Cladistics

Since the 1970s, taxonomists have increasingly used cladis-tics in their attempts to determine natural relationships. Cladistics is a method of examining natural relationships among organisms, based on features shared by the organisms. The relationships are portrayed in straight line diagrams (evolutionary trees) called cladograms (Fig. 16.5).

In cladistics, the value or form of a feature is referred to as a character state. In a feature such as flower color, for example, the character could be flower color, and the state could be purple or red. After the specific chemical or other nature of a character is determined, hypotheses (assumptions) are made about which states are primitive (ancestral) and which are derived (have evolved from something else).

Obviously, the hypotheses need to be tested, and to test them, we often try to find additional characters that may support one hypothesis more than they support others. Those hypotheses that are not supported can be eliminated, and those that are supported can be subjected to further testing.

In trying to choose the best of several to many clado-grams, taxonomists use the principle of parsimony. Parsimony, as it applies to cladistics, is based on a principle of logic called Occam's razor, which states that "one should not make more assumptions than the minimum needed to explain anything." The best cladogram is usually interpreted as that which requires the fewest evolutionary changes in the taxa involved to arrive at the present situation.

Cladistics has been used to infer evolutionary relationships among many different groups of organisms in all the kingdom survey chapters that follow in this book.

Species A

Species B

Species C

Species B

Branch representing the common ancestor of species A and B.

Branch representing the common ancestor of species A,B, and C.

Evolutionary Cladogram Palnts

Node representing a divergence between two lines of evolution (i.e., one line to species C and one line to the A-B species pair).

Figure 16.5 A simple cladogram showing relationships of three species derived from a common ancestor. The cladogram indicates that Species A and Species B are more closely related to each other than either is to Species C.

Branch representing the common ancestor of species A and B.

Branch representing the common ancestor of species A,B, and C.

Node representing a divergence between two lines of evolution (i.e., one line to species C and one line to the A-B species pair).

* Terminal branches representing the evolution of individual species after divergence from ancestors shared with other species.

Figure 16.5 A simple cladogram showing relationships of three species derived from a common ancestor. The cladogram indicates that Species A and Species B are more closely related to each other than either is to Species C.

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  • donnamira
    What is cladistics in plants?
    8 years ago

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