17.2 Classification Based on Evolutionary Relationships
 Cladistics is classification based on common
ancestry.
•  Phylogeny is the evolutionary history for a group of
species.
–  evidence from living
species, fossil record,
and molecular data
–  shown with branching
tree diagrams
17.2 Classification Based on Evolutionary Relationships
•  Cladistics is a common method to make evolutionary
trees.
–  classification based on common ancestry
–  species placed in order that they descended
from common ancestor
17.2 Classification Based on Evolutionary Relationships
  A cladogram is an evolutionary tree made using
cladistics.
–  A clade is a group of species that shares a
common ancestor.
–  Each species in
a clade shares
some traits with
the ancestor.
–  Each species in
a clade has
traits that have
changed.
17.2 Classification Based on Evolutionary Relationships
•  Derived characters are traits shared in different
degrees by clade members.
1 Tetrapoda clade
–  basis of arranging
species in
cladogram
–  more closely related
species share more
derived characters
–  represented on
cladogram as hash
marks
2 Amniota clade
3 Reptilia clade
4 Diapsida clade
5 Archosauria clade
FEATHERS &
TOOTHLESS
BEAKS.
SKULL OPENINGS IN
FRONT OF THE EYE &
IN THE JAW
OPENING IN THE SIDE OF
THE SKULL
SKULL OPENINGS BEHIND THE EYE
EMBRYO PROTECTED BY AMNIOTIC FLUID
FOUR LIMBS WITH DIGITS
DERIVED CHARACTER
17.2 Classification Based on Evolutionary Relationships
•  Nodes represent
the most recent
common
ancestor of a
clade.
CLADE
1 Tetrapoda clade
2 Amniota clade
3 Reptilia clade
•  Clades can be
identified by
snipping a
branch under a
node.
4 Diapsida clade
5 Archosauria clade
FEATHERS AND
TOOTHLESS
BEAKS.
SKULL OPENINGS IN
FRONT OF THE EYE AND
IN THE JAW
OPENING IN THE SIDE OF THE
SKULL
SKULL OPENINGS BEHIND THE EYE
EMBRYO PROTECTED BY AMNIOTIC FLUID
NODE
FOUR LIMBS WITH DIGITS
DERIVED CHARACTER
17.2 Classification Based on Evolutionary Relationships
 Molecular evidence reveals species’ relatedness.
•  Molecular data may confirm classification based on
physical similarities.
•  Molecular data may lead scientists to propose a
new classification.
•  DNA is usually given the last word by scientists.