10 Migrants Every 10 Generations

1
10 Migrants Every 10 Generations
2
Some Points/Issues from Last Slide

• The interval between migrations has an effect on
  Fst Fst will increase with time as a result of
  genetic drift following a migration event,until
  the next migration event when it will drop
  (although it has no effect on the average Fst
  over the long term)
• The smaller the Ne of subpops and/or the larger
  the interval between migrations, the wilder the
  ocillation
• Migration rate should be kept as constant as
  possible especially in small populations where
  Fst can increase rapidly over a short time span
  without migration

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Inbreeding and Outbreeding
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What is Inbreeding?

• When the parents of an individual share one or
  more common ancestors, the individual is inbred
• Inbred matings include self-fertilization, mating
  of brother-sister, father-daughter, mother-son,
  etc.
• Also known as consanguineous breeding

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Inbreeding

• The consequences of matings between relatives is
  that offspring have an increased probability of
  inheriting alleles that are recent copies of the
  same DNA sequence.
• Recent copies of the same allele are referred to
  as identical by descent or autozygous
• - Differs from homozygosity in that not all
  homozygous alleles are identical by descent can
  be allozygous (Adh copies 7 and 8 differ at 9
  bases but are homozygous for Adh-F)

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Two ways that identical alleles can be sampled
A1A2
A3A4
A5A6
A7A8
Generation t - 1
A7
A1
A5
A2
A1
A4
A8
Gene Pool
A3
A6
A6
A8
A5
A4
A7
A2
A3
A6A6
A3A7
A4A6
A1A5
Generation t
1/2N alleles identical
1- 1/2N alleles distinct X Ft-1 inbreeding in
previous generation
Probability
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Taken together the probability of creating a
zygote in generation t with both alleles
identical by descent (Ft) is
Ft 1/2Ne 1- 1/2Ne Ft-1
Inbreeding coefficient in generation t
Inbreeding coefficient in generation t-1
Effective population size
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A calculation with and without prior inbreeding
Ft 1/4 1-1/4 0 0.250 Ft 1/4
1-1/4 .25 0.437
9
We can also predict accumulated inbreeding over
several generations when the initial population
is inbred
1- Ft 1- 1/(2Ne)t (1-Fo) 1-Ft 1-(1/8)10
(1- 0.25) 1-Ft (0.263) (0.75) 0.197 Ft
0.803
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And when the initial population is not inbred
Ft 1- 1 1/(2Ne)t Ft 1- 1 (1/8)10
Ft 1- 0.263 0.737
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Conservation Concerns with Inbreeding

• 1. Inbreeding coefficient increases rapidly in
  small closed populations

Generation 0 F0 Generation 1 F1-
(1-1/2N)1 1- (1-1/8)1 0.125 Generation 2
F1- (1-1/2N)2 1- (1-1/8)2
0.234 Generation 4 F1- (1-1/2N)4 1-
(1-1/8)4 0.410 Generation 10 F1-
(1-1/2N)10 1- (1-1/8)100.737
12
Conservation Concerns with Inbreeding

• 2. Inbreeding also accumulates with time at a
  rate dependent upon their population sizes.
  Inbreeding increases more rapidly in small than
  large populations

N5
1.0
N25
F
0.5
N100
N500
0
0 10 20 30 40 50
Generations (t)
F (N5) after 50 gen 1.00 F (N500) after 50
gen 0.049
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Conservation Concerns with Inbreeding

• 3. Inbreeding is unavoidable in small
  populations as all individuals become related by
  descent over time

0.3
1200
Average F
0.2
800
F
N
0.1
400
N
0
0
Przewalskis Horse
1910 1930 1950 1970 1990
(Mongolian Wild Horse)
Year
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Conservation Concerns with Inbreeding

• 4. Inbreeding reduces reproductive fitness
  known since the time of Darwin (1876)

Characters Sp. Exp. OgtI OltI
Similar Difference

(O-I) Height 54 83
57 8 18
13 Weight 8 11
8 1 2 Flowering 32 58
44 9 5 Seed Set
23 33 26 2 5
41
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Conservation Concerns with Inbreeding

• Inbreeding reduces reproductive fitness in
  essentially all well-studied populations of
  outbreeding plants and animals and is known as
  inbreeding depression

..
1.0
.
..

Outbred Mortality
..
Ralls and Ballou (1983) Inbreeding depression for
41 of 44 captive mammal pops with a 33 reduction
in juvenile survival
0.5
..
.
0
0 0.5 1.0
Inbred Mortality
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Inbreeding depression in the wild

• There is now irrefutable evidence despite earlier
  skepticism
• Crnokrak and Roff (1999 Heredity) reviewed 35
  papers on inbreeding depression in nature for 34
  taxa that included 157 data sets. In 141 cases
  (90) inbred individuals did poorer than outbreds

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Linearity of inbreeding depression with F
Inbreeding depression in Maize
Height
1
Mean Inbred Outbred
.6
Yield
.2
0 .2 .4 .6 .8 1.0
F
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Inbreeding depression is greater in more
stressful conditions
A. Selfing
Rose Pink Plant
Relative Fitness
(Dudash 1990)
.45
.25
B. Outcross
Relative Fitness
.99
.95
Field Green house
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Variation in inbreeding different loci will
become homozygous in different individuals
Peromyscus polionotus 3 subsp.
8
Litter Mass
7.5
7

0 0.2 0.4 0.6
F
F
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Effects of Ploidy on Inbreeding Depression

• Absent in haploids as there are no hidden
  deleterious alleles
• Expect less inbreeding depression in tetraploids
  than diploids for similar degrees of inbreeding
  true for plant Epilobium angustifolium,
  inbreeding depression due to selfing was 0.95 in
  diploids and 0.68 in tetraploids (Husband and
  Schemske 1997)

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Inbreeding and Extinction A Threshold
EffectFrankham (1995)
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Detecting and Measuring Inbreeding Depression

• Comparing survival of inbred and outbred
  offspring in the wild (Jimenez et al. 1994
  white-footed mice)

1
Survival Inbred Outbred
0.5
0
0 4 8
12
Weeks after Introduction
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An alternative approach for detecting inbreeding
depression

• To outcross populations suspected of suffering
  inbreeding depression
• If the outcrossed progeny display increased
  fitness (heterosis) then the original population
  is suffering inbreeding depression

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Inbreeding depression, environmental stress, and
population size variation in scarlet gilia
(Ipomopsis aggregata)Heschel and Paige (1995)
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Small Small Large N61
N42 N2435
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An alternative approach for detecting inbreeding
depression

• Use genetic markers to infer the degree of
  inbreeding, comparing inbreds and outbreds
• Comparison of allozyme heterozygoisties from 25
  loci of Isle Royale gray wolves to Mainland
  wolves indicates inbreeding possible inbreeding
  depression
• F 1- (0.039/0.087) 0.55

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An alternative approach for detecting inbreeding
depression

• In Heschel and Paige 1995 RAPD markers
  demonstrated a significantly higher percentage of
  band sharing within two of the small populations
  than two of the large populations
• 62.3 1.23 66.3 1.65 versus 53.6 1.76
  49.1 3.54

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Natural Selection should often favor matings
between individuals of intermediate genetic
similarity

• - Matings between very similar individuals may
  lead to inbreeding depression (deleterious
  recessive alleles are exposed)
• - Matings between very dissimilar individuals may
  disrupt favorable gene complexes and lead to
  outbreeding depression

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Hybridization between different local populations
can sometimes destroy locally adapted gene
complexes

• - Tatra Mountain Ibex - Capra ibex ibex
• - Nubian Ibex - Capra ibex nubiana
• - Bezoars - Capra ibex aegagrus
• - Fremont and Narrowleaf Cottonwood Trees -
  Populus angustifolia, P. fremontii

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Price and Waser (1979) showed that outbreeding
depression will often occur on a much finer scale
than previously recognized, especially in plants
subject to restricted pollen and seed dispersal
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Such plants are likely to show pronounced
microgeographic genetic differentiation resulting
from drift in subpopulations isolated by distance
or from adaptation to local edaphic and biotic
conditions
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Optimal Outcrossing
Optimal Outcrossing
Repro. Success
Inbreeding Depression
Outbreeding Depression
Genetic Dissimilarity
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Delphinium nelsoni
b
bc
bc
c
Seeds/ Flower
a
Selfed 1 10 100 1000
Outcrossing Distance (M)
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Optimal Outcrossing in Agave schottii - Field and
Genetic StudiesTrame et al. (1995)
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b
ab
ab
Relative Fitness
a
1 M 10M 100M 2500M Outcrossing
Distance
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a
ab
b
Proportion Bands Shared
b
1M 10M 100M 2500M Outcrossing
Distance