Systematics

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Cladistics And Systematics

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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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