Download - 1 BSCI 363: read the rest of chapter 9 CONS 670: read the rest of chapter 7, and chapter 9
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BSCI 363: read the rest of chapter 9
CONS 670: read the rest of chapter 7, and chapter 9
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stabilizing directional disruptive
As natural selection begins
After selection has occurred
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P decreasesH depends on genotype favored by selection
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stabilizing directional disruptive
As natural selection begins
After selection has occurred
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P
H
P
H
P
H? ? ?
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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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Emigration / Immigration
Donor populationRecipient population
Pollen grains
emigration from one population and immigrationinto 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 e.g., Madagascar (source - sink model)
2. Equivalent island modele.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. *
* assume thorough gene flow between populations
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Beforegene flow:
After:
A = .7
A = .6
A = .5A = .4
A = .55
A = .55 A = .55
A = .55.7.6.5.4
X = .55
m
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Changes in allele frequency due to migrationmij = 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 AfterRecipient i qi qi’ = (1-mij) qi + mij qj
Donor j qj qj
j i“jump” “into”
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Gene flow example
Before Ne = 200 Ne = 300qj = 0.9 qi = 0.5
After qj’ = 0.9 qi’ = 0.51
mij = 5 = 0.0167 300qi’ = (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)
Donor population (j) Recipient (i)
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5 individuals
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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
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Cold-adapted favored
Warm-adapted favored
m
m
m
m
m
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Effect of inbreeding on HSelfing: In a population with f (a) = f (A) = 0.5 At Hardy-Weinberg equilibrium, genotypic frequencies are
p2 + 2pq + q2 = 1AA 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% witheach generation of selfing. But there is no loss of allelic diversity: f (a) = f (A) = 0.5 43
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(1-s)HS = --------------- 2pq
(1- s/2)
HS = equilibrium heterozygote frequency (random + selfing) s = proportion of selfing
The case of selfing with some random mating too
The frequency of heterozygotes will always fall between 2pq and 0
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.5
0
.1
.2
.3
.4
Ht
Time in generations0 20
Brother-sister (sibs)
Selfing
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The loss of heterozygosity through time caused by inbreeding
18Generations
521 43
Los
s of
Ht
0.75
0.50
0.25
Full-sibs
Half-sibs
Doublefirstcousins
Firstcousins
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Genetic consequences of inbreeding1) 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
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Genetic load
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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 locus1 - F = probability that an individual will be allozygous at a given locus
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Calculate Junior’s inbreeding coefficients from this pedigree:
AB CDMom Dad
AC
CC
C = .5
C = .5
C = .5Sis
Junior (or couldbe DD from Dad)
Probability of C from Dad to Sis to Junior = .25Probability of C from Dad to Junior = .50 Probability of Jr. inheriting CC from Dad = .25 X .50 = .125Probability of Junior inheriting DD from Dad = .125
F = .125 + .125 = .25 = probability of Jr. being autozygous
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