Phylogenetic Systematics

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Phylogenetic Systematics

• aka Cladistics
• A Modern Approach to Reconstructing Phylogenetic
  History

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Goals of Cladistic Analysis

• To reconstruct the phylogenetic history of taxa
• To recreate the true branching pattern of
  evolution
• To understand the evolutionary relationships
  between taxa
• Necessary for evolutionary study of taxa

human
ape
monkey
Cladogram
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Vocabulary of Cladistics

• Apomorphy (apomorphic)
• A derived character state for some trait
• Plesiomorphy (plesiomorphic)
• A primitive or ancestral character state for some
  trait
• Synapomorphy (synapomorphic)
• A derived trait shared by some taxa
• Symplesiomorphy (symplesiomorphic)
• A primitive trait shared by some taxa
• Autapomorphy (autapomorphic)
• A derived trait unique to a single taxon

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More Vocabulary

• Homology (homologous)
• Characters in different taxa that are either the
  same or primitive/derived modifications of the
  same trait
• Examples include the forelimb of monkeys,
  pectoral fin of whales, and wing of birds
• Homoplasy (homoplastic)
• Characters that dont meet the criteria of
  homology
• Often they have a similar function but different
  evolutionary origin
• Examples include wing of birds, butterflies and
  bats

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Natural Artificial Taxa

• A Taxon (pl. taxa) is a group of organisms that
  is given a name
• Each taxon has a place in the Linnaean hierarchy
• A Natural Taxon is a group of organisms that
  exists in Nature.
• Species is a lineage
• Monophyletic group or a Clade is a group of
  species that includes an ancestral species and
  all its descendants
• An Artificial Taxon does not correspond to a unit
  of evolutionary history
• Paraphyletic groups do not include all
  descendants of an ancestor
• Polyphyletic groups place a common ancestor in a
  different taxon

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Ingroups and Outgroups

• Ingroup refers to the group being studied in a
  cladistic analysis
• Sister group (taxon) is the taxon that is
  genealogically most closely related to the
  ingroup
• Sister taxa make the best Outgroups
• Outgroup refers to a group used in a cladistic
  analysis that is not part of the taxon under
  study
• Usually included to determine morphocline or
  polarity of characters
• The Outgroup Method for determining polarity
• Assumes that character state found in outgroup is
  PRIMITIVE for the ingroup
• This assumption is based on parsimony argument

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Traits used in Cladistics

• Only homologies are used
• Only synapomorphies are used
• Only homologous synapomorphies tell us about
  evolutionary relationships
• Homoplasies are ignored
• Homoplasies may arise thru convergent or parallel
  evolution
• They tell us nothing about evolutionary
  relationships
• Only homologous synapomorphies reveal
  phylogenetic history