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Phylogeny
Ever since Darwin, biologists have attempted to arrange living organisms into classifications that reflect their evolutionary relationships, putting those with a common ancestor near each other, as in a family tree. The study of the evolution of organisms and how they are related to each other through evolution is called phylogeny. The result of phylogenetic research is a phylogenetic tree (called a dendrogram), which can be used to create a classification. Phylogenies can be developed for groups of families, genera, or species.
Because they cannot go back and watch evolution occur, scientists can only guess at what occurred. But there are ways they make their guesses more educated. One method is called cladistics. In cladistics it is assumed that evolution has occurred, and that the evolutionary history of living organisms can be portrayed as branching trees. The method involves analyzing each of the characteristics of the group under study and determining which characteristics have changed, and the sequence of change. The branching diagram is formed by grouping together species with similar characteristics, and particularly special kinds of similar characteristics called synapomorphies (shared derived characteristics). The result of a cladistic study is a cladogram. As often happens in life, cladograms are never so simple. Plants often evolved similar structures at different times and in different ways. These similarities should not be used in grouping plants based on evolution. In order to weed out this extraneous information, phylogenists assume that the shortest tree is the best-shortest meaning the tree with the fewest character changes.
An example of a cladogram is shown at right. This cladogram portrays the divisions of seed plants. Point A represents the hypothetical common ancestor of all seed plants. Each additional node (B, C, and D) represents the hypothetical ancestor for each group above (B is the hypothetical ancestor for Ginkgophyta, Pinophyta, Gnetophyta, and Magnoliophyta; C is the hypothetical ancestor of Pinophyta, Gnetophyta, and Magnoliophyta; and D is the hypothetical ancestor of Gnetophyta and Magnoliophyta). Between each node some evolutionary change has occurred represented by a synapomorphy. For example, between B and C (step 2) the means of fertilization changed; it no longer was necessary for the sperm to swim to the egg and so the sperm lost its flagella. At step 3 the means of fertilization changed again and double fertilization occurred, in which one sperm nucleus fertilizes the egg and a second nucleus fertilizes a different cell in the female gametophyte.
These branching diagrams, in addition to being useful for classifying organisms, are also very valuable for studying the structure of the organisms and for predicting attributes of poorly known organisms. It is known that Taxus brevifolia (the Pacific yew) has an important medical compound, taxol. By knowing how this species of Taxus is related to other yews, it is possible to predict which other yews also contain taxol.