Chapter%2025:%20Tracing%20Phylogeny

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Chapter 25Tracing Phylogeny
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Phylogeny

• Phylon tribe, geny genesis or origin
• The evolutionary history of a species or a group
  of related species.

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Phylogeny

• Found in fossils and the fossil record.

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Fossils

• Any preserved remnant or impression of a past
  organism.

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Types of Fossils

• 1. Mineralized
• 2. Organic Matter
• 3. Trace
• 4. Amber

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Mineralized Fossils

• Found in sedimentary rock.
• Minerals replace cell contents.
• Ex bone, teeth, shells

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Organic Matter Fossils

• Retain the original organic matter.
• Ex plant leaves trapped in shale.
• Comment can sometimes extract DNA from these
  fossils.

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Trace Fossils

• Footprints and other impressions. No organic
  matter present.

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Amber

• Fossil tree resin.
• Preserve whole specimen.
• Usually small insects etc.

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Fossils - Limitations

• Rare event.
• Hard to find .
• Fragmentary.
• Dating.

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Fossil Dating Methods

• 1. Relative - by a fossil's position in the
  strata relative to index fossils.
• 2. Absolute - approximate age on a scale of
  absolute time.

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Absolute - Methods

• 1. Radioactive
• 2. Isomer Ratios

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Radioactive

• Estimated from half-life products in the fossil.
• Ex Carbon - 14 Potassium - 40

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Isomer Ratios

• Ratio of L- and D- amino acid isomers.
• L- used by living things.
• D- not used by living things.

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Death

• L- form ? D- form
• Age can be calculated from the ratio of L-/D- as
  long as the temperature of the area is taken into
  account.

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What do fossils tell us?

• That the geographical distribution of organisms
  has changed over time.
• Reason? The land formations of the earth have
  changed.

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Continental Drift

• The movement of the earth's crustal plates over
  time.
• Drift is correlated with events of mass
  extinctions and adaptive radiations of life.

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Pangaea

• 250 million years ago.
• One super continent.
• Many life forms brought into contact with each
  other.

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Mesozoic era

• Pangaea began to break up.
• 180 million years ago.

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Result

• Geographical Isolation.
• New environments formed.
• Old environments lost.
• As the environments changed, so did Life.

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Example

• Australian fauna and flora are unique.
• Separated early and remained isolated for 50
  million years.

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Mass Extinctions

• The sudden loss of many species in geologic time.
• May be caused by asteroid hits or other disasters.

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Examples

• Permian Extinction
• Cretaceous Extinction

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Permian Extinction

• 250 million years ago.
• 90 of species lost.

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Cretaceous Extinction

• 65 million years ago.
• Loss of the dinosaurs.
• Good evidence that this event was caused by an
  asteroid that hit in the Yucatan, causing a
  nuclear winter.

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The crater
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Result of Mass Extinctions

• Areas are open for the surviving species to
  exploit.
• Rapid period of speciation (adaptive radiation).
• Many new species are formed in a very short
  period of time.

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Systematics

• The study of biological diversity.
• Uses evidence from the fossil record and other
  sources to reconstruct phylogeny.

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Systematics - concerns

• 1. Phylogeny- tracing of evolutionary
  relationships.
• 2. Taxonomy- the identification and
  classification of species.

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Taxonomy

• Natural to humans.
• Modern system developed by Linnaeus in the 18th
  century.

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Linnaeus Taxonomy

• 1. Binomial Nomenclature two names for each
  organism.
• Ex - Homo sapiens
• 2. Hierarchical System arranges life into
  groups. Ex - Kingdom ? Species

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Goal of Systematics

• To have Taxonomy reflect the evolutionary
  affinities or phylogeny of the organisms.

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Problem

• How to group taxa so that the phylogenetic
  relationships are correct ?

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Ideal Situation

• Monophyletic Grouping - a single ancestor gave
  rise to all species in the taxon.

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Other Possibilities

• Polyphyletic - grouping where members are derived
  from two or more ancestral forms.
• Paraphyletic - grouping that does not include
  all members from an ancestral form.

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Problem

• Not all likeness is inherited from a common
  ancestor.
• Problem is of homology vs analogy.

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Homology and Analogy

• Homology likeness attributed to shared
  ancestry.
• Ex forelimbs of vertebrates
• Analogy likeness due to convergent evolution.
• Ex wings of insects and birds

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Convergent Evolution

• When unrelated species have similar adaptations
  to a common environment.
• Ex Sharks and dolphins

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Only one is a cactus
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Need

• Methods to group organisms by similarities and
  phylogenies.
• One possible method is Molecular Systematics.

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Molecular Systematics

• Compares similarities at the molecular level.
• Ex DNA, Proteins

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DNA Comparisons

• A direct measure of common inheritance.
• The more DNA in common, the more closely related.

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DNA Comparison Methods

• 1. DNA-DNA Hybridization
• 2. Restriction Mapping
• 3. DNA Sequencing
• All three methods have been used, but DNA
  Sequencing is becoming the most common.

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Protein Comparisons

• Examines the Amino Acid sequence of homologous
  proteins.
• Ex Cytochrome C Study

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Schools of Taxonomy

• 1. Phenetics
• 2. Cladistics

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Phenetics

• Makes no phylogenetic assumptions.
• Taxonomic affinities based on measurable
  similarities.
• Ex Numerical Taxonomy

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Cladistics

• Branch points defined by novel characteristics.
• Branch pattern may not reflect evolutionary
  history.

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Cladistics
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Problem ?
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Classical Evolutionary Taxonomy

• Balances Phenetics and Cladistics with overall
  homology.

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Result

• Taxonomy will become Genealogies, reflecting the
  organisms "Descent with Modification.

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Summary

• Recognize the use and limits of fossils.
• What happens to evolution in mass extinctions.
• What is phylogeny?

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Summary

• What is a phyletic tree?
• How is molecular systematics used in phylogeny?