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Molecular Clocks

Molecular Clocks. Polymers (molecules made of small repeating units) can be used as molecular clocks if their building blocks are replaced at a known and constant rate.

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Molecular Clocks

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  1. Molecular Clocks

  2. Polymers (molecules made of small repeating units) can be used as molecular clocks if their building blocks are replaced at a known and constant rate. • Nuclear DNA: Humans and chimpanzees differ in 5 % of their bases. Substitutions occur at a rate of 1% per 1 million years. Therefore, 5 million years have presumably passed since the two species diverged. • Mitochondrial DNA (mtDNA) can also be used.

  3. Parsimony Analysis - Computers put together evolutionary trees based on statistical evidence, fossil evidence, molecular clocks, DNA comparisons, and protein comparisons using the fewest possible number of mutational events. Because mutations are rare events, the tree that requires the fewest mutations is more likely to reflect reality. • Different genes can change at different rates, so the more genes studied, the more accurate the information.

  4. Neanderthals • Analyzing ancient DNA and mtDNA indicate that Neanderthals diverged from us more than half a million years ago – much farther back than the fossil evidence. • Neanderthal DNA and Human DNA are so different that it is unlikely that they interbred. • Extrapolation from mutation rates indicate that the last shared ancestor between modern humans and Neanderthals lived from 690,000 to 550,000 years ago.

  5. Mitochondrial DNA and Y chromosomes • MtDNA and DNA sequences on the Y chromosome can be used to reveal which parent passes on a particular gene. • MtDNA traces maternal lineages, because it passes from mothers to offspring. • MtDNA mutates faster than nuclear DNA because it lacks repair systems, so it is useful as a clock. • MtDNA is very abundant and easy to extract from cells (1 nucleus; 1,000’s of mito.)

  6. Cont’d • Drawback: Rare paternal mitochondria enters cell and recombines with maternal DNA. • Y chromosome is used to trace paternal lineage. • Most parts of the Y chromosome are virtually identical in the male population, however there are particular segments that change and can therefore be used for molecular clock studies. • Y chromosome and mtDNA studies can be combined for a better picture of the past.

  7. Mitochondrial “Eve-or Eves” • Theoretically, if a particular sequence of mtDNA could have given rise to different mtDNA sequences in modern humans, then that ancestral sequence may represent an early humanlike female or first woman. • The more diverse the collection of mtDNA mutations the longer those humans line has existed.

  8. Replacement Hypothesis • H. sapiens evolved from H. erectus about 200,000 ya in Africa. Descendents spread from Africa.

  9. Multiregional Hypothesis • Human traits originated in several places, and H. erectus migrated, mixing and sharing genes, gradually evolving into H. sapiens.

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