BIOE 109
Summer 2009
Lecture 6- Part I
Microevolution – Random genetic drift
Random genetic drift
Definition: random changes in the frequencies
of neutral alleles from generation to generation
caused by “accidents of sampling”
Random genetic drift
Definition: random changes in the frequencies
of neutral alleles from generation to generation
caused by “accidents of sampling”
Q: What is a neutral allele?
Random genetic drift
Definition: random changes in the frequencies
of neutral alleles from generation to generation
caused by “accidents of sampling”
Q: What is a neutral allele?
A: A neutral allele has no effect on fitness.
Random genetic drift
Definition: random changes in the frequencies
of neutral alleles from generation to generation
caused by “accidents of sampling”
Q: What is a neutral allele?
A: A neutral allele has no effect on fitness.
Q: How can alleles be neutral?
How can alleles be neutral?
How can alleles be neutral?
1. Mutations among very similar amino acids
How can alleles be neutral?
1. Mutations among very similar amino acids
(leu)
CUC

(val)
GUC
How can alleles be neutral?
1. Mutations among very similar amino acids
(leu)
CUC

(val)
GUC
2. Mutations involving silent (synonymous) changes
How can alleles be neutral?
1. Mutations among very similar amino acids
(leu)
CUC

(val)
GUC
2. Mutations involving silent (synonymous) changes
(leu)
CUC

(leu)
CUU
How can alleles be neutral?
1. Mutations among very similar amino acids
(leu)
CUC

(val)
GUC
2. Mutations involving silent (synonymous) changes
(leu)
CUC

(leu)
CUU
3. Mutations in non-coding (nonfunctional) DNA
Codon bias shows that silent changes may not be
neutral!
Random genetic drift
Definition: random changes in the frequencies
of neutral alleles from generation to generation
caused by “accidents of sampling”
What are RANDOM changes in frequencies?
What is sampling error?
Some properties of random genetic drift
Some properties of random genetic drift
1. Magnitude inversely proportional to effective population
size (Ne).
Some properties of random genetic drift
1. Magnitude inversely proportional to effective population
size (Ne).
2. Ultimately results in loss of variation from natural
populations.
Some properties of random genetic drift
1. Magnitude inversely proportional to effective population
size (Ne).
2. Ultimately results in loss of variation from natural
populations.
3. The probability of fixation of a neutral allele is equal to its
frequency in the population.
Some properties of random genetic drift
1. Magnitude inversely proportional to effective population
size (Ne).
2. Ultimately results in loss of variation from natural
populations.
3. The probability of fixation of a neutral allele is equal to its
frequency in the population.
4. Will cause isolated populations to diverge genetically.
Some properties of random genetic drift
1. Magnitude inversely proportional to effective population
size (Ne).
2. Ultimately results in loss of variation from natural
populations.
3. The probability of fixation of a neutral allele is equal to its
frequency in the population.
4. Will cause isolated populations to diverge genetically.
5. Is accentuated during population bottlenecks and founder
events.
What is effective population size?
What is effective population size?
• in any one generation, Ne is roughly equivalent to the
number of breeding individuals in the population.
What is effective population size?
• in any one generation, Ne is roughly equivalent to the
number of breeding individuals in the population.
• this is equivalent to a contemporary effective size.
What is effective population size?
• in any one generation, Ne is roughly equivalent to the
number of breeding individuals in the population.
• this is equivalent to a contemporary effective size.
• Ne is also strongly influenced by long-term history.
What is effective population size?
• in any one generation, Ne is roughly equivalent to the
number of breeding individuals in the population.
• this is equivalent to a contemporary effective size.
• Ne is also strongly influenced by long-term history.
• this is equivalent to a species’ evolutionary effective
size.
Factors affecting effective population size
Factors affecting effective population size
1. Fluctuations in population size
Factors affecting effective population size
1. Fluctuations in population size
- here, Ne equals the harmonic mean of the actual
population numbers:
Factors affecting effective population size
1. Fluctuations in population size
- here, Ne equals the harmonic mean of the actual
population numbers:
1/Ne = 1/t(1/N1 + 1/N2 + 1/N3 + … 1/Nt)
Factors affecting effective population size
1. Fluctuations in population size
- here, Ne equals the harmonic mean of the actual
population numbers:
1/Ne = 1/t(1/N1 + 1/N2 + 1/N3 + … 1/Nt)
Example: Over 3 generations, N = 2000, 30, 2000
Factors affecting effective population size
1. Fluctuations in population size
- here, Ne equals the harmonic mean of the actual
population numbers:
1/Ne = 1/t(1/N1 + 1/N2 + 1/N3 + … 1/Nt)
Example: Over 3 generations, N = 2000, 30, 2000
Arithmetic mean = 1343.3
Factors affecting effective population size
1. Fluctuations in population size
- here, Ne is equal to the harmonic mean of the actual
population numbers:
1/Ne = 1/t(1/N1 + 1/N2 + 1/N3 + … 1/Nt)
Example: Over 3 generations, N = 2000, 30, 2000
Arithmetic mean = 1343.3
Harmonic mean = 87.4
Factors affecting effective population size
2. Unequal numbers of males and females
Factors affecting effective population size
2. Unequal numbers of males and females
• let Nm = No. of males, Nf = No. of females:
Factors affecting effective population size
2. Unequal numbers of males and females
• let Nm = No. of males, Nf = No. of females:
Ne =
4NmNf
Nm + Nf
Factors affecting effective population size
2. Unequal numbers of males and females
• let Nm = No. of males, Nf = No. of females:
Ne =
4NmNf
Nm + Nf
Example: Breeding populations of northern elephant seals:
Factors affecting effective population size
2. Unequal numbers of males and females
• let Nm = No. of males, Nf = No. of females:
Ne =
4NmNf
Nm + Nf
Example: Breeding populations of northern elephant seals:
• 15 alpha males each controlling a harem of 20 females:
Factors affecting effective population size
2. Unequal numbers of males and females
• let Nm = No. of males, Nf = No. of females:
Ne =
4NmNf
Nm + Nf
Example: Breeding populations of northern elephant seals:
• 15 alpha males each controlling a harem of 20 females:
census size (N) = 315
Factors affecting effective population size
2. Unequal numbers of males and females
• let Nm = No. of males, Nf = No. of females:
Ne =
4NmNf
Nm + Nf
Example: Breeding populations of northern elephant seals:
• 15 alpha males each controlling a harem of 20 females:
census size (N) = 315
effective size (Ne) = 57.1
Factors affecting effective population size
3. Large variance in reproductive success
• reduces Ne because a small number of individuals have a
disproportional effect on the reproductive success of the
population.
Factors affecting effective population size
4. Genetic Bottlenecks
• genetic bottlenecks refer to severe reductions in effective
population size.
Ne
Time
Genetic Bottlenecks
• genetic bottlenecks refer to severe reductions in effective
population size.
Genetic Bottlenecks
• genetic bottlenecks refer to severe reductions in effective
population size.
Examples: the northern elephant seal (Mirounga
angustirostis) and the cheetah (Acinonyx jubatus).
Founder effects
• occur when a new population is founded from a
small number of individuals.
Founder effects
• occur when a new population is founded from a
small number of individuals.
Consequences:
Founder effects
• occur when a new population is founded from a
small number of individuals.
Consequences:
1. New population has a fraction of genetic variation
present in the ancestral population.
Founder effects
• occur when a new population is founded from a
small number of individuals.
Consequences:
1. New population has a fraction of genetic variation
present in the ancestral population.
2. Initial allele frequencies differ because of chance.
Founder effects
• occur when a new population is founded from a
small number of individuals.
Consequences:
1. New population has a fraction of genetic variation
present in the ancestral population.
2. Initial allele frequencies differ because of chance.
Example: the silvereye, Zosterops lateralis
Founder effects in the silvereye, Zosterops lateralis
Timing of island hopping…
… and reductions in allelic diversity
The interplay between drift, migration,
and selection
1. Gene flow vs. drift
The interplay between drift, migration,
and selection
1. Gene flow vs. drift
• random drift and gene flow act in opposition to each other!
The interplay between drift, migration,
and selection
1. Gene flow vs. drift
• random drift and gene flow act in opposition to each other!
random drift  allow genetic divergence of two populations.
The interplay between drift, migration,
and selection
1. Gene flow vs. drift
• random drift and gene flow act in opposition to each other!
random drift  allow genetic divergence of two populations.
gene flow  prevents divergence.
The interplay between drift, migration,
and selection
1. Gene flow vs. drift
• random drift and gene flow act in opposition to each other!
random drift  allow genetic divergence of two populations.
gene flow  prevents divergence.
if Nem > 1, gene flow overrides drift and prevents
divergence
The interplay between drift, migration,
and selection
1. Gene flow vs. drift
• random drift and gene flow act in opposition to each other!
random drift  allow genetic divergence of two populations.
gene flow  prevent divergence.
if Nem > 1, gene flow overrides drift and prevents
divergence
if Nem < 1, random drift can lead to genetic divergence.
The interplay between drift, migration,
and selection
2. Gene flow and selection
The interplay between drift, migration,
and selection
2. Gene flow and selection
• gene flow and selection usually act in opposition.
The interplay between drift, migration,
and selection
2. Gene flow and selection
• gene flow and selection usually act in opposition.
selection  favor different alleles in different populations
(“local adaptation”).
The interplay between drift, migration,
and selection
2. Gene flow and selection
• gene flow and selection usually act in opposition.
selection  favor different alleles in different populations
(“local adaptation”).
gene flow  prevents adaptive divergence.
The interplay between drift, migration,
and selection
2. Gene flow and selection
• gene flow and selection usually act in opposition.
selection  favor different alleles in different populations
(“local adaptation”).
gene flow  prevents adaptive divergence.
if m > s, gene flow can overpower local adaptation
The interplay between drift, migration,
and selection
2. Gene flow and selection
• gene flow and selection usually act in opposition.
selection  favor different alleles in different populations
(“local adaptation”).
gene flow  prevent adaptive divergence.
if m > s, gene flow can overpower local adaptation
if s > m, then selection can allow local adaptation
The interplay between drift, migration,
and selection
3. Drift and selection
The interplay between drift, migration,
and selection
3. Drift and selection
- random genetic drift and natural selection act in
opposition.
The interplay between drift, migration,
and selection
3. Drift and selection
- random genetic drift and natural selection act in
opposition.
selection  deterministic change in allele frequency
The interplay between drift, migration,
and selection
3. Drift and selection
- random genetic drift and natural selection act in
opposition.
selection  deterministic change in allele frequency
random drift  random changes in allele frequency
The interplay between drift, migration,
and selection
3. Drift and selection
- random genetic drift and natural selection act in
opposition.
selection  deterministic change in allele frequency
random drift  random changes in allele frequency
if Nes > 10, selection controls the fate of the allele
The interplay between drift, migration,
and selection
3. Drift and selection
- random genetic drift and natural selection act in
opposition.
selection  deterministic change in allele frequency
random drift  random changes in allele frequency
if Nes > 10, selection controls the fate of the allele
if Nes < 1, drift will overpower the effect of selection