Cladistics in taxonomy.ggggggggddccc@%12367>ppt

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Cladistics and Phylogenetic
Revolution

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Cladistics(fromGreek, "branch")is an approach
tobiological classificationin whichorganismsare
categorized based on shared derived characteristics
that can be traced to a group'smost recent common
ancestorand are not present in more distant
ancestors.
Therefore, members of a group are assumed to share a
common history and are considered to be closely related.
“clade” is defined as a given common ancestor plus all
descendants, including two or more lineages,
PHYLOGENETIC SYSTEMATICS

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Cladistics
•Derived characteristic:similarity that is
inherited from the most recent common
ancestor of an entire group
•Ancestral:similarity that arose prior to the
common ancestor of the group
•In cladistics, only shared derived characters
are considered informative about
evolutionary relationships
•To use the cladistic methodcharacter
variation must be identified as ancestral or
derived

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•Characters can be any aspect of the
phenotype
–Morphology -Physiology
–Behavior -DNA
•Characters should exist in recognizable
character states
–Example: Teeth in amniote vertebrates
has two states, presentin most
mammals and reptiles and absencein
birds and turtles
Cladistics

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Examples of ancestral versus derived
characters
•Presence of hair is a shared derived feature
of mammals
•Presence of lungs in mammals is an
ancestral feature; also present in
amphibians and reptiles
Cladistics

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•Determination of ancestral versus derived
–First step in a manual cladistic analysis is
to polarizethe characters (are they
ancestral or derived)
•Example: polarize “teeth” means to
determine presence or absence in the
most recent common ancestor
Cladistics

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–Outgroup comparisonis used to
assign character polarity
•A species or group of species not a
member of the group under study is
designated as the outgroup
–Outgroup species do not always
exhibit the ancestral condition
Cladistics

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•When the group under study exhibits
multiple character states, and one of
those states is exhibited by the
outgroup, then that state is ancestral
and other states are derived
•Most reliable if character state is
exhibited by several different outgroups
Cladistics

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•Following the character state-outgroup
method
–Presence of teeth in mammals and
reptiles is ancestral
–Absence of teeth in birds and turtles
is derived
Cladistics

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Construction of a cladogram
•Polarize characteristics
•Clade:species that share a common
ancestor as indicated by the possession of
shared derived characters
•Clades are evolutionary units and refer to a
common ancestor and all descendants
•Synapomorphy:a derived character
shared by clade members
Cladistics

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•A simple cladogram is a nested set of
clades
•Plesiomorphies:ancestral states
•Symplesiomorphies:shared ancestral
states, not informative about phylogenetics.
Cladistics

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Cladistics

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•Homoplasy:a shared character state that
has not been inherited from a common
ancestor
–Results from convergent evolution
–Results from evolutionary reversal
•If there are conflicts among characters, use
the principle of parsimonywhich favors
the hypothesis that requires the fewest
assumptions
Cladistics

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Systematics and Classification
•Classification:how we place species and
higher groups into the taxonomic hierarchy
–Genus, family, class..
•Monophyletic group:includes the most
recent common ancestor of the group and
all of its descendants (clade)
•Paraphyletic group:includes the most
recent common ancestor of the group, but
not all its descendants

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•Polyphyletic group: does not include
the most recent common ancestor of all
members of the group
•Taxonomic hierarchies are based on
shared traits, should reflect evolutionary
relationships
•Why should you refer to birds as a type
of dinosaur?
Systematics and Classification

17. 17Monophyletic Group
Systematics and Classification

18. 18Paraphyletic Group
Systematics and Classification

19. 19Polyphyletic Group
Systematics and Classification

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Old plant classification system
Systematics and Classification

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New plant classification system
Systematics and Classification

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RECENCY OF COMMON ANCESTRY