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Individual fitness is the reproductive success over an individual life cycle, with
respect to other members of a population. Fitness of a zygote depends on
the probability of survival to reproductive age, number of gametes
contributing to the production of descendant, and the number of descendant
which succeed in passing their own genes to the next generation;
There is a positive correlation between fitness and level of heterozygosity
It is generally not easy to measure overall fitness. Fitness components are
easier to quantify. These are generally characteristics linked to physiology,
metabolism, survival and reproduction of an organism.
Grey wolves from Isle Royale
Population established in 1949
Ht / H0 = 0.039 / 0.087 = 0.45
Reduced reproductive output
Reduced survial of pups
Inbreeding depression
N
40
15
♀♂
Low levels of genetic diversity at both
functional and selectively neutral loci
Adder (Vipera berus)
Reduction in clutch size
Abnormalities in youngsters
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Florida panther
N
60 – 70
Low variability recorded at nucelar loci and mitochondrial DNA
sequences with respected to other populations and other big cats
Asiatic lion
Gir
Serengeti
Ngorongoro
Panthera leo persica
Panthera leo azandica and nubica
Individual fitness reduction
• 41 nuclear loci
• mtDNA
• DNA fingerprints (multilocus, minisatellites)
Mantle hair loss (higher susceptibility to patogens)
Tail malformation
Heart defects (atrial septal defects)
Cryptorchidism (undescended testicle)
Low semen production
Defect in the head and tail of the spermatozoon
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Cheetah
• 52 nuclear loci
• mtDNA
• Major histocompatibility complex (MHC)
• DNA fingerprints (singlelocus, microsatellites)
• Acceptance of allogenic skin grafts from unrelated individuals
High susceptibility to infections (felin infectious peritonitis)
Abnormal sperm morphology
High natal mortality (30%)
Highest fitness occurs when expression of recessive, deleterious alleles is
masked by dominant alleles;
This is the dominance hypothesis used to explain reduction in fertility,
progeny body mass, growth and survival rate, and higher patogen
susceptibility recorded in populations with high inbreeding coefficients;
According to the dominance hypothesis, inbreeding increases the
probability that recessive deleterious alleles are found in homozygote state
and can therefore be expressed.
Purging selection
Recessive lethal or deleterious alleles become evident through inbreeding,
and can therefore be eliminated (purged) via natural selection;
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Aldabra giant tortoise
Colonization:
80,000 years before present
Single mitochondrial line
Low allelic variability at eight microsatellite loci
N = 100,000
N
California elephant seal
20 – 30
(probably just one harem left)
N  100,000
Reduced genetic variability at 20 loci
Two mitochondrial lines
In small, non isolated populations allelic frequencies vary because of the
interaction of migration and genetic drift;
Genetic drift results in a decrease of genetic variability of a population
and an increase of genetic differentiation among populations;
Gene flow, on the other hand, leads to an increase of genetic diversity and
to a homogeneization of allele frequencies among populations.
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After t generations, an equilibrium is reached whereby the number of
alleles lost to genetic drift is counter‐balanced by the alleles gained from
migrants;
The probability of autozygosity expressed by the coefficeint of inbreeding
will have a different value when, from one generation to the next, allele
frequencies change because of gene flow among populations.
When the number of alleles lost to genetic drift equals the number of alleles
gained from migrants, Fe denotes a relation of dynamic equilibrium between
genetic drift and gene flow
Fe =
1
1 + 4Nem
• In a small population, gene flow will have to be relatively higher to
counteract genetic drift than in larger populations
• The inbreeding coefficient decreases very fast with respect to single
increment of gene flow. A small number of migrants can result in a drastic
inbreeding (F) decrease.
1
0.8
Nm = 0.25 (one migrant every 4 generations)
Fe = 0.50
Nm = 1
Fe = 0.20
0.6
Fe
0.4
1
Fe =
1 + 4Nem
0.2
0
0
1
2
3
4
5
6
7
8
9
10
Number of migrants per generation (Nem)
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Fitness rebound
Introduction of individuals from nearby
populations results in fitness increase)
Adder (Vipera berus)
Florida panther
(Puma concolor coryi)
N
60 – 70
Fitness rebound thanks to translocation of invididuals Puma concolor
coguar from central North America
Factors affecting population genetic structure
• Population (small) size and genetic drift
• Population structure
• Mating system (deviation from random mating)
• Mutation
• Migration (gene flow)
• Selection - different gamete fertility
different survival of zygotes
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Allelic frequencies p and q, for alleles A and a, respectively, under
HWE:
p (A)
q (a)
AA
Aa
p+q=1
p (A)
q (a)
p
pq
2
aA
aa
qp
q
2
Expected genotipic frequencies:
AA: p
Aa: 2p q
2
AA
aa: q
2
aa
A random sample of the population results in equal allele frequencies
(0.5). The actual population structure is unknown and the expected
genotype frequencies under Hardy-Weinberg equilibrium will be:
p2 = 0.25
q2 = 0.25
2pq = 0.5
The actual genotype frequencies are
p2 = 0.5
q2 = 0.5
2pq = 0
There is a deficit of overall heterozygotes even with random mating in
each deme. This is due to the absence of gene flow between demes.
AA
aa
Genotype frequencies reach equilibrium when the barrier is eliminated
and a panmictic regime is established in the population (isolate breaking
or Wahlund effect)
Genotype frequencies change
from
½ AA, 0 Aa, ½ aa
to
¼ AA, ½ Aa, ¼ aa
The frequency of homozygotes decreases from 1 to ¼ + ¼ = ½.
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hn
h3
h1
h2
HS - HO
FIS =
HS
h1
h2
HS1
FST =
FST =
HS2
HT - HS
HT
HT - HS
HT
FST is the reduction in heterozygotes in the average subpopulation
with respect to the total, panmictic population;
That is the probability of two alleles of a subpopulation being
identical by descent (IBD) with respect to the probability of two
alleles drawn from the entire set of subpopulations (the total
population) being IBD;
FST is a measure of the level of genetic differentiation among
(sub)populations.
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p
q
HE (2pq)
Population
A1A1
A1A2
A2A2
1
0.25
0.50
0.25
0.5 0.5
0.5
2
0.04
0.32
0.64
0.2 0.8
0.32
HS = 0.41
0.35 0.65
p
HT = 0.455
q
m
x
HT = 1 -
2
i
i =1
h1
h2
HS1
FST =
HS2
HT - HS
HT
HS3
HS1
FST =
HS2
HT - HS
HT
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HS3
HS1
FST =
HS2
HT - HS
HT
An example of population divergence estimated by  , using the software Arlequin
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Results are shown in the default browser in html format
Compare FST values and check for statistical significance
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Galàpagos giant tortoises
Pinta
ISABELA
Marchena
Santiago
Fernandina
Genovesa
Pinzón
St Cruz
St Cristóbal
St Fé
Española
Floreana
St. Salvador
St Cruz
Pinzon
St Cristobal
Sierra Negra
Cerro Azul
Isabela
Espanola
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ISABELA
V. ALCEDO
SIERRA NEGRA
CERRO AZUL
EAST
WEST
CABO
ROSA
Volcano
CR
20 km
LP
LT
LC
PEG
ISABELA
Sierra Negra (CR)
–
Cerro Azul (W)
LP
0.303
–
LT
0.205
0.002
–
LC
0.173
0.191
0.229
PEG
0.957
0.846
0.747
Cerro Azul (E)
–
0.004
–
Genetic distances between tortoise popolations of volcan Cerro Azul and
Sierra Negra based on FST values
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