Cladistics

The points at which groups of organisms have separated are difficult to determine. Cladistic analysis attempts to identify these branching points and to establish evolutionary relationships between a diverse group of organisms. Closely related groups have shared characters termed "shared derived characters". Features shared by the entire group of organisms are termed "primitive". Characters may be called primitive or derived depending on the level of classification. For example, feathers would be considered a derived character in the vertebrates but a primitive character in the birds. Shared characters are shown on a branching diagram called a cladogram (Fig. 2.3). Each shared character is evaluated and a decision is made, by reference to a more distant relative (termed an outgroup), as to whether the state is primitive or derived. Cladograms are then constructed using a computer program. The program will produce a number of possible cladograms. These are then evaluated by the cladist.

Although superficially similar to evolutionary trees, clado-grams show only the relationships between organisms, not their distribution in time. Taxa are shown on the tips of each branch. Groups with a unique common ancestor are known as sister groups. Each branch is supported by a shared character from a hypothetical common ancestor. Ancestors are rarely physically identified. This is partly because of the incompleteness of the fossil record but also because by focusing on shared evolutionary characters, rather than searching for ancestors, the pattern of relationships between groups of organisms can been discovered.

Lizard Horse Seal Lion Domestic cat

Lizard Horse Seal Lion Domestic cat

Cladogram For Seal
Fig. 2.3 A simplified cladogram. The characters next to the arrows are the shared characters that define the branching points. This cladogram shows that lions and domestic cats share a common ancestor and are therefore sister groups.

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