All organisms are given two names (a binomial name); the first is the genus name and the second is the species name, for example Homo sapiens, the name for humans. The Linnaean classification system places all organisms into a hierarchy of ranked groups. The genus includes one or more related species, while a group of similar genera are placed in the same family. Similar families are grouped into the same order, similar orders in the same class, and similar classes in the same phylum.
Organisms are assigned to the higher ranks of the Linnaean classification scheme largely on the basis of shared similarities (syna pomorphisus). Species are identified on the basis of an organism's ability to interbreed, in addition to its morphological, behavioral, and biochemical characters. Although species are defined as interbreeding populations, taxonomists rarely have information on an organism's breeding behavior and therefore often infer interbreeding groups on the basis of reproductive system morphology, and other shared characters.
In the last 20 years, modern molecular techniques such as DNA hybridization have allowed biologists to gain extensive information on the genetic distance between organisms, which they use to construct hypotheses about the relatedness of organisms. From this information researchers hypothesize as to whether or not the populations are genetically close enough to interbreed.
While the biological species concept has historically been the most widely used definition of a species, more recently the phylogenetic and ecological species concepts have taken the forefront as a more inclusive and useful definition. Whereas the biological species concept defines a species as a group of organisms that are reproductively isolated (able to successfully breed only within the group), the phylogenetic species concept considers tangible (and measurable) differences in characteristics. This idea, also called the cladistic species concept, examines the degree of genetic similarity between groups of related individuals (called clades) as well as their similarities in physical characteristics. For instance, the biological species concept might group coyotes and wolves together as one species because they can successfully breed with one another. In contrast, the phylogenetic concept would definitively split coyotes and wolves into two species based upon the degree of divergence in genetic characters and larger observable traits (e.g., coat color). In contrast to these, the ecological species concept might classify wolves and coyotes as different species by comparing the differing environmental resources that they exploit, called adaptive zones. Currently, the precise definition of a species is a topic under constant scientific debate and likely will never fully be resolved. Rather, the definition may change with the perspectives and needs of each sub-discipline within biology (ecology versus zoology, for example). A pluralist approach combines some or all of these species concepts to arrive at a more inclusive definition.