BIOE 109 Summer 2009 Lecture 6- Part I Microevolution – Random genetic drift

BIOE 109Summer 2009

Lecture 6- Part IMicroevolution – Random genetic drift

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?



Q: What is a neutral allele?A: A neutral allele has no effect on fitness.



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?


1. Mutations among very similar amino acids



(leu) (val)CUC GUC



(leu) (val)CUC GUC

2. Mutations involving silent (synonymous) changes



(leu) (val)CUC GUC


(leu) (leu)CUC CUU



(leu) (val)CUC GUC


(leu) (leu)CUC CUU

3. Mutations in non-coding (nonfunctional) DNA

Codon bias shows that silent changes may not be neutral!



What are RANDOM changes in frequencies?

What is sampling error?

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.





4. Will cause isolated populations to diverge genetically.

5. Is accentuated during population bottlenecks and founder events.

What is effective population size?


• in any one generation, Ne is roughly equivalent to the

number of breeding individuals in the population.



number of breeding individuals in the population. • this is equivalent to a contemporary effective size.




• Ne is also strongly influenced by long-term history.




• Ne is also strongly influenced by long-term history.

• this is equivalent to a species’ evolutionary effective size.

Factors affecting effective population size


1. Fluctuations in population size


1. Fluctuations in population size - here, Ne equals the harmonic mean of the actual

population numbers:



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


1. Fluctuations in population size - here, Ne is equal to the harmonic mean of the actual



Arithmetic mean = 1343.3 Harmonic mean = 87.4


2. Unequal numbers of males and females


2. Unequal numbers of males and females • let Nm = No. of males, Nf = No. of females:



Ne = 4NmNf

Nm + Nf



Ne = 4NmNf

Nm + Nf

Example: Breeding populations of northern elephant seals:



Ne = 4NmNf

Nm + Nf


• 15 alpha males each controlling a harem of 20 females:



Ne = 4NmNf

Nm + Nf



census size (N) = 315



Ne = 4NmNf

Nm + Nf



census size (N) = 315effective size (Ne) = 57.1


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.


4. Genetic Bottlenecks • genetic bottlenecks refer to severe reductions in effective population size.

Time

Ne

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


Consequences:

Founder effects


Consequences:

1. New population has a fraction of genetic variation present in the ancestral population.

Founder effects


Consequences:


2. Initial allele frequencies differ because of chance.

Founder effects


Consequences:


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



• random drift and gene flow act in opposition to each other!




random drift allow genetic divergence of two populations.





gene flow prevents divergence.





gene flow prevents divergence.

if Nem > 1, gene flow overrides drift and prevents

divergence





gene flow prevent divergence.

if Nem > 1, gene flow overrides drift and prevents

divergence

if Nem < 1, random drift can lead to genetic divergence.


2. Gene flow and selection


2. Gene flow and selection • gene flow and selection usually act in opposition.


2. Gene flow and selection • gene flow and selection usually act in opposition. selection favor different alleles in different populations (“local adaptation”).


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.


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


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


3. Drift 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




random drift random changes in allele frequency if Nes > 10, selection controls the fate of the allele




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

BIOE 109 Summer 2009 Lecture 6- Part I Microevolution – Random genetic drift

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