Classification of Living Things

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Cladistic Systematics

• Now that we know how to read phylogenetic
  trees.how are they made?
• Based on shared anatomical characteristics
• Cladogram a branching diagram that shows the
  relationship among species in regard to their
  shared derived characteristics
• A cladogram is a special type of phylogenetic
  tree

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Cladistic Systematics

• Cladogram Traces evolutionary history of the
  group under study
• Uses shared derived characters to
• Classify organisms, and
• Arrange taxa into a cladogram
• A clade is an evolutionary branch that includes
• A common ancestor, together with
• All its descendent species

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Constructing a Cladogram

• When constructing a cladogram, the first step is
  to make a chart with the characteristics to be
  compared.
• Notice in the chart on the right all of the
  organisms have a notochord, but not all of them
  are vertebrates.

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Constructing a Cladogram

• Only the newt, snake and lizard have lungs and a
  3-chambered heart.
• Snakes and lizards have internalization
  fertilization and amniotic membranes in their
  eggs.
• Reptiles (snakes and lizards) are a clade
• Clade an evolutionary branch that includes a
  common ancestor together with all its descendant
  species

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Constructing a Cladogram

• The newt (an amphibian) and the eel (fish) are
  all evolutionary descendants of reptiles
• The next step is to draw a cladogram showing
  these relationships
• The goal is to have the simplest drawing possible
• What is the deal then with bony limbs and long
  cylindrical body?

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Constructing a Cladogram

• parsimony the fewest number of assumptions is
  the most logical.
• results in the simplest cladogram possible

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Parsimony

• Cladists are always guided by the principle of
  parsimony
• The arrangement requiring the fewest assumptions
  is preferred
• This would
• Leave the fewest number of shared derived
  characters unexplained
• Minimize the number of assumed evolutionary
  changes
• The reliability of a cladogram is dependent on
  the knowledge and skill of the investigator

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Alternate, Simplified Cladograms

• X, Y and Z share the same characteristics.
• b. c. b. and c. are unlikely examples as the
  shared characteristics in Y and Z would have had
  to evolve independently

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Cladistic Versus Traditional View ofReptilian
Phylogeny
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Phenetic Systematics

• Assumes it will never be possible to construct a
  truly phylogenetic classification system
• Species are classified according to the total
  number of shared similarities
• Disregards assumed phylogenetic considerations
• Ignores issues of convergent or parallel evolution

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Traditional Systematics

• Mainly uses anatomical data
• Classify organisms using assumed phylogeny with
  emphasis on phenotype
• Stress both common ancestry and degree of
  structural difference among divergent groups
• Construct phylogenetic trees by applying
  evolutionary principles to categories
• Not strict in making sure all taxa are
  monophyletic