Transcript PPT

Genetic omelettes and the death
of evolution of new species
Maladaptation
1
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.
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43e-1
Genetic consequences of inbreeding
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)
-e.g., sickle cell anemia, cystic fibrosis, Tay-Sachs,
hemophilia, phenylketonuria, etc.
- Genetic load
4) increase in phenotypic variability resulting from a deviation
from the mean genotypes in non-inbred individuals
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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 = alleles that are 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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43e-2
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 (through Sis) 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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Calculation of F from sib mating
AB
parents
CD
A = .5
A = .5
sibs
A = .5
A = .25
A = .5
-- --
What is F?
Identical by descent
AA
BB
CC
DD
Probability
.25 x .25 = .0625
“
“
“
F = 4 x .0625 =6 .25
31e
Calculating F in a non-inbred population
C1C2
1 grandparent
Non-inbred gene pool,
F1 generation
Inbred F2
C1C1
Non-inbred
autozygous
Ne = number of breeding individuals
2 Ne = number of alleles in the gene pool
Probability of drawing any first allele, say C1, = 100%
Probability of drawing the same allele again = F = 1
2Ne
C1C2
allozygous
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Calculating F in a Non-inbred population, cont.
Fnoninbred = 1 which is approximately 0 in an ideal pop.
2Ne
Probability of drawing autozygous alleles = 1 = Ft
2Ne
= p (C1) * p (C1)
Probability of drawing allozygous alleles = 1 - 1
2Ne
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51-2
Relationship of F and H
*
*
When H0 = 1 (i.e., no initial inbreeding), F = 0
so:
Ft = 1 - Ht
I.e., inbreeding and heterozygosity are inversely
related.
Bottom line: all real-world populations tend to become
• completely homozygous because of genetic drift
• AND completely inbred
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50
Outbreeding depression due to regional adaptation
Hunting results in extinction of Czech ibex
Translocation of ibex from nearby Austria
IbexTurkey X IbexCzech-Austria
(fall rut)
(spring rut)
fertile hybrids that rutted in fall,
gave birth in February (coldest month)
extinction of population
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51A
OUTBREEDING:
Outbreeding = crossing of unrelated invididuals.
Hybrid vigor = Heterosis = increased fitness due
to outbreeding.
which is why:
• stray dogs look like mutts and not like AKC poodles
• you see wild-type fruitflies on your rotting apple
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Consequences of inbreeding:
Results of an early experiment on inbreeding
in rats (Ritzema-Bos 1894)
Year
% of unsuccessful Average litter % mortality by
matings
size
4 weeks of age
1887
1888
1889
1890
1891
1892
0
2.6
5.6
17.4
50.0
41.2
7.50
7.14
7.71
6.58
4.58
3.20
3.9
4.4
5.0
8.7
36.4
45.5
45.5
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55-top
Juvenile mortality increases after 1 generation
Species
% juvenile mortality
Inbred* Noninbred Sign test
Ind. elephant
50
15
+
Zebra
50
26
+
Pygmy hippo
40
32
+
Giraffe
60
21
+
Sable
100
18
+
Oryx
100
5
+
Wildebeest
25
14
+
Dorcas gazelle
44
28
+
* F=0.25; e.g., wild-caught male x daughter
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Ralls and Ballou: Examination of zoo pedigrees
Infant mortality in 41 of 44 species
was higher in the inbred animals
(7 orders, 21 families and 36 genera)
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Summary
Inbreeding:
1) Inbreeding depression
a) decrease in fertile matings
b) decrease in litter size
c) increase in juvenile mortality
2) Inbreeding does not always result in inbreeding depression
a) selfing plants
b) Tamil tribes of India
c) European Bison
3) Positive aspects
a) derive offspring without deleterious alleles
b) fix alleles (domestic stock)
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Usual outcome of inbreeding: THE F VORTEX
declining Ne
increased F
(decreased H)
(increased genetic drift)
Inbreeding depression
decreased N
Extinction
decreased r (reproductive rate)
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How much inbreeding is tolerable?
If
Then
F = 1
2Ne
F=
and Ne = 4 M F
M+F
1
2 4MF
M+F
F = 1 +
8F
Important!
1
8M
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60a
How much inbreeding is tolerable?
F = 1 +
8F
1
8M
Research on domestic farm animals:
natural selection for performance can balance inbreeding
depression if the ΔF is no more than 1% per generation.
So,
F = 0.01 is a tolerable level of inbreeding
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60a
How much inbreeding is tolerable?
F = 1 +
8F
If
1
8M
F = 0.01 is a tolerable level of inbreeding, then
.01 = 1
8F
+
1
8M
so F = 25 and M = 25
or, Ne = 50
Magic number!
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60a
What happens to the ‘magic number’ when sex ratios are unequal?
Number of females
F=
1
1
+
8Nm
8Nf
Conclusion:
15 = smallest number of effective
individuals of one sex
.005 tolerance
25
.01 tolerance
15
Number of males
20
Population size
Population bottlenecks
H=1- 1
2Ne
bottleneck
Time
= expected proportion of Ho retained after a
1-generation bottleneck
Ht = Ho 1 - 1 t = proportion of Ho retained t generations after
2Ne
a bottleneck
if Ne at t=0 = 4, then Ht=1 = 1 -
1 = 7
2x4
8
i.e. 1/8 of original H
was lost in 1
21
generation
61A