III.

Classification Methods

Grouping organisms according to shared characteristics is not a simple task, and scientists often disagree about the best way to classify organisms. Some think that organisms should be grouped according to differences or similarities in the way they look or act. Other scientists argue that classification should be based on characteristics derived from a shared evolution. Conflicting philosophies about classification have resulted in a variety of classification methods, each with their own set of assumptions, techniques, and results.

The classification of insects, birds, and bats illustrates a traditional classification process. Insects, birds, and bats are all animals—that is, they are multicellular organisms that obtain energy from food. Scientists group these organisms into the Kingdom Animalia.

Birds and bats both have spinal cords, causing scientists to classify both birds and bats in the phylum Chordata. Within the phylum Chordata, key features cause scientists to separate birds and bats. Birds are placed in the class Aves, which includes egg-laying animals, while bats are placed in the class Mammalia, which includes animals that give birth to live young and nurse their young from mammary glands. Insects, which lack spinal cords, are classified in the phylum Arthropoda, the taxon that includes animals with jointed legs and a skeleton on the outside of the body. Insects are further divided based on such broad physical features as the presence or absence of wings.

Scientists using the classical approach must judge the relative importance of characteristics. They may decide, for example, that wing structure is more important than the presence of fingernails in certain cases of classification. Some critics argue that this interpretation and evaluation is too subjective. To introduce more objectivity into classification, some scientists devised the phenetics approach to classification.

In the phenetics approach, scientists rely on quantitative methods and consider only the observable characteristics of modern organisms. Pheneticists identify a set of characteristics to measure and assign a certain numerical value to each characteristic. The tally is used to determine the extent of similarity between organisms. For example, pheneticists may find that, overall, birds and reptiles have a 77 percent similarity of body structure, or morphology, compared to a 55 percent morphological similarity between birds and mammals. From this measurement, pheneticists would suggest a classification that grouped birds and reptiles more closely than birds and mammals.

A third classification method is the cladistic approach, which strives to classify organisms by natural evolutionary relationships, known as phylogeny. Cladists use the fossil record, molecular genetics, and other techniques to create an evolutionary tree called a cladogram. This branched diagram shows the relationship of a group of species based on the fewest number of shared changes that have occurred from generation to generation.