Mapping the course of evolution with the cladogram

MAMMALIA

warm-bloodedness fur- or hair-bearing

Figure 3.8. A cladogram showing humans within the larger group Mammalia. Mammalia is diagnosed by warm-bloodedness and possession of fur (or hair); many other characters unite the group as well. Carnivora, a group of mammals that includes bears and dogs (among others) is shown to complete the cladogram. Carnivores all uniquely share a special tooth (the carnassial) and humans all uniquely share, among many other features of the skull and skeleton, a large cranium. Note that all mammals (including humans and carnivores) are warm-blooded and have fur (or hair), but only humans have the gracile skeletal features, and only members of Carnivora have the carnassial tooth.

Because cladograms are hierarchical, they are an excellent way to map the hierarchical distributions of characters in nature. Derived characters are evidence of monophyletic groups because, as newly evolved features, they are potentially transferable from the first organism that acquired them to all its descendants: in short, they characterize the bifurcations at each node on the cladogram. Primitive characters - those with a much more ancient history - provide no such evidence of monophyly.

To illustrate this, we resort for the last time(!) to mammals and their fur. Mammalian fur, we said, is among the shared, derived characters that unite mammals as a monophyletic group. On a cladogram, therefore, we look for characters that mark a node in the diagram. All organisms characterized by shared, derived characters are linked by the cladogram into monophyletic groups. Reflecting the hierarchy of character distributions in nature, the cladogram documents monophyletic groups within larger monophyletic groups. In Figure 3.8, a small part of the hierarchy is shown: humans (a monophyletic group, possessing shared, derived characters) are nested within mammals (another monophyletic group possessing other shared, derived characters). Notice that the character of warm-bloodedness is primitive for Homo sapiens, but derived for Mammalia. As we have seen, features can be derived or primitive, all depending upon what part of the hierarchy one is investigating.

The cladogram need not depict every organism within a monophyletic group. If we are talking about humans and carnivores, we can put them on a cladogram and show the derived characters that diagnose them, but we might (or might not) include other mammals (for example, a gorilla). So we said with regard to Figures 3.6 and 3.7, if the hierarchical relationships that we have established are valid, the addition of other organisms into the cladogram should not alter the basic hierarchical arrangements established by the cladogram. Figure 3.9 shows the addition of one other group into the cladogram from Figure 3.8. The basic relationships established in Figure 3.8 still hold, even with the new organism added. The cladogram is likely correct.

MAMMALIA

warm-bloodedness fur- or hair-bearing

Figure 3.8. A cladogram showing humans within the larger group Mammalia. Mammalia is diagnosed by warm-bloodedness and possession of fur (or hair); many other characters unite the group as well. Carnivora, a group of mammals that includes bears and dogs (among others) is shown to complete the cladogram. Carnivores all uniquely share a special tooth (the carnassial) and humans all uniquely share, among many other features of the skull and skeleton, a large cranium. Note that all mammals (including humans and carnivores) are warm-blooded and have fur (or hair), but only humans have the gracile skeletal features, and only members of Carnivora have the carnassial tooth.

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