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9/13/2008
Mechanisms of NonNon-adaptive Evolution
• Mutation
Alteration of DNA during or after synthesis
• Genetic Drift
Loss of alleles due to random processes
• Gene Flow
Movement of alleles between populations in
different selective environments
Mutation
• Mutation can introduce deleterious
alleles in a population.
• But since mutation of specific genes
occurs very infrequently, even weak
selection can overcome the effect of
mutation on evolution.
Genetic Drift
… a loss of alleles due to random processes in a
population of finite size.
Example:: Dealing Cards
Example
C
Deal out a set of cards from a full deck and some suites or
numbers will not be represented in the hand. The elimination
of cards occurred by random sampling, not by any process
of selection.
An analogous process involving the chance loss of alleles
occurs during reproduction.
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Causes of Genetic Drift
• independent assortment of alleles
• random variation in survival
• random variation in mating success
• random variation in fecundity
Fisher’s Model
Assume::
Assume
Each set of parents give birth to two offspring.
A new allele (A
(A2) arises, which has no effect on fitness
((fitness of A1A2 = fitness of A1A1)).
Then::
Then
Within each couple, there is a 25% chance the new
allele will be lost during reproduction.
[Source: Fisher 1930, The Genetical Theory of Natural Selection,
Selection, Clarendon Press]
Father’s alleles
Mother’s alleles
A1
A2
A1
A1A1
A1A2
A1
A1A1
A1A2
For two offspring, there is a 25% chance of losing A2 (= 0.5(0.5).
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• In each case,
case one allele
outperformed the other, but the
path of evolution differed
among populations.
• Genetic drift had a greater
effect on evolution in smaller
populations.
Allele Frequency
• Replicated populations of fruit
flies and observed changes in
alleles for body color.
Small population (N=18)
Generation
Large population (N=100)
Allele Frequency
Experimental Evidence
for Genetic Drift
Generation
[Source: Buri 1956, Evolution 10, 367367-402]
Experimental Evidence
for Genetic Drift
• Distribution of alleles in
Buri’s flies started out
normal but ended up
bimodal.
• The final distribution of
alleles closely matches
that predicted by models
of genetic drift.
[Source: Hartl & Clark 1989, Principles of
Population Genetics,
Genetics, Sinauer]
As it is difficult to imagine that eyes, though
useless, could be in any way injurious to
animals living in darkness, I attribute their loss
wholly
h ll tto di
disuse.
Charles Darwin, 1859
The Origin of Species
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Blind Cavefish of Mexico
(Astyanax mexicanus)
mexicanus)
Loss of Eyes
• Eyes have been lost independently in
flatworms, arthropods, fish, amphibians,
reptiles, and mammals.
• Eyes begin to develop but then regress via
apoptosis (cell death).
• The loss of eyes results from mutations in
regulatory genes, which normally stimulate
the expression of other genes required for eye
development.
Regression of Eye Structures in Cavefish
Surface fish
Cave fish
(La Cueva Chica)
Cave fish
(Cueva de la Curva)
[Source: Jeffrey and Martasian 1998, American Zoologist 38, 685685-696]
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Curing the blind
[Source: Borowsky 2008, Current Biology 18: R23-R24]
Necessary Conditions for Genetic Drift
• Variation among individuals
• Heritability of variation
• Small population size
Balance between Drift and Selection
When selection is weak or the population
is very small, genetic drift can oppose
natural selection.
In other words, an allele that confers a
fitness advantage can be lost when a
population is small.
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Experimental Studies of Genetic Drift
Frequency
y of A1
Locus controlling color (2 alleles) in flour beetles (Tribolium
(Tribolium spp.)
N = 10
(per population)
N = 100
Generation
Rich et al. (1979), Evolution 33: 579-584
Gene Flow
If selective environment varies spatially, deleterious
alleles can enter population from other populations.
This process leads to a migration load (the
maintenance of deleterious alleles by migration).
Migration load depends on the following:
• Strength of selection on alleles
• Rate of migration
• Size of populations
Balance between Migration and Selection:
Metal Tolerance in Sweet Vernal Grass
Mine
Study sites
Distances between
sites (feet)
1
Contaminated
pasture
2
60
3 45
65
20 10 35
Pasture
6
7
50
8
60
[Source: Antovonics et al. 1971, Advances in Ecological Research 7, 1
1--85]
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Evidence for Migration Load
• Selection against transplanted individuals
within a region of dispersal
• Degree of local adaptation increases with
degree of separation between populations
Summary
•
Mutation, genetic drift, and migration are
additional mechanisms by which populations
evolve.
•
Under certain conditions, genetic drift and
migration can oppose selection.
•
Mutation is the only process that increases
genetic variation. All others either decrease or
redistribute genetic variation.
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