied from 20% to 60% and left most
survivors with disfiguring scars. The case-fatality rate in
infants was even higher, approaching 80% in London and
98% in Berlin during the late 1800s.” Reidel (2005).
The WHO certified that smallpox was eradicated in 1979
Relationships Between Smallpox and HIV
1.
“Smallpox, on the other hand, was a continuous presence in Europe for 2,000
years, and almost everyone was exposed by direct person-to-person contact.
Most people were infected before the age of 10, with the disease's 30 percent
mortality rate killing off a large part of the population before reproductive age.”
ScienceDaily (Nov. 20, 2003)
2.
The HIV epidemic in Africa began as vaccination against smallpox waned in
the 1950’s – 1970’s. Perhaps vaccinations for smallpox were working against
HIV, too.
3.
In vitro studies of wbc’s from vaccinated people had a 5x reduction in infection
rate of HIV compared to unvaccinated controls. Weinstein et al. 2010
So, it may have been selected for in Europe, and now confer some resistance to
HIV.
Population Genetics
I. Basic Principles
A. Definitions:
B. Basic computations:
C. Hardy-Weinberg Equilibrium:
D. Utility
1. If no real populations can explicitly meet these assumptions, how can the
model be useful? It is useful for creating an expected model that real populations can be
compared against to see which assumption is most likely being violated.
2. Also, If HWCE is assumed and the frequency of homozygous recessives can
be measured, then the number of heterozygous carriers can be estimated.
Example:
Cystic fibrosis (cc) has a frequency of 1/2500 = 0.0004 in people of northern
European ancestry.
More than 1,000
different mutations in
the CFTR gene have
been identified in cystic
fibrosis patients. The
most common mutation
(observed in 70% of
cystic fibrosis patients)
is a three-base deletion
in the DNA sequence,
causing an absence of a
single amino acid in the
protein. = 0.0004 x 0.7 =
0.00028
Water follows salt flow by osmosis
and dilutes mucus
Example:
Cystic fibrosis (cc) has a frequency of 1/2500 = 0.0004 in people of northern
European ancestry; common allele = 0.00028.
Mucus in lungs reduces
respiration, increases
bacterial infection
In pancreas/liver,
reduces flow/efficacy of
digestive enzymes
In intestine, reduces
nutrient uptake
Example:
Cystic fibrosis (cc) has a frequency of 1/2500 = 0.0004 in people of northern
European ancestry, common allele = 0.00028
How many carriers are there?
q2 = 0.00028, so
q2 = q = 0.017.
p + q = 1, so p = 0.983
So, the frequency of heterozygous carriers for this allele = 2pq = 0.033
This calculation can only be performed if HWE is assumed.
Population Genetics
I. Basic Principles
II. Deviations from HWE
A. Mutation
II. Deviations from HWE
A. Mutation
1. Basics:
II. Deviations from HWE
A. Mutation
1. Basics:
a. Consider a population with:
f(A) = p = .6
f(a) = q = .4
II. Deviations from HWE
A. Mutation
1. Basics:
a. Consider a population with:
f(A) = p = .6
f(a) = q = .4
b. Suppose ‘A' mutates to ‘a' at a realistic rate of:
μ = 1 x 10-5
II. Deviations from HWE
A. Mutation
1. Basics:
a. Consider a population with:
f(A) = p = .6
f(a) = q = .4
b. Suppose ‘A' mutates to ‘a' at a realistic rate of:
μ = 1 x 10-5
c. Well, what fraction of alleles will change?
‘A' will decline by: μp = .6 x 0.00001 = 0.000006
‘a' will increase by the same amount.
II. Deviations from HWE
A. Mutation
1. Basics:
a. Consider a population with:
f(A) = p = .6
f(a) = q = .4
b. Suppose ‘A' mutates to ‘a' at a realistic rate of:
μ = 1 x 10-5
c. Well, what fraction of alleles will change?
‘A' will decline by: μp = .6 x 0.00001 = 0.000006
‘a' will increase by the same amount.
d. So, the new gene frequencies will be:
q1 = q + μp = .400006
p1 = p - μp = p(1-μ) = .599994
At this realistic rate, it takes thousands of generations to cause appreciable
change. Mutation is the source of new alleles, but it does not change the
frequency of alleles very much. Were the mutationists wrong?
II. Deviations from HWE
A. Mutation
1. Basics:
2. Other Considerations:
II. Deviations from HWE
A. Mutation
1. Basics:
2. Other Considerations:
- Selection:
Selection can BALANCE mutation... so a deleterious allele
might not accumulate as rapidly as mutation would predict,
because it is eliminated from the population by selection each
generation.
II. Deviations from HWE
A. Mutation
1. Basics:
2. Other Considerations:
- Selection:
- Drift:
The probability that a new allele (produced by mutation)
becomes fixed (q = 1.0) in a population = 1/2N (basically, it's
frequency in that population of diploids). In a small population,
this chance becomes measureable and likely. So, NEUTRAL
mutations have a reasonable change of becoming fixed in small
populations... and then replaced by new mutations.
II. Deviations from HWE
A. Mutation
B. Migration
1. Basics:
- Consider two populations:
p2 = 0.7
p1 = 0.2
q1 = 0.8
q2 = 0.3
II. Deviations from HWE
A. Mutation
B. Migration
1. Basics:
- Consider two populations:
p2 = 0.7
p1 = 0.2
q2 = 0.3
q1 = 0.8
suppose migrants immigrate at a rate
such that the new immigrants
represent 10% of the new population
II. Deviations from HWE
A. Mutation
B. Migration
1. Basics:
- Consider two populations:
p1 = 0.2
q1 = 0.8
p2 = 0.7
q2 = 0.3
suppose migrants immigrate at a rate
such that the new immigrants
represent 10% of the new population
II. Deviations from HWE
IMPORTANT EFFECT, BUT MAKES
POPULATIONS SIMILAR AND INHIBITS
DIVERGENCE AND ADAPTATION TO LOCAL
CONDITIONS (EXCEPT IT MAY INTRODUCE
NEW ADAPTIVE ALLELES)
A. Mutation
B. Migration
1. Basics:
- Consider two populations:
p1 = 0.2
q1 = 0.8
p(new) = p1(1-m) + p2(m)
P(new) = (0.2).9 + (0.7)0.1 = 0.25
p2 = 0.7
q2 = 0.3
suppose migrants immigrate at a rate
such that the new immigrants
represent 10% of the new population
Frequency of the ‘B’ allele of the ABO blood group locus,
largely as a result of the Mongol migrations following the
fall of the Roman Empire
II. Deviations from HWE
A. Mutation
B. Migration
C. Non-Random Mating
1. Positive Assortative Mating
"like phenotype mates with like phenotype"
II. Deviations from HWE
A. Mutation
B. Migration
C. Non-Random Mating
1. Positive Assortative Mating
"like phenotype mates with like phenotype"
a. Pattern:
offspring
F1
AA
Aa
aa
.2
.6
.2
II. Deviations from HWE
A. Mutation
B. Migration
C. Non-Random Mating
1. Positive Assortative Mating
"like phenotype mates with like phenotype"
a. Pattern:
offspring
F1
AA
Aa
aa
.2
.6
.2
ALL AA
1/4AA:1/2Aa:1/4aa
ALL aa
II. Deviations from HWE
A. Mutation
B. Migration
C. Non-Random Mating
1. Positive Assortative Mating
"like phenotype mates with like phenotype"
a. Pattern:
offspring
F1
AA
Aa
aa
.2
.6
.2
ALL AA
1/4AA:1/2Aa:1/4aa
ALL aa
.2
.15 + .3 + .15
.2
.35
.3
.35
a. Pattern:
offspring
F1
AA
Aa
aa
.2
.6
.2
ALL AA
1/4AA:1/2Aa:1/4aa
ALL aa
.2
.15 + .3 + .15
.2
.35
.3
.35
b. Effect:
- reduction in heterozygosity at this locus; increase in homozygosity.
Groth, J. 1993. Call matching and
positive assortative mating in Red
Crossbills. The Auk 110L: 398-401.
male
female
Type 2
Type 1
II. Deviations from HWE
A. Mutation
B. Migration
C. Non-Random Mating
1. Positive Assortative Mating
2. Inbreeding
- reduction of heterozygosity across the entire genome, at a rate that
correlates with the degree of relatedness.
- full sibs, parent/offspring: lose 50%of heterozygosity each generation.
BigCatRescue
White tigers in the U.S. are all descendants of a brother-sister pair from
the Cincinnati Zoo. The AZA has outlawed captive breeding of white
tigers.
II. Deviations from HWE
A. Mutation
B. Migration
C. Non-Random Mating
1. Positive Assortative Mating
2. Inbreeding
- reduction of heterozygosity across the entire genome, at a rate that
correlates with the degree of relatedness.
- full sibs, parent/offspring: lose 50%of heterozygosity each generation.
CAN INCREASE PROBABILITY OF DIVERGENCE BETWEEN
POPULATIONS, AND CAN ALSO BE A WAY TO PURGE
DELETERIOUS ALLELES (ALTHOUGH AT A COST TO
REPRODUCTIVE OUTPUT).
II. Deviations from HWE
A. Mutation
B. Migration
C. Non-Random Mating
D. Genetic Drift - Sampling Error
1. The organisms that actually reproduce in a population may not be
representative of the genetics structure of the population; they may vary just due to
sampling error (chance).
D. Genetic Drift - Sampling Error
1. The organisms that actually reproduce in a population may not be
representative of the genetics structure of the population; they may vary just
due to sampling error (chance).
- most dramatic in small samples.
2. effects:
D. Genetic Drift - Sampling Error
1. The organisms that actually reproduce in a population may not be
representative of the genetics structure of the population; they may vary just
due to sampling error (chance).
- most dramatic in small samples.
2. effects:
1 - small pops will differ more, just by chance, from the original
population
D. Genetic Drift - Sampling Error
1. The organisms that actually reproduce in a population may not be
representative of the genetics structure of the population; they may vary just
due to sampling error (chance).
- most dramatic in small samples.
2. effects:
1 - small pops will differ more, just by chance, from the original
population
2 - small pops will vary more from one another than large populations
D. Genetic Drift - Sampling Error
1. most dramatic in small samples.
2. effects
3. circumstances when drift is very important:
D. Genetic Drift - Sampling Error
1. most dramatic in small samples.
2. effects
3. circumstances when drift is very important:
- “Founder Effect”
The Amish, a very small, close-knit
group decended from an intial
population of founders, has a high
incidence of genetic abnormalities such
as polydactyly
- “Founder Effect” and Huntington’s Chorea
HC is a neurodegenerative disorder caused by
an autosomal lethal dominant allele.
The fishing villages around Lake Maracaibo in
Venezuela have the highest incidence of
Huntington’s Chorea in the world, approaching
50% in some communities.
- “Founder Effect” and Huntington’s Chorea
HC is a neurodegenerative disorder caused by
an autosomal lethal dominant allele.
The fishing villages around Lake Maracaibo in
Venezuela have the highest incidence of
Huntington’s Chorea in the world, approaching
50% in some communities.
The gene was mapped to chromosome 4, and
found the HC allele was caused by a repeated
sequence of over 35 “CAG’s”. Dr. Nancy Wexler
found homozygotes in Maracaibo and described
it as the first truly dominant human disease
(most are incompletely dominant and cause
death in the homozygous condition).
- “Founder Effect” and Huntington’s Chorea
HC is a neurodegenerative disorder caused by
an autosomal lethal dominant allele.
The fishing villages around Lake Maracaibo in
Venezuela have the highest incidence of
Huntington’s Chorea in the world, approaching
50% in some communities.
By comparing pedigrees, she traced the
incidence to a single woman who lived 200
years ago. When the population was small, she
had 10 children who survived and reproduced.
Folks with HC now trace their ancestry to this
lineage. Also a nice example of “coalescence” –
convergence of alleles on a common ancestral
allele.
D. Genetic Drift - Sampling Error
1. most dramatic in small samples.
2. effects
3. circumstances when drift is very important:
- “Founder Effect”
- “Bottleneck”
- “Genetic Bottleneck”
If a population crashes (perhaps as the result of a plague) there will be both
selection and drift. There will be selection for those resistant to the disease
(and correlated selection for genes close to the genes conferring resistance),
but there will also be drift at other loci simply by reducing the size of the
breeding population.
European Bison, hunted
to 12 individuals, now
number over 1000.
Cheetah have very
low genetic diversity,
suggesting a severe
bottleneck in the
past. They can even
exchange skin grafts
without rejection.
Fell to 100’s in the 1800s,
now in the 100,000’s