Classification in General
The species is the basic unit in the classification of living organisms. In the most general terms, members of the same species can mate and produce viable offspring. Similar species are grouped into genera, and related genera comprise the next unit of classification, the family. Orders are larger classification units, made up of families of related animals or plants. Orders of similar organisms are grouped into a still larger unit, called a class. Organisms belonging to the various classes differ markedly in their evolutionary histories, and, usually, in their structures and in how they go about their lives. A phylum is composed of related classes of organisms. To further express relationships between living things, these groups are sometimes further divided into subfamilies, subclasses, and so forth.
Animals of different, but closely related, species can sometimes interbreed. A common example of this phenomenon is the mule, which is the offspring of two distinct species of animal, the horse and the donkey. Offspring born as the result of an interspecies mating are generally sterile. In natural situations, such hybridization rarely, if ever, occurs.
All of the organisms that have been scientifically identified and described are given a Latin name, consisting of two parts. The first part of the name represents the animal or plant's genus, while the second denotes its species. Unique or isolated populations within a species are sometimes designated as subspecies. In this case, a third Latin name will be added to the usual two. The current trend in taxonomy, however, is to eliminate the subspecies characterization and to classify such populations as species whenever possible.
