There are more than five million different species on Earth. While each species represents the end point of a unique evolutionary path, biologists do not treat each one as completely unrelated to the others. Instead, they have created biological classification systems, or taxonomies, to reflect similarities and differences among species. A taxonomy is hierarchical, meaning that groups, or taxa , are themselves members of larger groups. For example, birds and mammals are grouped together as vertebrates because they both have backbones. Jellyfish do not belong to the vertebrate taxon because they lack backbones. However, birds, mammals, and jellyfish all belong to the taxon Animalia.
The fundamental group in a biological taxonomy is the species. The most widely accepted definition of a species is "a group of actually or potentially interbreeding natural populations that are reproductively isolated from (unable to mate with) other such groups." In the eighteenth century, a Swedish naturalist named Linnaeus developed a biological taxonomy called "binomial nomenclature." Linnaeus grouped very similar species together into genera (singular, genus). Thus, every species is known by two labels: the name of the genus to which it belongs, and a specific modifier to distinguish it from other species in the genus (e.g., Homo sapiens).
Today, species are organized into groups at many levels higher than genus, including family, order, class, phylum, and kingdom. Systematists, who are biologists specializing in taxonomy, have developed two different methods to organize these groups: cladistics, which considers phylogeny (evolutionary history), and numerical phenetics, which considers phenotypic (outwardly observable) similarity. Cladists group taxa according to how long ago they diverged from a common ancestor, using only characteristics that provide information about phylogeny. They group together taxa that are closely related. Pheneticists group taxa according to their overall similarity in appearance, using as many characteristics as possible, regardless of phylogeny. These methods contradict one another when closely related groups appear to be very different. For example, crocodiles are more closely related to birds than to turtles, snakes, or lizards, but look more like turtles, snakes, or lizards than like birds.
Systematists now use molecular techniques to analyze protein and DNA sequences for information on relatedness and similarity. Data at the level of molecules have the advantage of being more easily quantifiable, or reducible to numbers, than much phenotypic data. Furthermore, certain types of DNA and protein are thought to evolve at constant rates over long periods of time, providing " molecular clocks " for establishing phylogenetic relatedness. However, the accuracy of these "clocks" is difficult to determine without independent knowledge of phylogeny, such as a detailed fossil record.
While there remains considerable debate over the merits of the various taxonomic methods, there is no doubt about the scientific importance of biological classification. Suppose we want to know how flight evolved. Taxonomic methods tell us that bats, birds, and insects each evolved flight independently. Thus, any other characteristics that bats, birds, and insects share can tell us something about the evolution of flight in general. Without a taxonomy, we might assume that flight evolved just once, making it difficult to draw any conclusions. A more pressing reason to develop taxonomies is the rapid loss of biological diversity, since conservationists may prefer to focus their efforts on unique species with few close relatives, rather than on species that are more similar to others. Taxonomies can help them prioritize the species that are least replaceable.
see also Kingdoms of Life; Linnaeus, Carolus; Phylogenetic Relationships of the Major Groups.
Brian R. West
Campbell, Neil A. Biology, 2nd ed. Redwood City, CA: Benjamin Cummings Publishing Company, Inc., 1990.
Curtis, Helena, and N. Sue Barnes. Biology, 5th ed. New York: Worth Publishers, 1989.