Download - Neutral Evolution and Molecular Clocks
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Neutral Evolution &
Molecular Clock
Presented By Miss Aditi PatilM.Sc. Part I24/01/2009
Under the Guidance ofDr. Swati Bhavsar
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Neutral Theory Of Molecular Evolution
•Motoo Kimura (1960-1970)•Does not deny natural selection•Genetic polymorphism•Neutral alleles with respect to
fitness•Random fixation of selectively
neutral mutants•Mutation and genetic drift
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Relative importance of drift and selection
• Molecular
evolution
• Evolution at
phenotypic
levelFigure 1 : Balance between natural selection and random drift
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Mathematical Expression Of The Neutral Theory
• If population size is N and µ is neutral mutation rate per gamete per generation at a locus.
No. of new mutations = µ X 2N
(E.g.: mutation in protein ‘x’ in experimental rat)
- given µ for the rat is 2.5 X 10 -9
- consider population size N = 50 No. of new mutations = 1.25X 10 -7
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• According to drift theory,
P = 1/(2N)
where P = probability that a mutation will be fixed by drift
Thus, P for protein ‘x’ , P = 1/(2X50) = 0.01
• No. of mutations fixed per generation is,
= µ X 2N X 1/(2N) = µ
Where µ = Neutral mutation rate per gamete per generation at a locus
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• No. of mutations in protein ‘x’ fixed per generation is, = 2.5 X 10 -9 X100 X 1/100 =2.5X10 -9 mutation per gamete per generation.
• Therefore, the rate of neutral evolution is theoretically constant and is equal to the rate of mutation.
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Applications Of Neutral Theory
•The faster rates
•The degrees of polymorphism
•Constancy of molecular
evolution
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•Zuckerkandl and Pauling (1965)
•The rate of molecular evolution
•Molecular clock
•Species divergences and constructing phylogenies
The Molecular Clock Hypothesis
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Molecular Clock And Neutral Theory
•The molecular clock hypothesis
•The neutral theory prediction•Rate of evolution per
generation•Rate of mutation•Rate variation among
lineages•Constancy of the molecular
clock
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Figure 2 : Proteins evolving at different rates
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Causes of Rate Variation Among Lineages
• The Generation-time Effect• Nucleotide substitution rates • Substitution fixation rates• species with shorter generation
times • Higher rates of evolution• Effect of near-neutrality• offset the generation time effect
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Limitations Of Molecular Clock• Changing generation times• Population size • Species-specific differences • Changes in the intensity of
natural selection
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Applications Of Molecular Clock
• Molecular systematics• Correct scientific classification
of organisms• Dates of phylogenetic events• Divergence of living taxa• Formation of the phylogenetic
tree• Divergences times
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References• M. Kimura, The neutral theory of molecular evolution: a
review of recent evidence, Japanese Journal of Genetics (1991), 66: 367-86.
• http://understandingevolution.com/evosite/evo101/IIIE5bNeutraltheory.shtml
• http://plato.stanford.edu/entries/evolutionary-genetics/figure2.gif
• www.pbs.org/wgbh/evolution/library/05/1/pdf/l_051_06.pdf
• http://en.wikipedia.org/wiki/Neutral_theory_of_molecular_evolution
• http://en.wikipedia.org/wiki/Molecular_clock
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Thank you