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    Patterns 2a: Evidence in the Earth

    We see repeating patterns in the earth.
    • fossils that change with depth
    • fossils that show a progression of change
    • evidence that the continents move over geologic time

    By seeking empirical, observable evidence of patterns,
    scientists can proceed to seek the cause of these patterns.

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    The Fossil Record

    The fossil record is not the most powerful line of evidence for evolution,
    but it does provide a record of earth's past in which patterns are evident.
    • Species appear.
    • New species appear.
    • Species go extinct.
    • Later species resemble earlier species.

    Paleontology and Geology were the first scientific disciplines to reveal that earth was more than a few thousand years old.

    Mountain ranges filled with marine fossils tell us that extant species are very different those of the past.

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The oldest layers of sedimentary rock comprising the mountains around Convict Lake in the Sierra Nevada
(more than 200 miles inland from coastal San Francisco, CA) contain 400-million year old marine fossils.

    Fossils are Rare

    Most organisms are consumed and destroyed relatively soon after they die.
    But sometimes, properties of the organism and environment promote fossilization.

    If you're going to become a fossil, it helps to have...

    • hard body parts
    • a rapid burial
    • protection from scavengers/bacteria
    • protection from oxygen

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    Types of Fossils: Unaltered Body Parts

    Fossil bodies or body parts can be unaltered (composed of actual matter from the organism):
    • encased in preserving material, such as amber
    • mummified (arid conditions)
    • frozen

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    Types of Fossils: Altered Body Parts

    Organisms can be altered after death.

    Permineralization is the process by which water surrounding the body seeps into tissues,
    gradually replacing organic matter with aqueous mineral salts.

    • A mold is the impression left by the outside of an organism.
    • A cast is the entire body, replaced by minerals.

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    Types of Fossils: Trace Fossils

    • burrows
    • tracks
    • castings (worm excrement deposited on the surface of the substrate)
    • gastroliths (stones swallowed by the animal (bird or reptile) to aid digestion)
    • coprolites (fossilized excrement)

    ...are fossils of behavior, made by organisms that were moving, feeding, and burrowing in their habitats.

    Trace fossils can

    • confirm presence of a species in absence of its own fossils
    • reveal species past ecological impact

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    Determining Fossil Age: Stratigraphy

    Stratigraphy is the study of layers (strata) of
    • sedimentary layered rock
    • volcanic (=igneous) layered rock

    As erosion wears away rock and washes them into bodies of water,
    particles settle to the bottom, where they solidify.

    As layers accumulate, lower layers compress and turn to stone.

    Early paleontologists estimated fossil age via relative dating of the layers.

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A stratigraphic column showing the relative ages of sedimentary rock.

    Geological Principles

  • Principle of Superposition
      In undisturbed sedimentary rock, the oldest layers
      are at the bottom and the youngest/most recent layers are at the top.

  • Principle of Fossil Succession
      Any pebbles, fossils, or other fragments embedded
      in sedimentary rock must be older than the rock matrix itself.

  • Principle of Cross-cutting Relationships
      Intrusions that cut through sediments must be younger/more recent
      than the sedimentary layers they cross.

  • Principle of Deformation
      Sedimentary layers that have been deformed by folding, lifting,
      or other malformation must be older than the event that changed them.

    Using these principles, geologists can determine the relative age of fossils embedded in sedimentary rock.

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    Determining Fossil Age: Radiometry

    Only radiometric dating techniques can reveal the absolute age of a fossil.

    A sample's relative proportions of

    • radioactive isotope
    • its decay products

      ...reveal the time elapsed since the sample was formed.

    The greater the proportion of decay product to radioactive isotope,
    the older the sample.

    Different elements and isotopes are used to date different types of fossils.

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    Absolute (Radiometric) Dating

    Elements can exist as isotopes. These are atoms of the same element
    • same chemical properties
    • same number of protons
    • same number of electrons
    • different numbers of neutrons

    An isotope may be

    • stable
    • unstable (radioactive)

    A radioactive isotope emits ionizing radiation

    • &alpha particles
    • β particles
    • γ rays

      ...to decay to a stable form.

      When this occurs, the radioactive element becomes
      the stable isotope of a different element.

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    Half life

    The half life of a radioactive isotope is the time necessary for half of its nuclei to decay to the stable form.
    • one half-life: 50% of the original sample will still be radioactive
    • two half-lives: 25% of the original sample will still be radioactive
    • three half-lives: 12.5% of the original sample will still be radioactive
    • etc.

      Isotopes with long half lives are used to date very old specimens.
      Isotopes with relatively short half lives are used to date relatively recent specimens.

      Unlike relative dating, radiometric dating establishes a discrete age of the specimen.

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    Carbon Dating

    Carbon-dating Carbon dating can be used to assign an absolute age to specimens ~50,000 years or younger.

    Carbon can exist in the atmosphere (CO2) as

    • stable carbon-12 (12C - 6 protons + 6 neutrons)
    • unstable carbon-14 (14C - 6 protons + 8 neutrons)

    Atmospheric [14CO2] : [12CO2] is about 1 : 10,000,000,000,000.

    • Living organisms constantly take up CO2.
    • Thus, they will have the same 14C : 12C ratio as the atmosphere.
    • Once they die, they stop taking up atmospheric CO2
    • The 14C in their tissues begins to decay into stable 14N.
    • Compare [sample 14C : 12C ratio] to [atmospheric 14C : 12C ratio]
    • Presto! This yields the specimen's age.

    Carbon dating was used to date the iconic Bog People of Europe.

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(start at 41:25)

    Dated Fossils Demonstrate Change Over Time

    Fossils dated with both relative and radiometric methods can be compared.
    Often, they will show a series of intermediate characters.

    The illustration shows evolution of skull morphology in the mammal lineage.

    • synapsids (300mya) had
      • two jaw hinge bones, quadrate and articular
      • squamosal and dentary bones are part of skull
      • undifferentiated teeth

    • therapsids (280mya) had
      • modified jaw hinge bones
      • differentiated canines

    • early cynodonts (260 mya) had
      • further modified jaw hinge bones
      • differentiated canines and cuspid teeth

    • late cynodonts (220 mya) had
      • squamosal and dentary bones separate to form a new jaw hinge
      • articular-quadrate hinge is still functional, but smaller
      • quadrate and articular are moving to the lower/inner skull
      • teeth differentiated into incisors, canines, molars

    • very late cynodonts (195 mya) had
      • squamosal/dentary bones expanded to form the only jaw hinge
      • quadrate and articular moved to the inner skull, near the ear
      • these would evolve into the inner ear bones (incus and malleus)

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    Pattern 2b: Biogeography

    Biogeography is the study the distribution and diversity of living things and ecosystems over time and space.

    The effects of geographical separation on the evolution of populations can be observed at any many levels:

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    Distributions Mirror Continental Drift

    The original supercontinent, Pangaea, split into

    Gondwanaland to the south,
    which further split to give rise to
    • Africa (Columbian "Old World")
    • South America (Columbian "New World")
    • Antarctica
    • Australia
    • India
    • Arabian peninsula
  • Laurasia to the north,
    which further split to give rise to
    • Eurasia (Columbian "Old World")
    • North America (Columbian "New World")

  • Living organisms rode the land masses as they drifted apart.
    Separated populations became reproductively isolated.
    In isolation, the populations genetically diverged.

    The evolution of related lineages can be traced by comparing
    the descendants' current geographic distributions with their genetic makeup.

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    Island Biogeography: Repeated Cladogenesis

    The same patterns can be observed on a smaller scale
    on islands derived from a single land mass that split.

    In this example...

      1. Ancestral taxon X lives on the orignal island.
      2. The island splits, dividing the X population.
      3. Ancestral X populations evolve into descendant taxa A and B.
      4. The island housing B splits, dividing the B population.
      5. Ancestral B populations evolve into descendant taxa C and D.
      6. A mountain range on D's island splits the D population.
      7. Ancestral D populations evolve into descendant taxa E and F.
      8. Some individuals from population E migrate to a new island.
      9. Ancestral E populations evolve into descendant taxa G and H.

      (Note: Once an ancestral taxon splits into descendant taxa
      the ancestor is considered to be extinct. Remember that it existed
      in the past, and its descendants are different individuals.)