Transcript PPT

BSCI 363: read the rest of chapter 9
CONS 670: read the rest of chapter 7, and chapter 9
1
stabilizing
directional
disruptive
As natural selection begins
P decreases
H depends on genotype favored by selection
After selection has occurred
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stabilizing
directional
disruptive
As natural selection begins
After selection has occurred
P
H
P
?
H
P
?
H
?3
Dynamic Effects:
Natural Selection
maintains allele frequencies in equilibrium
with environmental demands
vs.
Genetic Drift
pulls allele frequencies away from environmental
equilibrium
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5 causes of microevolution
1) genetic drift - stochastic variation in inheritance
2) Assortative mating
3) Mutation
4) Natural selection
5) Migration (gene flow)
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31a-2
Emigration / Immigration
Pollen grains
Recipient population
Donor population
emigration from one population and immigration
into the other; breeding = Gene flow
Migration (m) of breeding individuals results in
increased H and increased P
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Models of gene flow based on population structure
metapopulation
subpopulation
1. Continent to island model
(source - sink model)
e.g., Madagascar
2. Equivalent island model
e.g., Philippines
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3. Stepping-stone model
e.g., Hawaiian Islands
4. Isolation by distance model (continuous habitat)
e.g., Amazon forest
Genetic
neighborhood
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Gene flow results in homogenization of allele frequencies on
“islands” of equivalent size. *
A = .6
Before
gene flow:
A = .7
A = .5
A = .4
After:
A = .55
A = .55
A = .55
m
A = .55
* assume thorough gene flow between populations
.7
.6
.5
.4
X = .55
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30e
Changes in allele frequency due to migration
mij = gene flow = # breeding immigrants from donor population j
size of recipient population i
migrants (m) moving from donor (j) to recipient (i)
Change in allele frequency (q) in population i:
Before
Recipient i
qi
After
qi’ = (1-mij) qi + mij qj
Donor
qj
j
j
“jump”
qj
i
“into”
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41A
Gene flow example
Donor population (j)
Before
After
Ne = 200
qj = 0.9
qj’ = 0.9
Recipient (i)
Ne = 300
qi = 0.5
5 individuals
qi’ = 0.51
mij = 5 = 0.0167
300
qi’ = (1 - mij) qi + mij(qj)
= (1 - 0.0167) (.5) + (0.0167) (0.9)
= (0.5067) = 0.51
(If number of immigrants = 50, then qi’ = 0.57)
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41e
Gene flow: major points
1) High mij homogenizes allele frequencies in two populations
2) Rate of gene flow influences Ne of recipient population and
metapopulation
3) A small amount of gene flow may counteract genetic drift
and conserve genetic diversity in small populations
4) Allele frequency in the donor population is assumed to be
unchanged after gene flow to recipient population
5) Size of donor population does not influence allele
frequencies in recipient populations
6) Applications: calculate number of individuals needed to
introduce into recipient population of known size to
maintain its genetic diversity.
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Directional selection in peppered moths
(Biston betularia) in England
2 phenotypes: black moth, mottle white moth
Prior to 1600 (Industrial revolution)
black form approximately 1%
white form approximately 99%
After 1600 (widespread industrial pollution, smoke and soot)
black form approximately 90%
white form approximately 10%
Now (local pollution from smokestacks)
Near pollution source
Away
black form
50%
10%
white form
50%
90%
Outbreeding depression?
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Selection and gene flow:
colonization along an environmental gradient
Cold-adapted favored
m
m
m
m
m
Warm-adapted favored
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Effect of inbreeding on H
Selfing: In a population with f (a) = f (A) = 0.5
At Hardy-Weinberg equilibrium, genotypic frequencies are
p2 + 2pq + q2 = 1
AA
Aa
aa
Parental genotypic frequencies:
.25
.50
.25
F1 homozygotes
.25
.25
F1 heterozygotes
.125
.25
.125
F1 genotypes
.375
.25
.375
Conclusion: Frequency of heterozygotes is reduced by 50% with
each generation of selfing.
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But there is no loss of allelic diversity: f (a) = f (A) = 0.5
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The case of selfing with some random mating too
The frequency of heterozygotes will always fall between 2pq and 0
(1-s)
HS = --------------- 2pq
(1- s/2)
HS = equilibrium heterozygote frequency (random + selfing)
s = proportion of selfing
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The loss of heterozygosity through time caused by inbreeding
Brother-sister (sibs)
.5
.4
Ht
Selfing
.3
.2
.1
0
0
Time in generations
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0.75
Full-sibs
Half-sibs
0.50
Double
first
cousins
0.25
First
cousins
1
2
3
4
Generations
5
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Genetic consequences of inbreeding
1) decrease in heterozygosity, no change in P (allelic diversity)
(the more related the individuals, the faster the loss of H)
2) increases the probability of a zygote receiving identical alleles
(homologous alleles), which will result in increased expression
of recessive alleles.
3) increased phenotypic expression of deleterious alleles (strongly
selected against)
- often results in decreased size, reproduction, vigor, etc.,
which decrease fitness (i.e., inbreeding depression)
4) increase in phenotypic variability resulting from a deviation
from the mean genotypes in non-inbred individuals
Genetic load
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43e-1
Inbreeding coefficient
Sewall Wright (1923)
F = the probability that an individual will receive two equal
alleles, at a specific locus, that are from the same ancestor.
Autozygous = identical by descent
allozygous = not identical by descent
F = probability that an individual will be autozygous at a given
locus
1 - F = probability that an individual will be allozygous at a
given locus
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Calculate Junior’s inbreeding coefficients
from this pedigree:
Mom
AB
CD
Sis
Dad
AC
CC
Junior (or could
be DD from Dad)
Probability of C from Dad to Sis to Junior = .25
Probability of C from Dad to Junior = .50
Probability of Jr. inheriting CC from Dad = .25 X .50 = .125
Probability of Junior inheriting DD from Dad = .125
F = .125 + .125 = .25
= probability of Jr. being autozygous
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