Major Groups of Organisms

Faced with the multiplicity of organisms in the world, classifying them into groups with like characteristics makes talking about them, identifying them, and describing them easier. Utilitarian groupings—poisonous plants, trees that produce edible fruits, animals that sting, and so forth—are useful today as in the past, however, they provide information about only one aspect of organisms' lives. For decades researchers have accumulated libraries of information about creatures large and small. Systematists (the biologists concerned with classification and naming of organisms) have arranged much of this information into schemes that provide not only descriptions and names for organisms, but also show hereditary relationships among and between members of the various groups. Indeed, one of the principal objectives of systematics is to determine as best as possible—using all available information—the evolutionary history of life on Earth.

Not everyone agrees on how to organize and classify the existing information, and with researchers constantly producing new data, opinions change often to accommodate the new material. Authors of current textbooks reflect the differences of opinion. Which is the correct classification? Obviously, the one used by your instructor. This classification scheme, or any of the others, is neither completely right nor completely wrong. Any classification scheme is a hypothesis proposed by knowledgeable experts and subject to change and interpretation by other knowledgeable experts. Tomorrow's classifications will be different and probably closer to the truth than are today's.

The first column in Table shows the 250‐year‐old Linnaean view of the living world. For centuries, if something was alive, it was either a “plant” or an “animal”, or, later, a “higher” animal (such as a human being). When microscopes revealed a new world of tiny organisms living in concert with the more familiar larger ones, classifiers added and refined categories—and have been doing so ever since to accommodate large and small discoveries.

TABLE 1Three Historical Classification Schemes of Organisms