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Transcript
Mutationism,
Neutralism,
Selectionism
1
Deleterious mutations
Neutral mutations
Advantageous mutations
Overdominant mutations
2
New mutations may be:
1. Advantageous
2. Overdominant
3. Deleterious
4. Neutral
3
Probability of fixation:
1. Advantageous - low
2. Overdominant – close to zero
(genetic load)
3. Deleterious - very very low
4. Neutral - very low
4
Conditional time to fixation:
1. Advantageous - fast
2. Overdominant - extremely
slow
3. Deleterious - fast
4. Neutral - slow
5
Deleterious mutations
Neutral mutations
Advantageous mutations
Overdominant mutations
6
Amount of variability created:
1. Advantageous - none
2. Overdominant - a lot
3. Deleterious - none
4. Neutral - some
7
THE DRIVING FORCES IN
EVOLUTION
8
3 types of evolutionary
explanations:
• Mutationism = evolutionary
phenomena are explained by the
joint effects of mutational input
and random genetic drift.
9
3 types of evolutionary
explanations:
• Neutralism = evolutionary
phenomena are explained by the
joints effects of mutation, random
genetic drift, and purifying
selection.
10
3 types of evolutionary
explanations:
• Selectionism = evolutionary
phenomena are explained by the
joints effects of advantageous
selection and balancing selection.
11
Selectionism
• Gene substitutions occur as a consequence
of selection for advantageous mutations.
Polymorphism is maintained by balancing
selection.
• Gene substitution and polymorphism are
two separate phenomena driven by different
evolutionary forces.
12
Selectionism
Gene substitution is the end result of an adaptive
process whereby a new allele takes over future
generations of the population if and only if it
improves the fitness of the organism.
Polymorphism is maintained when the coexistence of
two or more alleles at a locus is advantageous for the
organism or the population.
13
•Two separate evolutionary
forces!
•Genetic polymorphism is
permanent - the same alleles are
maintained at constant
frequencies for long periods of
evolutionary time.
14
Adaptation
An adaptation is a heritable character state that
enhances the fitness of the organisms that bear it
relative to alternative character states, especially
ancestral character states in the population in
which the adaptation evolved.
15
Honey creepers
16
Adaptation
Translation into Molecular Lingo:
An adaptation is a mutation that enhances the
fitness of the organisms that carry it relative to the
other alleles in the population.
17
Extreme selectionism leads to
The “Panglossian paradigm”
“It is proved… that things cannot be other than they are, for since
everything was made for a purpose, it follows that everything is
made for the best purpose.”
Dr. Pangloss in
Candide by Voltaire
18
The Panglossian paradigm:
“Observe, for instance, the nose is formed for
spectacles, therefore we wear spectacles.”
19
The Panglossian paradigm:
“Legs were clearly intended for breeches ,
and we wear them.”
Dr. Pangloss in
Candide by Voltaire
20
Abbott Handerson Thayer
1849-1921
21
22
23
24
25
1911
1901–1909: President of the United States.
1911: A 116-page article on animal coloration.
26
Kimura M. 1968. Evolutionary rate at the molecular level". Nature 217: 624–626.
King JL, Jukes TH. 1969. Non-Darwinian Evolution. Science 164: 788–798.
The Neutral
Theory of
Molecular
Evolution
Motoo Kimura
27
The majority of evolutionary
changes and much of the variability
within species are caused by
random genetic drift of mutant
alleles that are selectively neutral or
nearly so.
28
Neutrality, in the sense of the
neutral theory, does not
imply strict equality in fitness
for all alleles. It only means
that the fate of alleles is
determined largely by
random genetic drift.
29
Selection may operate, but its
intensity is too weak to offset
the influences of chance effects.
s< 1/(2Ne)
Ne = effective population size.
30
That is, either the selection
coefficient is small, or the effective
population size is small.
s< 1/(2Ne)
Ne = effective population size.
31
According to the neutral theory, the
frequency of alleles is determined
by purely stochastic rules, and the
picture that we obtain at any given
time is merely a transient state
representing a temporary frame
from an ongoing dynamic process.
32
The neutral theory regards substitution
and polymorphism as two facets of the
same phenomenon.
Gene substitution is a long and gradual
process whereby the frequencies of
mutant alleles increase or decrease
randomly, until the alleles are
ultimately fixed or lost by chance.
33
Polymorphic loci consist of
alleles that are either on their
way to fixation or are about to
become extinct.
Thus, at any given time, some loci
will possess alleles at frequencies
that are neither 0% nor 100%.
These are the polymorphic loci.
34
All molecular manifestations that
are relevant to the evolutionary
process should be regarded as the
result of a continuous process of
mutational input and a
concomitant random extinction
or fixation of alleles.
35
•A single evolutionary force.
•Genetic polymorphism is
transient - allele frequencies
fluctuate with time and the
polymorphic alleles themselves
are continuously replaced.
36
A population that is free from selection can
accumulate many polymorphic neutral
alleles. Then, if a change in ecological
circumstances occurs, some of the neutral
alleles will no longer be neutral but
deleterious, against which purifying
selection may operate. After these alleles are
removed, the population will become more
adapted to its new circumstances than
before.
Kimura (1983)
37
38
39
The neutral theory does not
preclude adaptation.
Adaptive evolution may occur
without adaptive selection.
40
The Slightly Deleterious Model of
Molecular Evolution +
The Slightly Advantageous Model of
Molecular Evolution =
The Nearly Neutral Model of Evolution
41
Mean fitness
selectionism
neutralism
nearly neutral model
time
42
Leigh Van Valen’s “Red Queen
Hypothesis”
(based “Through the LookingGlass, and What Alice Found
There” by Lewis Carroll (1871).
Alice complains to the Red
Queen that she is exhausted from
running, only to find she is still
under the tree where she started.
The Red Queen replies:
“Now, here, you see, it takes all
the running you can do to keep
in the same place. If you want
to get somewhere else, you
must run at least twice as fast
43
as that.”
Neutralist-selectionist perceptions
44
Neutralist-selectionist perceptions
A total resolution of the dispute
between neutralists and
selectionists would require
knowledge of the distribution of
fitness effects of mutant alleles.
45
46
The distribution of fitness effects of random mutations in vesicular stomatitis virus.
Random mutations were introduced into the virus, and the fitnesses of the mutants
were compared against the unmutated wild type. A fitness of less than one indicates
that the mutant was less fit than the wild type, so the mutation was deleterious. A
fitness of zero indicates that no mutated progeny were recovered, and that the
mutation was therefore lethal.
From: Eyre-Walker A, Keightley PD. 2007. The distribution of fitness effects of
47 new
mutations. Nature Rev. Genet. 8:610-618.
Yeast
48
Fact 1:
The cheetah is about to
become extinct.
49
Fact 2:
Cheetah
populations are
devoid of genetic
variation.
50
Two explanations:
1. Selectionist explanation:
The cheetahs are depauperate of
genetic variation and, therefore,
they are on the verge of
extinction.
51
Two explanations:
2. Neutralist explanation:
The cheetahs are on the verge of
extinction and, therefore, they
are depauperate of genetic
variation.
52
53
54
55
If genetic variation reflects
adaptive needs, then
advantageous selection will
quickly restore genetic
variability to its prebottleneck levels.
56
If genetic variation is
merely random sampling,
then post-bottleneck
genetic variability will
remain low for very long
periods.
57
Northern elephant seal, Mirounga angustirostris
1922, Baja California, ~20 animals
2000, Baja California, ~120,000 animals
Genetic variation = ~ 0
58
Helix aspersa (brown garden snail)
H = 0.121
Europe
California
few individuals
59
Helix aspersa (brown garden snail)
H = 0.121
Europe
California
H = 0.000
Large populations.
60
Important pests of citrus trees.
Neutrality tests
(no need to know history)
61
62
63
64
65
66
67
polymorphic
fixed
synonymous
X
Y
nonsynonymous
Z
W
68
69
Numbers of synonymous and nonsynonymous fixed differences and
polymorphisms at the glucose-6-phosphate dehydrogenase locus between
Drosophila melanogaster and D. simulans
The comparisons are based on 32 sequences from D. melanogaster and 12
sequences from D. simulans, with an aligned length of 1,705 bp.
Type of change
Fixed Polymorphic
Synonymous
26
36
Nonsynonymous
21
2
70
polymorphic
fixed
synonymous
X
Y
nonsynonymous
Z
W
71