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Molecular clock, Neutral hypothesis

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Molecular clock, Neutral hypothesis

  1. 1. MOLECULAR CLOCK HYPOTHESIS! 1 “A technique in molecular evolution to relate the time that two species diverged to the number of molecular differences measured between the species DNA/ protein sequences”
  2. 2. Neutral theory by Kimura “Large fraction of new mutations DO NOT have an effect on evolutionary fitness, so natural selection neither favours nor disfavours them” 2 Mutations Fixed Lost Substitution Rate Genetic Drift Substitution Rate= Mutation Rate
  3. 3. Observations 3 Synonymous base substitution>Non-Synonymous base substitution Mutation rate is higher in Non-coding sequences Pseudogenes evolve at higher rate than functional genes
  4. 4. Calibrating molecular clock Absolute age of some divergence event - Fossil record - Geological event 4 5% difference 1% per million years over a period of 5 million years Five fold higher rate over a period of 1 million years
  5. 5. 5 r = D 2 t r= rate of nucleotide substitutions (per million years) D= the proportion of base pairs that differ between the two sequences t= time of most recent common ancestor in million years
  6. 6. 6 Using molecular clock, it was estimated that two species A and B must have diverged from their common ancestor about 9 x 106 years ago. If the rate of divergence per base pair is estimated to be 0.0015 per million years, what is the proportion of base pairs that differ between the two species now? a. 0.0270 b. 0.0135 c. 0.00017 d. 0.0035 Ans: D= r x 2T= 0.0015 x 2 x 9 =0.0270

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