Types of Classification Systems

The natural system of classification, which attempts to follow the evolutionary history— the phylogeny—of the organisms being categorized, is only one of several types of classi­fication system. Another fundamental type is an artificial classification system, in which several key characters, often very easy to observe, are chosen as the basis of classification. Good examples are roadside floras and picture guides to plants, birds, and mammals of national and state parks. The botanical classifications in these are often based primarily on flower color: All plants with white flowers are grouped together, as are all those with red flowers, and so on. Within each category, the next classification category might be the plant's habit: Trees are grouped together, as are shrubs, herbs, vines, and so on. These systems are described as artificial because many plants in a category are not closely related to each other by descent from a common ancestor; furthermore, they are separated from their close natural relatives in other categories simply because they differ in flower color or habit.

Both artificial and natural classification systems have distinct advantages. Natural sys­tems facilitate our study of evolution—not just the evolution of species, but also the evolution of various aspects of a species, such as its structure, metabolism, and reproduc­tive biology (Fig. 1). Artificial systems typically have the goal of easy plant identification by means of characters such as flower color and plant habit. Alternatively, the artificial system may be designed to group together plants with economically or scientifically im­portant features. From a practical standpoint, carpenters and woodworkers are more inter­ested in color, texture, grain, and hardness of a wood than its phylogeny. Gardeners might classify plants according to their ability to tolerate shade, full sun, frost, or alkaline soils.

FIGURE 1: This diagram represents one interpretation of the evolutionary relationships of the monocots; each enclosed space represents an order (a group of related families). The size of the space represents he number of species in the order: The small order Typhales contains only one family, Typhaceae, one genus, Typha, and three species (cattails), whereas the large order Poales contains the grass family Poaceae with several thousand species. Orders drawn in the lower right, such as Arales and Alismatales, are thought to be relatively unchanged from the ancestors of the monocots, whereas all orders farther to the left or top have undergone a great deal of modification. Chapter 25 gives common names for many of these orders. A map of the phenotype of treelike or shrub-like body is given in green. The entire order Arecales (palms) and most of the order Pandanales ("screw pines") are treelike, whereas a few members of Asparagales are treelike (Joshua trees, Australian grass trees). In Poales, bamboos are treelike grasses. The woody, treelike growth of palms, screw pines, and Cyclanthales is similar, so probably these three orders are closely related and their close placement in this diagram may be correct. The growth of bamboos is not at all like that of palms; their treelike, woody growth has evolved separately as a type of convergent evolution. Although bamboos and palms have a somewhat superficial resemblance, they are not really closely related, so Ute separation of the two orders in this diagram is probably justified. Chemical features are also analyzed in this way; the presence of tricin, a chemical known as flavone, is given in brown. It is almost universally present in the grasses (Poales) and is very common in sedges (Cyperales) and palms (Arecales). Could this also be a case of convergent evolution? It could, because tricin also occur sporadically in the Asparagales, Liliales, Orchidales, and even a few dicots; these species are so distantly related that the genes for the tricin-synthesizing metabolic pathway must have arisen sewed times as independent mutations in each group. The possibility of such convergent evolution makes it difficult to determine whether the presence of tricin in Poales, Cyperales, and Arecales is evidence of natural relatedness or convergence.

Although artificial classifications are extremely useful, they can only be adjuncts to natural systems of classification which take evolution into account. As physiologists exam­ine the phylogeny of all species with C₄ metabolism as revealed by a natural classification, they find that C₄ metabolism has evolved several times, so all C₄ metabolisms are not expected to be identical. Similarly, someone studying the metabolism of petal pigmentation might classify flowers artificially according to color, deciding to investigate the synthesis of red pigment first. It would he necessary to check the phylogeny of the experimental plants because some plants produce red anthocyanin pigments, but a different evolutionary line produces red betalains that are metabolically very different . If this phylo­genetic difference were not known, the scientist would get inexplicably conflicting results.

Because we do not yet know all details of plant phylogeny, we cannot be certain how to construct the definitive natural system that is correct in every detail. In many cases botanists must make educated guesses, and of course they do not all agree. Because investiga­tors are free to publish their own views and opinions, numerous "natural" systems have been proposed. At present, most agree with each other on most major points, and the areas of disagreement are the groups of plants that need more study.

A third type of classification, used for fossil organisms, combines features of both artificial and natural systems. The goal is to understand the evolution of the fossil and to identify both its ancestors and its relatives that later evolved into another species. This requires a natural system. Typically, we do not know much about the fossil, so superficially similar ones are grouped together for convenience—a basically artificial system. The groupings are form genera: All fossils with the same basic form or structure are classified together. For example, a piece of fossil wood similar to the wood of modern pines, spruces, and larches is classified in the form genus Pityoxylon. If the piece of fossil wood was part of a branch with leaves and cones attached, there would probably be enough characters present to allow us to assign the wood to a natural genus. But if the fossil contains only wood, not enough characters are present to determine whether it came from an ancient pine, spruce, larch, or some other group that has since become extinct.