Seminars on Science: Diversity of Fishes
Cladistic Reasoning
by Dr. Adriana Aquino
We’re going to construct a cladogram to show relationships between vertebrates. We start off by
choosing a group of taxa and then a group of relevant characteristics. This allows us to see a
variable distribution across the taxa. Here’s our data matrix:
If we use a simple binary code where 0 = absence of a character and 1 = presence of a character,
and then substitute the character name with a number, we can rewrite our data matrix so it looks
like this:
© 2000 American Museum of Natural History
1
The original reasoning of cladistics proceeds as follows:
• The presence of each apomorphic (derived) character state (in the matrix as “1”) defines a
relationship. For example, the PRESENCE of amnion defines the group [turtle + human + gecko +
snake + alligator + budgie].
• However, the ABSENCE of a character CANNOT define a group (e.g., absence of amnion does
NOT define the group [ladybug + perch + coelacanth + salamander + frog]) as having descended
from a common ancestor exclusive of the group [turtle + human + gecko + snake + alligator +
budgie].
Character 1 (presence of amnion) defines the group:
© 2000 American Museum of Natural History
but not the group:
2
So THE FIRST STEP is, for each one of the characters, to propose hypotheses of groups supported
by the apomorphic state:
Character 1:
presence of
amnion
Character 2:
presence of
bony limbs
Character 3:
presence of
warm blood
© 2000 American Museum of Natural History
3
Character 4:
presence of
internal nostrils
Character 5:
presence of
atrial septum
Character 6:
presence of 2
temporal openings
© 2000 American Museum of Natural History
4
Character 7:
presence of
hemipenes
Character 8:
presence of a
gizzard
Character 9:
presence of
pedicellate teeth
Character 10:
presence of
feathers
Character 11:
presence of
bony wings
© 2000 American Museum of Natural History
5
Our second step is to group our taxa according to whether or not each of the characters we have
identified is present. We take the characters one at a time and apply them to our group of taxa.
The taxa are grouped according to whether a character is present. We can now construct a
branching tree diagram to show this diagrammatically. As this builds, we create a tree that shows
groups nested within groups.
For example, let’s group our organisms according to whether characters 4 (internal nostrils) and 5
(atrial septum) are present. These two characters are shared by all the taxa except for the ladybug
and the perch. The presence of these two characters in all the other taxa allows us to hypothesize
that all the other vertebrates are more closely related to each other than any of them are to perch.
The cladogram below expresses this relationship.
4
5
Next, let’s re-examine our larger group of vertebrates, excluding the perch, using character 2, the
presence of bony limbs. This allows us to create a second, more inclusive group that excludes the
ladybug, the perch, and also the coelacanth. Now we can conclude that the larger group of
vertebrates is more closely related to each other than any of them are to fishes or invertebrates.
4
5
2
© 2000 American Museum of Natural History
6
We can form a third, more inclusive group by considering our group in the light of character 1, the
presence of an amnion. Now we have excluded the ladybug, the perch, the coelacanth, the
salamander and the frog. Here’s what our new subgroups look like:
4
5
2
1
We create a fourth, yet more inclusive group based on character 6, the presence of two temporal
openings in the skull. This new group [gecko, snake, alligator and budgie], is a subset of our
previous group, but this time we have excluded human and turtle because they lack these openings.
4
5
2
1
6
© 2000 American Museum of Natural History
7
Now, we look at this group to see if characters 7 [presence of hemipenes] and 8 [presence of
gizzard] are present. Character 7 creates a group that includes the gecko and the snake. If we look
at character 8, it includes the alligator and the budgie.
4
5
7
2
1
6
8
© 2000 American Museum of Natural History
8
Now let’s take the group that includes the coelacanth, the salamander and the frog, and look for
character 9, the presence of pedicellate teeth. Here we see the monophyly of the subgroup - the
salamander and the frog.
9
4
5
2
7
1
6
8
© 2000 American Museum of Natural History
9
Characters 10, the presence of feathers, and 11, the presence of bony wings, support the
monophyly of the group represented by the budgie, i.e., birds are the only group within our chosen
taxa that have feathers and wings.
Character 3, the presence of warm blood, occurs more than once within the branches we’ve
constructed on our tree. We can make a hypothesis that this character evolved twice within the
evolution of our group of taxa. This is an example of CONVERGENCE, or HOMOPLASY.
Character 12, the presence of vertebrae, is shared by all the groups and therefore defines the entire
taxon Vertebrata, exclusive of the ladybug.
9
12
4
5
2
7
3
1
6
8
10
11
3
© 2000 American Museum of Natural History
10
Above is the final branching tree: compare it to the first ones, noting how adding characters
improves the resolution of the relationships among taxa included in the analysis. From this
cladogram we can make statements as such: the gecko and the snake are more closely related to
each other than either is to the alligator or the budgie.
What else can we say about the relationships shown in the final cladogram?
© 2000 American Museum of Natural History
11