Mutationism, Neutralism, Selectionism

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Mutationism, Neutralism, Selectionism
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Deleterious mutations
Neutral mutations
Overdominant mutations
Advantageous mutations
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New mutations may be 1. Advantageous 2.
Overdominant 3. Deleterious 4. Neutral
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Probability of fixation 1. Advantageous -
low 2. Overdominant close to zero (genetic
load) 3. Deleterious - very very low 4. Neutral
- very low
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Conditional time to fixation 1. Advantageous -
fast 2. Overdominant - extremely slow 3.
Deleterious - fast 4. Neutral - slow
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Amount of variability created 1. Advantageous -
none 2. Overdominant - a lot 3. Deleterious -
none 4. Neutral - some
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THE DRIVING FORCES IN EVOLUTION
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3 types of evolutionary explanations

• Mutationism evolutionary phenomena are
  explained by the joint effects of mutational
  input and random genetic drift.

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3 types of evolutionary explanations

• Neutralism evolutionary phenomena are explained
  by the joints effects of mutation, random genetic
  drift, and purifying selection.

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3 types of evolutionary explanations

• Selectionism evolutionary phenomena are
  explained by the joints effects of advantageous
  selection and balancing selection.

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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.

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The Synthetic Theory of Evolution or
Selectionism or Experimental Neo Darwinism
From left to right Julian Huxley, Ernst Mayr,
Theodosius Dobzhansky, George Gaylord Simpson,
and George Ledyard Stebbins
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The Synthetic Theory of Evolution (Selectionism)
considers advantageous selection and balancing
selection as the main driving forces of the
evolutionary process.
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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.
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• Two separate evolutionary forces!
• Genetic polymorphism is permanent - the same
  alleles are maintained at constant frequencies
  for long periods of evolutionary time.

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Adaptation
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Honey creepers
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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
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The Panglossian paradigm Observe, for
instance, the nose is formed for spectacles,
therefore we wear spectacles.
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The Panglossian paradigm Legs were clearly
intended for breeches , and we wear
them. Dr. Pangloss in Candide by Voltaire
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Abbott Handerson Thayer 1849-1921
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The Neutral Theory of Molecular Evolution
Motoo Kimura
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The Neutral Theory of Evolution (Neutralism)
considers mutation, random genetic drift, and
purifying selection to be the main driving forces
of the evolutionary process.
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The neutral theory of molecular evolution
contends that at the molecular level 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.
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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.
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Selection may operate, but its intensity is too
weak to offset the influences of chance effects.
?s?lt 1/(2Ne) Ne effective population size.
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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.
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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.
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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.
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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.
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• A single evolutionary force.
• Genetic polymorphism is transient - allele
  frequencies fluctuate with time and the
  polymorphic alleles themselves are continuously
  replaced.

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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)
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The neutral theory does not preclude adaptation.
Adaptive evolution may occur without adaptive
selection.
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The Slightly Deleterious Model of Molecular
Evolution
Tomoko Ohta
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selectionism
fitness
neutralism
slightly deleterious model
time
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The neutralist-selectionistdebate
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Fact 1 The cheetah is about to become extinct.
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Fact 2 Cheetah populations are devoid of
genetic variation.
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Two explanations
1. Selectionist explanation The cheetahs are
depauperate of genetic variation and, therefore,
they are on the verge of extinction.
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Two explanations
2. Neutralist explanation The cheetahs are on
the verge of extinction and, therefore, they are
depauperate of genetic variation.
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Indirect Tests of Neutrality
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Drift may have very large effects on the genetic
makeup if the population size decrease is steep
and of long duration.
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If genetic composition reflects adaptive needs,
then advantageous selection will quickly restore
genetic variability to its pre-bottleneck levels.
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If genetic composition reflects random sampling,
then post-bottleneck genetic variability will
remain low for very long periods.
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1922, Baja California, 20 animals 2000, Baja
California, 120,000 animals Genetic variation
0
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Helix aspersa (brown garden snail)
H 0.121
Europe
1850
California
few individuals
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Helix aspersa (brown garden snail)
H 0.121
Europe
1990
California
Large populations. Important pests of citrus
trees.
H 0.000
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Deleterious mutations
Neutral mutations
Overdominant mutations
Advantageous mutations
Theoretical Expectations
Empirical Tests?
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Neutrality tests
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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.
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Neutralism versus Selectionism
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A total resolution of the dispute between
neutralists and selectionists would require
knowledge of the distribution of fitness values
of mutant alleles.
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Both theories agree that most new mutations are
deleterious, and that these mutations are quickly
removed from the population so that they
contribute neither to the rate of substitution
nor to the amount of polymorphism within
populations.
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The difference concerns the relative proportion
of neutral mutations among nondeleterious
mutations. While selectionists claim that very
few nondeleterious mutations are selectively
neutral, neutralists maintain that the vast
majority of nondeleterious mutations are
effectively neutral.
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