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7/25/11
Neutralism
Molecular Evolution
Neutralist-Selectionist Controversy
Motoo Kimura
Kimura, M. (1968). "Evolutionary rate at the
molecular level". Nature 217: 624–626.
Neutral Theory of Molecular Evolution
Motivated by problem of “mutational load”:
reduction in population mean fitness (relative to
maximal fitness) due to mutation accumulation if
all mutations are visible to selection
Dr. Erica Bree Rosenblum
Asserted that calculated substitution rates would
produce an unsustainably high mutational load
unless the most substitutions are selectively neutral.
Neutralism
Neutralism
Motoo Kimura
Motoo Kimura
Kimura, M. (1968). "Evolutionary rate at the
molecular level". Nature 217: 624–626.
Kimura, M. (1968). "Evolutionary rate at the
molecular level". Nature 217: 624–626.
Neutral Theory of Molecular Evolution
Tomoko Ohta
1924-1994
Neutral Theory of Molecular Evolution
1924-1994
Ohta, T (1973). "Slightly deleterious mutant
substitutions in evolution". Nature 246: 96–98
1924-1994
King, J.L. and Jukes, T.H (1969).
"Non-Darwinian Evolution". Science 164: 788–798
Seen as oppositional and threatening.
Nearly Neutral Theory of Molecular Evolution
Kimura and Ohta 1974: Principles
Neutralist-selectionist debate
1.  Rate of amino acid substitution at a protein is constant as
long as mutations don’t affect protein function
Neutralism: most fixation events due to drift of
(nearly) neutral mutations
2.  Functionally less important parts of a molecule evolve faster
3.  Substitutions that don’t disrupt the function of a molecule
occur more frequently
4.  Gene duplication must precede the emergence of a gene
having a new function.
5.  Selective elimination of deleterious mutants and random
fixation of selectively (nearly) neutral mutants occur far more
frequently than positive selection on advantageous mutants
Neo-darwinism: merging of Darwinian and
Mendelian theory - gave central role to selection
in evolution
Pan-selectionism: extreme reliance on selection
as evolutionary explanation:
Fixation events due to positive selection
Polymorphisms due to balancing selection
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7/25/11
The neutralist position
The selectionist position
Neutralism: most fixation events due to drift of (nearly)
neutral mutations
Pan-selectionism: extreme reliance on selection as
evolutionary explanation:
Fixation events due to positive selection
Polymorphisms due to balancing selection
Under “neutralism” selection can still act, but
|s| must be less than 1/(2Ne)
Substitution and polymorphism are trails of the same
process (drift)
Substitution and polymorphism are evidence of different
types of selection (positive and balancing, respectively)
Polymorphism is generally stable and adaptive.
Polymorphisms are unstable and transient
Adaptation can even occur without positive selection
(purging of deleterious alleles)
Neutralist-selectionist debate
Dispute centers on role of drift vs selection
Also on the fitness of new mutations
Both agree that many new mutations are deleterious
But are the rest neutral or not?
“…roars of "The Neutral Theory Is Dead" and "Long Live the
Neutral Theory" will continue to reverberate, sometimes in the title
of a single article…”
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