MODERN CLASSIFICATION THEORY

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MODERN CLASSIFICATION THEORY

• Cladistics

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Goals of classification

• 1. Organize knowledge of groups (filing system)
• 2. Scientific hypothesis of relationship
  (phylogeny)Make classification match
  evolutionary history of groups.
• maximize information content
• maximize predictability

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Attempts to classify scientifically

• Classical
• Phenetics
• Cladistics

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Classical

• Grouping by unique characters and similar
  adaptation
• Homology
• Primitive characters (e.g. fish-shaped body)
• Convergence
• Bias of researcher, authority

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Phenetics

• 1950s1960s
• Use of computers.
• Calculate overall similarity
• Classify on basis of similarity.

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Phenetics Character matrix
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Phenetics Similarity matrix
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Phenetics Overall similarity
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Phenetics

• Grouping by overall similarity.
• Homology
• Primitive characters
• Convergence
• G.I.G.O

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Cladistics

• Grouping by common descent.
• 3-taxon statement
• A and B are more closely related to each other
  than either is to C.

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MONOPHYLETIC group(s)

• Grouping of species including all descendants of
  a common ancestor and the common ancestor.

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POLYPHYLETIC group(s)

• Include one or more species not descended from
  the same ancestor as the other members.
• TO BE AVOIDED!!!!!!!

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• BUT, lineages, true common ancestors, are
  unknown. ALL we really know are modern species.
• Fossil specimens could be ancestors, or extinct
  side branches, of the family tree.

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HOW TO RECOGNIZE A MONOPHYLETIC GROUP

• Classification by shared derived characters only.
  
• primitive /or convergent similarities will
  mislead.

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Shared derived character

• Homology
• Distinguishing character.

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Cladistics

• Avoids bias of researcher, authority.
• Grouping (classification) scientific
  hypothesis.
• Testable by congruence of characters
• Repeatable.