An Example of Cladogram Construction for Vertebrates
Trait
I Out-group Frog
| (Lobe| finned fish)
\e
1 Kangaroo 1 Mouse
]
f Human
|
1
Dorsal Nerve |Yes
Cord
Yes
lYes
Yes
Yes
Yes
Legs
jNo
Yes
iYes
Yes
Yes
Yes
Nature of
I Requires
I water
Requires Hard shell ! Develops
water
I prevents
inside the
1 mother
! drying
Develops
inside the
mother
Develops inside
the mother
]
j
egg
Nature of
\n egg
development
In egg
| In egg
1 Marsupial
Placental
Placental
Hair
(No
No
iNo
[Yes
Yes
Reduced
Presence of
pouch
[NO
No
(No
Yes
No
No
| No
j No
[NO
Yes
No
Yes
Bidpedal
: posture
>:
|
I
Steps for Constructing a Cladogram
1. Select a taxonomic group to be analyzed; for example, a group of vertebrates.
2. For each member of the group, determine some observable traits (characters),
and note their "states" (a "character state" is one of two [or more] possible forms
of that character). For example, for a character "fins," the possible states may be
"present" and "absent." For the character "number of forelimb digits," possible
states may be 1, 2, 3, 4, or 5
3. For each character, determine which state is ancestral (primitive or
plesiomorphic) and which is derived (apomorphic). This is usually done by
comparison with a more distantly related organism termed the "outgroup." It is
hypothesized that traits shared with the more distantly related organism(s) are
likely to be "ancient" or plesiomorphic traits. Similarly, traits that differ from the
outgroup are postulated to have arisen since the group being considered
branched from its shared common ancestor with the outgroup, and thus are likely
to be "derived" or apomorphic.
4. Group taxa by shared derived character states (synapomorphines).
5. When in doubt, choose the most parsimonious tree. While similar structures
may evolve independently in separate lineages facing similar selective pressures
(convergent evolution), this is assumed to be a rare event. Most major
structures (eyes, horns, tails, fur, etc.) are assumed to have evolved or to be lost
only rarely. Thus, when in doubt, choose a pathway that minimizes the number of
times a feature must be postulated to have arisen (or lost) separately.