Classification V: Phylogeny and Cladistics

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Classification V Phylogeny and Cladistics

• IB Biology HL

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Why Classify?

• Identify connections between ________ and
  ________ organisms
• Identifying unknown organisms
• Understanding evolutionary relationships and the
  development of __________ features through time
• Predicting shared characteristics in groups of
  organisms

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Phylogeny

• The study of the __________ past of a species
• Similar species are closely related and
  vice-versa
• Morphology use of similar features
• Molecular use of proteins or DNA sequence

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Evolutions Clock

• Differences in morphology take large periods of
  time, but molecular changes can record history
  more precisely (still gradually)
• The number or molecular changes can be used as a
  sort of clock to determine how long ago two
  related species divided from a common ancestor

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Comparing DNA

• DNA __________ combine the DNA of different
  species and see how closely they match up based
  on how well they recombine
• DNA ________ comparisons, usually done with
  powerful computers that can analyze code rapidly
  and look for differences

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Phylogenetic Trees

• The distances between organisms on a phylogenetic
  tree can be based on number of DNA differences

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Cladistics

• A system of classification which groups _______
  together according to the characteristics which
  have most recently evolved (natural
  classification)
• Primitive (plesiomorphic) traits evolved early
  on
• Derived (apomorphic) traits evloved more
  recently as modifications to a previous trait
• A clade is a monophyletic group (can be separated
  from others by the presence of __________ traits)

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Analogous vs. Homologous

• Analogous traits similar in function, but not
  from a common ancestor
• Fins on sharks and dolphins
• Homologous traits derived from the same part of
  a common ancestor (not always the same function)
• Arm and hand bones in most chordate quadrapeds
• Which are more useful in cladistics?

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Making Cladograms

• List organisms
• List characteristics (morphological, typically,
  but also molecular)
• Many of the characteristics will be derived, but
  one which is common to all (primitive
  characteristic) this is the ancestral trait to
  begin the cladogram
• Each new derived trait in succession will allow a
  new branch on the cladogram
• First, make a table (see following slide)

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Cladogram Planning Table
Spinal cord Jaws Lungs Claws/ nails Feathers Fur Opposable Thumbs
Hagfish
Perch
Salamander
Lizard
Pigeon
Mouse
Chimp
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Analyzing Cladograms

• One can determine evolutionary relationships
• One can determine which organisms are more
  closely related (i.e. lizard and bird, or lizard
  and salamander?)
• Sometimes surprises
• Hyenas closer to cats than dogs
• Birds are an offshoot of dinosaurs (theropods)
  with several similar structures