1. Cladistics is a method of classifying organisms based on shared derived characteristics and evolutionary relationships rather than physical similarity. It uses characteristics inherited from common ancestors to construct branching cladograms depicting evolutionary descent. 2. Determining whether a characteristic is ancestral or derived is important in cladistics. Ancestral characteristics predate the last common ancestor, while derived traits evolved more recently. Outgroup comparison is used to determine polarity. 3. A cladogram depicts nested clades of organisms sharing synapomorphies not found in outgroups. It shows the pattern of common ancestry and branching of life over time based on inheritance of characteristics from ancestors.

1. 1
Cladistics And Systematics

2. 2
Definition of Terms
•Systematics – study of the diversification of
living forms, both past and present, and
the relationships among living things through
time.
•Cladistics – (from ancient Greek, klados,
"branch"; originally called phylogenetic
systematics) is a taxonomical technique for
arranging organisms according to how they
branch in the evolutionary tree of life.

3. Introduction
All organisms:
Are composed of one or more cells
Carry out metabolism
Transfer energy with ATP
Encode hereditary information in DNA
Tremendous diversity of life
Bacteria-----whales----sequoia trees
Biologists group organisms based on shared
characteristics and newer molecular sequence
data
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4. SYSTEMATICS
Since fossil records are not complete,
scientists rely on other types of
evidence to establish the best
hypothesis of evolutionary
relationships
Systematics: the study of evolutionary
relationships
Phylogeny: a hypothesis about
patterns of relationship among species
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5. SYSTEMATICS
Darwin envisioned
that all species were
descended from a
single common
ancestor
He depicted this
history of life as a
branching tree.
Now called a
cladogram
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6. SYSTEMATICS
Twigs of a tree represent
existing species
Joining of twigs and branches
reflects the pattern of common
ancestry back in time to a
single common ancestor
Darwin called this process
“descent with modification”
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7. SYSTEMATICS
Phylogenies depict evolutionary relationships
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8. SYSTEMATICS
 Evolution can occur rapidly at one time and slowly at another
(punctuated and gradual evolution)
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9. SYSTEMATICS
Oscillating selection: Traits can evolve in
one direction, then back the other way
Evolution is not always divergent:
convergent evolution
Use similar habitats
Similar environmental pressures
Evolutionary reversal: process in which a
species re-evolves the characteristics of an
ancestral species
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10. 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 method character variation
must be identified as ancestral or derived
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11. CLADISTICS
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, present in most mammals and
reptiles and absence in birds and turtles
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12. CLADISTICS
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
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13. CLADISTICS
Determination of ancestral versus
derived
First step in a manual cladistic
analysis is to polarize the characters
(are they ancestral or derived)
Example: polarize “teeth” means to
determine presence or absence in
the most recent common ancestor
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14. CLADISTICS
Outgroup comparison is 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
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15. CLADISTICS
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
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16. CLADISTICS
Following the character state-outgroup
method
Presence of teeth in mammals and reptiles
is ancestral
Absence of teeth in birds and turtles is
derived
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17. CLADISTICS
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
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18. CLADISTICS
A simple cladogram is a nested set of clades
Plesiomorphies: ancestral states
Symplesiomorphies: shared ancestral states,
not informative about phylogenetics.
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19. CLADISTICS
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20. CLADISTICS
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 parsimony which favors the
hypothesis that requires the fewest assumptions
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21. CLADISTICS
Parsimony and Homoplasy
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22. CLADISTICS
A Cladogram; DNA
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23. CLADISTICS
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A Cladogram: DNA

24. OTHER PHYLOGENETIC METHODS
 Some characters evolve rapidly and principle of
parsimony may be misleading
 Rate at which some parts of the DNA genome evolve
 Mutations in repetition sequences, not deleted by
natural selection
 Statistical approaches
 Molecular clock: rate of evolution of a molecule is
constant through time
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25. 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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26. SYSTEMATICS AND CLASSIFICATION
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?
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27. SYSTEMATICS AND CLASSIFICATION
Monophyletic Group
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28. SYSTEMATICS AND CLASSIFICATION
Paraphyletic Group
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29. SYSTEMATICS AND CLASSIFICATION
Polyphyletic Group
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30. SYSTEMATICS AND CLASSIFICATION
Phylogenetic species concept (PSC)
Focuses on shared derived characters
Biological species concept (BSC)
Defines species as groups of interbreeding
population that are reproductively isolated
Typological species concept (TSC)
concept of a species as a group whose members
share certain characteristics that distinguish
them from other species
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