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10 Migrants Every 10 Generations. Some Points/Issues from Last Slide.

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some points issues from last slide
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
what is inbreeding
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”

inbreeding
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)

two ways that identical alleles can be sampled
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

A5

A8

A7

A4

A2

A3

Generation t

A6A6

A3A7

A4A6

A1A5

Probability

1/2N alleles

identical

1- 1/2N alleles

distinct X Ft-1 inbreeding in previous generation

slide7
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

a calculation with and without prior inbreeding
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

slide9
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

and when the initial population is not inbred
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

conservation concerns with inbreeding
Conservation Concerns with Inbreeding

1. Inbreeding coefficient increases rapidly in small closed populations

Generation 0 F=0

Generation 1 F=1- (1-1/2N)1 = 1- (1-1/8)1 = 0.125

Generation 2 F=1- (1-1/2N)2 = 1- (1-1/8)2 = 0.234

Generation 4 F=1- (1-1/2N)4 = 1- (1-1/8)4 = 0.410

Generation 10 F=1- (1-1/2N)10= 1- (1-1/8)10=0.737

conservation concerns with inbreeding12
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

1.0

N=5

N=25

F

0.5

N=100

0

N=500

0 10 20 30 40 50

Generations (t)

F (N=5) after 50 gen = 1.00

F (N=500) after 50 gen = 0.049

conservation concerns with inbreeding13
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

N

F

0.1

400

N

0

0

Przewalski’s Horse

1910 1930 1950 1970 1990

(Mongolian Wild Horse)

Year

conservation concerns with inbreeding14

Conservation Concerns with Inbreeding

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

Characters # Sp. # Exp. O>I O

(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%

conservation concerns with inbreeding15
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

inbreeding depression in the wild
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
linearity of inbreeding depression with f
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

inbreeding depression is greater in more stressful conditions
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

variation in inbreeding different loci will become homozygous in different individuals
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

effects of ploidy on inbreeding depression
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)
detecting and measuring inbreeding depression
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

an alternative approach for detecting inbreeding depression
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
slide24
Inbreeding depression, environmental stress, and population size variation in scarlet gilia (Ipomopsis aggregata)Heschel and Paige (1995)
slide27
Small Small Large

N=61 N=42 N=2435

an alternative approach for detecting inbreeding depression28
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
an alternative approach for detecting inbreeding depression29
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%
natural selection should often favor matings between individuals of intermediate genetic similarity
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

slide31
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

slide32
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
slide33
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
optimal outcrossing
Optimal Outcrossing

Optimal Outcrossing

Repro.

Success

Inbreeding

Depression

Outbreeding

Depression

Genetic Dissimilarity

delphinium nelsoni
Delphinium nelsoni

b

bc

bc

c

Seeds/

Flower

a

Selfed 1 10 100 1000

Outcrossing Distance (M)

slide37
b

ab

ab

Relative

Fitness

a

1 M 10M 100M 2500M

Outcrossing Distance

slide38
a

ab

b

Proportion

Bands

Shared

b

1M 10M 100M 2500M

Outcrossing Distance

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