For biologists, categorizing living things is the key to understanding life on earth. They do this through taxonomy, the classification of living things within hierarchically-organized categories. One of their main goals is to identify new species, the most specific of the categories they use to group organisms.

All organisms can be classified within seven levels of taxonomic categories. The wasp Encarsia arabica , for example, is part of the kingdom animalia, which includes all animals. This kingdom is divided into several phyla, such as Mandibulata , which is made up of animals that have hard, segmented bodies and characteristic jaw bones. This phylum is further divided into several classes. One of these classes, Insecta , includes organisms with antennae and a pair of compound eyes. The wasp's other traits allow biologists to place it into increasingly refined categories, including the most specific level of classification: species.

To determine whether specimens belong to the same species, biologists look at their body form, their body chemistry, how they develop, and how they behave. In general, only organisms that can produce healthy offspring together are part of the same species. Many types of orchids, though they look similar, are unable to breed with one another, and therefore don't belong to the same species. Dogs, on the other hand, come in a huge variety of shapes and sizes. But since they can all mate with one another, they share the same species name.

If defining a species is difficult, it may be because life is continually—although very slowly—changing. Scientists' current classifications are like snapshots of a moving scene. But by studying the species that exist today, biologists can better understand how the living world came to be the way it is, and how it might be changing.