BL 464. Evolution

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BL 464. Evolution

• Lecture 3 (1-23-08)

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Transitional forms Whales
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Transitional forms Birds
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Vestigial organs
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A transitional form
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Law of succession
Extant
Extinct
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Molecular Homologies

• Genetic similarities among organisms because of
  shared ancestry.

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Shared Genetic code
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A shared potential for negative selection
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Homologous pseudogenes
Genetic markers Immune from delterious mutations
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Relationship between phylogeny and age
of pseudogenes
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• Timeframe for evolution

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Relative dating stratigraphy
Absolute dating radiometric
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• Absolute dating fossils
• Radiometric dating
• Based on rates of radioactive decay of an element
  or isotope into a different element or isotope.
• Rates constant and independent of environmental
  factors
• The rates of decay are known
• Measuring amount of decay that has taken place
  from a parent element (or isotope) into a
  daughter element (or isotope) a geologic clock

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E.g., Potassium/Argon clock

• Decay of 40K produces 40Ar
• Best rocks for K/Ar dating have been formed at
  high temperatures
• e.g., those derived from volcanic activity
• Heat drives off previously accumulated Argon gas
• Sets the clock to zero
• As rock cools and solidifies, 40K continues to
  decay to 40Ar which is trapped inside the rock

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• To date the rock, it is reheated and the amount
  of 40Ar is measured.
• The ratio of 40Ar to the amount of 40K present
  reveals he time lapse since the rock was formed.
• 40K has a half-life of 1.3 billion years.
• In 1.3 billion years, 1/2 of the original 40K
  will have been converted to 40Ar
• In 2.6 billion years, 1/4 of the original 40K
  will remain.

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Radiometric Dating
1. Determine ratio of parent Isotope to daughter
isotope. 2. Convert ratio to number of
half- lives elapsed. 3. Number of half-lives
that have elapsed X the half-life
estimate of age of the rock.
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Relative dating of fossils