Random Genetic Drift as an Evolutionary force
affecting Genetic Variation, V
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Strength of Random Genetic Drift is
determined by (1/2N), the inverse of the 2N
numbers of breeding adults in a population.
Average Genetic variance WITHIN populations
DECREASES with RGD. This limits Natural
Selection.
Average Genetic Variance AMONG populations
INCREASES with RGD. This contributes to
Speciation.
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Ecological Opportunities for Random Genetic Drift.
1. Continuous drift; populations that are always small in size, N.
a. Endangered species, like California condor, Florida panther
b. Insular species (small islands, fragmented habitats)
c. Polygynous or Polyandrous mating systems - many individuals
but few breeders, e.g., polygynous elephant seals.
2. Intermittent drift: large fluctuations in population size, Nt, from one
generation to the next. It is the generations with LOW N that cause the
most drift.
3. Bottleneck effects when populations are reduced to near extinction
but then expand to large numbers. e.g. Northern elephant seals,
cheetahs. Like (2), but LOW N infrequent occurrence.
4. Founder effects at colonization (e.g., Human religious isolates)
a. a small group of individuals becomes geographically isolated
from the remainder of the species.
b. a small group of individuals colonize a new site.
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Range of Florida Panther
in 1,500 A.D.
Range of Florida Panther
in 2,001 A.D.
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Continuous RGD and the endangered Florida
Panther
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Population Bottleneck: hunted to near-extinction 50 years ago.
Now, there are about 50-80 animals.
Evolutionary Genetic Consequences
– Little Genetic Variation in the species:
– Lower variation than other panther species.
– Lower variation today than in museum samples from 1900.
– Reproductive problems symptomatic of Inbreeding
 80% of males have low sperm counts
 93% of males have abnormal sperm, highest of all large
cats.
 High frequency of “kinked tail,” a recessive trait
 Congenital Heart defect - a hole in the heart which is the
result of inbreeding.
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RGD and the endangered Florida Panther,
Genetic Remedy:
How do you counter act RGD?
Answer: Artificial Migration or gene flow: bring in
genetically different panthers from a Texas
subspecies and OUTBREED! When Outbred to
the Texas subspecies the hybrid individuals are
heterozygous, and the reproductive problems
are absent in the hybrid offspring.
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Bottleneck and Mating System cause RGD in the
endangered Northern Elephant Seal
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Hunted for the oil in their blubber, by 1890 there
were fewer than 20 animals left: Bottleneck.
2005, there are more than 30,000 animals, but they
have a Harem Polygynous mating system.
Currently, there is Little Genetic Variation in the
population at any gene.
Now, the population experience massive dies offs
owing to Increased Susceptibility to Disease owing
to homozygosity of genes in the immune system.
E.g., 40% of yearlings seals die of a skin disease.
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Male, Northern elephant seal, with harem
Harem size: 20+ females
When 1 male has 20 mates, then 19 males have 0 mates =
Extreme variation in male reproductive fitness.
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Random Genetic Drift and Mating Systems
Each Offspring gets (1/2) of its genes from Mother
and the other (1/2) of its genes from Father.
The Number of Mothers, Nfemales, determines
HALF the Strength of Random Genetic Drift.
The Number of Fathers, Nmales, determines
HALF the Strength of Random Genetic Drift.
How do we determine the N for RGD in species
with separate males and females?
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Random Genetic Drift and Mating Systems
The Number of breeding mothers = Nfemales.
The Number of breeding fathers = Nmales.
How do you COMBINE these to measure RGD?
1] Arithmetic Mean number of breeders:
NAverage = {Nfemales + Nmales}/2
2] Harmonic Mean number of breeders:
1/H = (1/2)(1/Nfemales + 1/Nmales)
Important: H is always less than NAverage
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Random Genetic Drift and Mating Systems
The Number of breeding Mothers = Nfemales.
The Number of breeding Fathers = Nmales.
How do you combine these to measure RGD?
1] Arithmetic Average number of breeders:
NAverage = {Nfemales + Nmales}/2
2] Harmonic Mean number of breeders:
1/H = (1/2)(1/Nfemales + 1/Nmales)
H is always less than NAverage
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Same number of adult males and females but
different amounts of RGD because of differences in
the Mating System
Population of Nmales = Nfemales = 400
Monogamy: one mate per male, H = 400
1/H = (1/2)(1/400 + 1/400) = (1/2)(2/400)
Polyandry: four mates per female, H = 160
1/H = (1/2)(1/400 + 1/100) = (1/2)(5/400)
Polygyny: 20 mates per male,
H = 38
1/H = (1/2)(1/20 + 1/400) = (1/2)(21/400)
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RGD owing to Bottleneck
in
Cheeta, Acinonyx jubatus
“King Cheeta” phenotype
With a black stripe :
homozygous for a
recessive allele causing the stripe.
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RGD and the endangered Cheeta,
Acinonyx jubatus
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Population Bottleneck: 100,000 in the early 1900's but
near-extinction 10,000 to 12,000 years ago. Today there
are fewer than 10,000 animals.
Little Genetic Variation in the species:
a. genetically identical and homozygous at
histocompatibility genes, the most variable
genes
in other mammals.
b. skin grafts between two cheetahs are accepted.
c. Blood samples from 55 cheetahs from 2 widely
separated populations were almost genetically identical.
Only in highly inbred strains of laboratory mice has such
genetic uniformity ever been observed!
Reproductive problems symptomatic of Inbreeding
e.g., very low sperm counts and high incidence of
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abnormal sperm.
The Effect of a Population Bottleneck is long
lasting.
Generation
Number of
Breeding
Pairs
When population size varies:
1
100
2
10,000
3
1,000,000
4
100,000,000
Arithmetic
Mean N
1/H = (1/4)(1/N1 + 1/N2 + 1/N3 +1/N4)
1/H = 0.005050
H = 198
H <<< Naverage
25,252,525
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Founder Effect causes RGD in the
Lancaster, Pennsylvania
Amish: a religious isolate population
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Founder Effect: Founded in in 1720-1770 by
fewer than 200 people.
2001, there are now 18,000 Amish in
Lancaster County, PA.
Tend to marry within the religion with few
converts to the religion after 1800. Converts
into the religion are like migrants into a
population; they represent gene flow.
(Pedigree data exist for nearly entire group.)
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Founder Effect and Subsequent Inbreeding in
the Lancaster Amish
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Inbreeding is defined as matings between
genetically related individuals. It increases
homozygosity and leads to a high incidence of
recessive, genetic diseases.
 Recessive genetic disease: homozygotes are
the affected individuals.
Lack of Allele Variation at Histocompatibility Genes:
Human immune system genes function best when
heterozygous. Increased homozygosity at these
genes causes the population to be more susceptible
to communicable diseases. Rubella Outbreak in
1991-92 in Amish in 6 states.
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Founder Effect and Subsequent Inbreeding in the
Lancaster Amish
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Inbreeding: high incidence of recessive, genetic diseases:
1. Ellis-van Creveld syndrome (dwarfism, heart trouble,
extra digits). Only 50 cases observed in the entire world in
past 100 years: 43 of these cases are among Amish of
Lancaster. All 43 cases trace back to lineage of Mr. and Mrs.
Samuel King, who joined group in 1744.
2. Pyruvate kinase deficiency: a blood disorder; all
cases trace back to Strong Jacob Yoder, who joined group in
1742.
3. Hemophilia, sex-linked bleeding disorder in which it
takes a long time for the blood to clot; all cases trace back to
2 sisters who converted in 1820.
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Inbreeding has occurred frequently in royal families.
Royal incest was common in ruling families of
Egypt, Incan Mexico, Hawaii, and Europe.
Connections between royalty and inbreeding
1) Kings are often polygynous.
2) Inbreeding preserves royal blood line.
3) Inheritance of material possessions.
4) Inheritance of position both matrilineal and patrilineal.
5) Prescribed in origin legends. E.g., Incan kings married
sisters because the Sun married his sister, the Moon
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Inbreeding is the cause of the large number of
cases of hemophilia, X-linked recessive blood disorder,
in royal families of Europe.
Queen Victoria of England was the original carrier for the
hemophilia allele, 20 members of her family inherited the
disease over several generations.
The disease spread through Europe, as her children and
grandchildren married into different royal houses to create
political alliances, e.g. Russia and Spain.
However, the birth and subsequent death of male heirs affected
by hemophilia lead to political unrest.
In Russia, to the firing-squad execution of the royal family.
In Spain, generated anti-British sentiment for ‘polluting’ the
Spanish blood line and contributed to origins of World War I.
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Inbreeding in the flour beetle, Tribolium castaneum
male
X
female
Eight Generations of
Brother-Sister Mating
20
100% Families Survive
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Surviving Families
12
10
8
6
C2-3
4
C2-2
C2-1
C2-4
2
0
0
1
2
3
4
5
6
Generation of Inbreeding
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8
9
10
Highly Inbred
Only 25% Families Left
75% have gone extinct! 21
At the start, the average was 150 offspring/family with no inbreeding
5.5
Ln(Family Size)
5
4.5
4
3.5
3
2.5
C2 Treatment
0
1
2
Highly Inbred
35 offspring/family
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4
5
6
7
8
9
10
Generation of Inbreeding
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Experimental
inbreeding
in inbreeding
flour beetles
by
In flour beetle,
experimental
by
Brother- mating results in:
Brother-sister
1) Loss of 75% of all family lineages
in 8 generations.
1) A 77% Decrease in family size of the surviving
lineages in 8 generations.
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How can Obligate Inbreeding species exist?
Dandelion, Taraxacum officinalis
Clarkia xantiana parviflora
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Interaction of Mutation, Selection, and RGD
Only naive theories about evolution assume that
Natural Selection leads a population
to achieve an optimal level of adaptation.
Because Mutation introduces harmful alleles into
populations and because they can become fixed
by Random Genetic Drift,
Natural Selection simply cannot produce the
best of all possible worlds.
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Fundamental Differences about the Evolutionary
Process
S. Wright
R. A. Fisher
Important Forces:
Selection, Random Drift,
Mutation
Important Forces:
Selection and Mutation
Population Size:
Small Populations
Population Size:
Large Populations
N ~ 500 - 10,000
N ~ 1,000,000,000,000
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R. A. Fisher’s World
Large, Panmictic Populations
S. Wright’s World
Metapopulation
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s, selective effect
of an allele
+/-
Natural Selection ~ s
0.1000
0.0100
Alleles with s in this range
have a history determined
largely by natural selection
Population Size
Threshold
Ne = 500
0.0010
0.0001
0.00001
Genes with s in this range
are effectively neutral
have a history determined
largely by RGD
Random Genetic Drift
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s, selective effect
of an allele
+/-
R. A. Fisher’s View
Natural Selection
0.1000
0.0100
0.0010
Alleles with s in this range
have an evolutionary history
determined largely by
natural selection
0.0001
0.00001
Random Genetic Drift
Population Size
Threshold
Ne > >1,000,000 29
s, selective effect
of an allele
+/-
S. Wright’s View
Natural Selection
0.1000
Population Size
Threshold
Ne = 50
0.0100
0.0010
Genes with s in this range
are effectively neutral
And have an evolutionary history
0.0001
determined
largely by RGD
0.00001
Random Genetic Drift
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