Download Random Genetic Drift

Random Genetic Drift : Chance as an
Evolutionary Force
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Random Genetic Drift is the random change
in allele frequencies from one generation to the
next that is caused by the finite size of the
breeding population of parents.
By chance, some parents have more offspring
than other parents.
By chance, some parents have fewer offspring
or no offspring at all.
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Random Genetic Drift occurs in ALL
countable, finite populations.
RGD is STRONGER in Small Populations (0 <
N < 500).
RGD is WEAKER in Large Populations (500 <
N).
The Strength of RGD is proportional to (1/2N)
in a diploid population and to (1/N) in a haploid
population.
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RGD and its Evolutionary Consequences
1) WITHIN populations RGD DECREASES GENETIC
VARIATION :: Random Genetic Drift makes a
population less genetically less variable and makes
the individuals in the population more Homozygous.
The ultimate outcome: ALL HERITABLE VARIATION
IS LOST!!
The population becomes either p* = 0.0 (allele is
lost) or p* = 1.0 (allele is FIXED).
2) BETWEEN populations RGD INCREASES GENETIC
VARIATION: Random Genetic Drift makes two
populations become genetically different from one
another.
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Generation 2
Generation 1
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No Chance Events
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Frequencies
Stay Exactly
the Same
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Chance Events
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Random Genetic
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Drift
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Generation 2
Random Genetic Drift
Generation 1
Different
Chance Events
Lead to
Different Outcomes
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One Starting Point
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Many Possible Outcomes5
RGD INCREASES Variation AMONG Populations
RGD for A Few
Generations
With NO
Migration
START: All Populations Genetically
Identical
END: Populations Genetically
Different from one another6
Generation 2
Generation 1
Average of MANY
Independent
Chance Events
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Average Effect of
Random Genetic Drift
On Allele Frequencies = ZERO
Average DP = 0
On AVERAGE
Frequencies
Stay the Same
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Generation 2
Generation 1
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5
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No Chance Events
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Frequencies
Stay Exactly
the Same
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Random Genetic 5
Drift
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On AVERAGE
Frequencies
Stay the Same
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Q: If the frequency of an allele, B, at time 0,
equals 0.65, what is probability that RGD
will result in Fixation of the B allele? That
is, B will go from a frequency of 0.65 to a
frequency of 1.00?
A) 0.00
B) 0.50
C) 0.65
D) 1.00
E) None of the above
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RGD INCREASES Variation AMONG Populations
RGD for A Few
Generations
With NO
Migration
START: All Populations Genetically
Identical
END: Populations Genetically
Different from one another10
RGD DECREASES Variation WITHIN Populations
RGD for Many
Generations
With NO
Migration
All Populations Genetically
Identical
Populations Genetically
Different from one another11
Migration OPPOSES RGD and LIMITS its effects
RGD and Migration
for Many
Generations
All Populations Genetically
Identical
Populations Somewhat
Genetically Different
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Random Genetic Drift : Effects
1. On average WITHIN one population, RGD DECREASES genetic
variation:
A) Random Genetic Drift makes INDIVIDUALS more homozygous,
more genetically similar.
B) the POPULATION becomes genetically less variable.
C) RGD causes ALLELES to become FIXED (Pa = 1) or LOST (Pa
= 0).
D) RGD diminishes the heritable variation within a population,
which limits NATURAL SELECTION.
2. On average AMONG a group of populations, RGD INCREASES
genetic variation:
A) RGD makes different populations genetically more
different from one another.
3. GENETIC FIXITY of SPECIES is NOT possible as long as there
are a lot of populations, because RGD will do something
different in each population. There is continual change of
allele frequencies in finite populations by chance, RGD.
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Generation 2
Random Genetic Drift
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Generation 1
Many
Different
Chance Events
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0
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5
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5
0
0
One Starting Point
0
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Three Equally Possible Outcomes14
Migration as an Evolutionary Force
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Migration is the Movement of Individuals between
populations.
When individuals move from one population to
another and then breed with the residents of the
new population, ‘Gene Flow’ can change frequency
of alleles in the new population.
– immigration
 movement of individuals INTO a population
– emigration
 movement of individuals OUT OF a
population
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Evolutionary Consequences of Migration (Gene Flow)
1) Gene Flow between populations reduces the genetic
differences between the populations.
2) Gene Flow can be a constraint on evolution when
immigrants carry genes into a population that are not
adapted for the ecological conditions of the population.
 alleles that are good in one population may be bad in
another population
3) Gene Flow can accelerate evolution when immigrants carry
genes into a population that are adapted for the conditions
of the population.
 beneficial alleles from other populations can be
brought in faster by migration than they can arise by
mutation.
Migration does NOT create new genetic variation; it moves
around already existing variation.
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Population 2
Population 1
Exchange Migrants
M = 0.20
pGreen = 1.00
pRED = 0.00
The two Populations
are genetically very
Different BEFORE
GENE FLOW
pGreen = 0.00
pRED = 1.00
The two Populations
are genetically more
similar AFTER
GENE FLOW
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What is Allele Frequency AFTER Migration in
Population 1? Population 1
Population 1
pGreen = 1.00
pRED = 0.00
p’Green = ???
p’RED = ???
p’Green = (1 – M)pGREEN,1 + (M)pGREEN,2
= 0.80*(1.00) + 0.20*(0.00) = 0.80
ΔpGREEN,1 = M (p2 – p1) < 0, Green DECREASED
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What is Allele Frequency AFTER Migration in
Population 2? Population 2
Population 2
pGreen,2 = 0.00
pRED,2 = 1.00
p’Green = ???
p’RED = ???
p’Green,2 = (1 – M)pGREEN,2 + (M)pGREEN,1
= 0.80*(0.00) + 0.20*(1.00) = 0.20
ΔpGREEN,2 = M (p1 – p2) > 0, GREEN INCREASED 19
Random Genetic Drift versus Migration
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RGD makes populations genetically
different.
Migration makes populations genetically
similar.
Together, RGD and Migration place a limit on
the degree of genetic differences between
populations.
Where is the limit or Balance between these
two evolutionary forces?
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Migration opposes RGD and LIMITS its effects
RGD and Migration
for Many
Generations
All Populations Genetically
Identical
Populations Somewhat
Genetically Different
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Could this happen with Migration? Answer: ????
RGD for Many
Generations
All Populations Genetically
Identical
Populations Genetically
Different from one another22
Random Genetic Drift and Migration: Effects
1. On average WITHIN one population, RGD DECREASES genetic
variation, Migration INCREASES genetic variation:
A) RGD makes INDIVIDUALS more homozygous, Migration
makes INDIVIDUALS more heterozygous.
B) the POPULATION reaches a STABLE LEVEL of genetic
variation where RGD and Migration are balanced.
C) Migration PREVENTS RGD from making ALLELES become
FIXED (Pa = 1) or LOST (Pa = 0).
D) Migration can replenish the heritable variation within a
population lost by RGD.
2. On average AMONG a group of populations, RGD INCREASES
genetic variation, Migration DECREASES genetic variation:
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