Cladograms

Cladograms (klados - branch; gramma - letter) are simply branching diagrams that show hierarchies of diagnostic characters. But, as we'll see, they're not just visual aids, they're the keys to understanding who's related to whom.

To understand how a cladogram works, we begin with two familiar animals; say, a cat and a dog. A cladogram of a cat and a dog is shown in Figure 3.5.

So we're looking for diagnostic characters for these animals. Here, we choose:

1. possession of fur;

2. possession of a backbone; and

3. possession of carnivorous teeth of a unique design

The cladogram links two separate objects - the cat and the dog - based upon the characters that they share. The features are listed on the clado-gram adjacent to the node, which is a split point (bifurcation) in the diagram (see Figure 3.5).

The issue becomes more complicated (and more interesting) when a third animal is added to the group (Figure 3.6), in this case a monkey. Now, for the first time, because none of the three animals is identical, two of the three will have more in common with each other than either does with the third. It is in this step that the hierarchy is established. The group that contains all three

Figure 3.5. A cladogram. The cat and dog are linked by the characters listed at the hatch mark (or bar), just below the node. The node itself defines the things to be united; commonly a name is attached to the node that designates the group. Here, such a name might be "mammalian carnivores."

Ekg Blood Draw

Figure 3.6. One possible distribution of three mammals. Members of the group designated by node 1 are united by the possession offur and a backbone; that group could be called Mammalia. Within the group Mammalia is a subset united by possession of carnivorous teeth (for example, "mammalian carnivores"). That subset is designated at node 2.

Figure 3.6. One possible distribution of three mammals. Members of the group designated by node 1 are united by the possession offur and a backbone; that group could be called Mammalia. Within the group Mammalia is a subset united by possession of carnivorous teeth (for example, "mammalian carnivores"). That subset is designated at node 2.

animals is diagnosed by certain features shared by all three (fur and possession of a backbone). Notice that a subset containing two animals (the cat and the dog) has also been established, linked together by a character (uniquely designed carnivorous teeth) that diagnoses them as being exclusive of the third animal (the monkey).

How the characters, and even the animals, are arranged on the cladogram, is controlled by the choice of characters. Let's try something different:

• Shortened snout

Based upon these characters, the clado-gram in Figure 3.7 contradicts the cladogram in Figure 3.6.

How do we choose? The cladogram that is most likely correct is the one that doesn't change when new characters are added. All characters that apply to these mammals support the cladogram in Figure 3.6. And we can infer from the cladogram in Figure 3.6 that a dog has much more in common with a cat than it does with a monkey.

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