Tracing Phylogeny: Macroevolution, the Fossil Record, and Systematics

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Tracing Phylogeny Macroevolution, the Fossil
Record, and Systematics
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The Fossil Record and the Geological Time Scale

• Fossils are reliable data only if they can be
  placed in time
• relative dating
• absolute dating

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Relative Dating

• Younger sediments are found on top of older ones
• strata at one location can be related to strata
  at another location by index fossils
• tells us the order in which groups of species
  present in a sequence of strata evolved
• establishes the geological time scale

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Pangea and Plate Tectonics

• Supercontinent
• Continental Drift

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Absolute Dating

• Radioactive dating
• L and D amino acid content

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L and D Amino Acid Isomers

• Isomers - same formula, different structure
• structural
• conformational
• geometric
• stereoisomerism

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Stereoisomerism

• 1815, Jean-Baptistse Biot (physicist) discovered
  light will interact with molecules in specific
  ways (Wingrove and Caret 1981)
• Pasteur noticed salt residue in wine kegs could
  be divided into right-handed and left-handed
  crystals (Wingrove and Caret 1981)
• The crystals were mirror images of each other.

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Mirror Images

• To understand mirror images, place left hand down
  on table. Imagine a mirror next to it. The
  mirror image of the left hand is the right hand.
  
• Try to superimpose left hand over right and see
  if the thumbs line up.
• Your hands are non-superimposable

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Dextrorotatory vs Levorotatory

• When Pasteur dissolved left-handed crystals in
  water, plane polarized light is bent to the left
• Right handed crystals bend plane polarized light
  to the right

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Horizontal vertical waves
Only vertical component comes through
polarizer
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Bends light to the right
Right-handed crystals in solution
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• By using a device to measure the angle of the
  rotation, the magnitude of rotation can be
  determined.
• Light bent to the right is called dextrorotatory
  (d)
• light bent to the left is called levorotatory (l)
• When you mix equal quantities of d and l
  crystals, there is not net rotation (they cancel
  each other out).

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Enantiomers

• Molecules that exhibit optical activity and are
  nonsuperimposable mirror images are said to be
  enantiomers
• Enantiomers are molecules with same formulas that
  the structures are (1) mirror images and (2) must
  not be superimposable on the other.

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MIRROR
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Racemic Mixtures

• Equal amounts of d and l means no optical
  activity
• This is a racemic mixture
• However, nature doesnt always produce equal
  amounts
• Life produces only l-amino acids
• When an organism dies, l forms are converted
  into a mixture of l and d
• Ratio of l and d can be used to date by
  racemation

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D-leucine
L-leucine
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Radioactive Dating and Half-Life

• Radioactivity discovered by Henri Becquerel
• some elements i.e. uranium and thorium are
  unstable
• they decay to form other elements or isotopes of
  the same element
• alpha particles
• beta particles
• gamma rays

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Alpha Particle

• Nucleus of a helium atom (or two protons and two
  neutrons) emitted in nuclear disintegration

Tracks made over period of 48 hours by
alpha particles emitted from a radioactive
particle of plutonium in lung tissue of ape.
(http//ccnr.org/alpha_in lung.html)
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Beta Particle

• Formed when a neutron splits into a proton and an
  electron in the nucleus

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Gamma Rays

• High energy photon (light) emission in
  radioactive disintegration

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Uranium Decay
238

• U has 92 protons and 146 neutrons
• unstable at 238 and so emits an alpha particle
• dropping two protons and two neutrons from U
  nucleus now makes it Thorium (daughter element)
  with a mass of 234
• 238U -----gt234Th 4He 92
  90 2
• Thorium is unstable and emits a beta particle

92
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Beta Decay

• A beta particle is formed when a neutron splits
  into a proton and an electron in the nucleus
• There is now an extra proton in the nucleus to
  form the element Protactinium 234Pa 91
• As alpha, beta and gamma rays are emitted to the
  surrounding materials, they heat up

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• Rate nuclear adjustments occur to form lead is
  independent of changes in
• temperature
• pressure
• chemical environment
• rate of decay of long-lived isotopes has not
  varied since earth came into existence
• each radioactive element has a particular mode of
  decay and unique rate of decay
• radiometric dating makes use of the rate of decay
  and mode of decay

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• Time zero begins when radioactive parent atoms
  become part of mineral from which daughter
  elements cannot escape
• retention of daughter elements essential - must
  be counted to determine original quantity of
  parent nucleotide
• ratio of parent to daughter nucleotide is by mass
  spectrometer (Geiger used for Carbon 14)

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• Igneous rocks date best
• sedimentary rocks rarely can be dated (only
  certain crystalline forms in rocks)
• metamorphic rocks require special care also

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Problems with Radiometric Dating

• Weathering of rock or leaching of minerals cause
  underestimation of age
• if material is gas, may diffuse out of rock
• older rocks may partially re-melt so age taken
  would be of second rock formation

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Half-Life

• Number of years needed for half of the original
  quantity of atoms to decay to its stable form
• every radionucleotide has its own unique
  half-life
• 235U 704 million years

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Some Half-Lives
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Potassium-Argon Method

• Occurs by electron capture (causes proton to be
  transformed into neutron)
• 11 of potassium changes to argon
• rest of potassium decays to 40Ca by beta emission
  (not useful)
• advantage is argon is inert
• another advantage is abundance of K in minerals

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Rubidium-Strontium Method

• Beta decay of 87Rubidium to 87Strontium
• Rubidium and potassium often found in same
  minerals, therefore, can use to check K dating
• Measure
• 87Rubidium
• 87Strontium
• 86Strontium
• Ratios of each against each other via mass
  spectrometer

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14Carbon Method

• Organic substances older than 50,000 years
  contain little 14C
• 14C is continuously made (cosmic radiation)
• when living organism quits assimilating carbon,
  addition of 14C stops
• decays to 14N by beta emission
• age determined by ratio of 14C to all other C in
  organism

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Thorium Decay

• Uranium brought to ocean by streams (in solution)
• decays to from 230Th
• 230Th precipitated as sediment on the ocean floor
• half-life 75,000 years

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How Do New Designs for Living Evolve?

• Predaptation
• most biological structures have an evolutionary
  plasticity that makes alternative function
  possible
• a structure evolves in one context and becomes
  used for another (birds wings)

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Designs for Living

• Developmental rates
• allometric growth
• differences in the relative rates of growth of
  various parts of the body
• gene mutation?

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Designs for Living

• Developmental rates
• timing of development
• paedomorphism
• when adults retain features that are juvenile
• heterochrony
• evolutionary changes in the timing or rate of
  development
• homeotic changes
• alter placement of different body parts

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Taxonomy

• Kingdom
• Phylum
• Class
• Order
• Family
• Genus
• Species

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Binomial Nomenclature

• Taxon
• monophyletic
• single ancestor gives rise to all species
• polyphyletic
• species derived from 2 or more ancestral forms
  not common to all members
• paraphyletic
• excludes species that share a common ancestor
  that gives rise to species included in the taxon

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Molecular Systematics

• Protein comparison
• DNA comparison
• Molecular clocks, i.e. cytochrome c evolution is
  quite constant with time.

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Plains zebra (Equus burchelli) - wider and fewer
- devlop 3 weeks after fertilization in embryo.
Grevys zebra (Equus grevyi) more numerous,
narrower - develop 5 weeks after fertilization
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Axolotl
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