In biology, a species (abbreviated sp., with the plural form species abbreviated spp.) is one of the basic units of biological classification and a taxonomic rank. A species is often defined as the largest group of organisms in which two individuals are capable of reproducing fertile offspring, typically using sexual reproduction. While in many cases this definition is adequate, the difficulty of defining species is known as the species problem. For example, a species complex is a group of closely related species that are very similar in appearance to the point that the boundaries between them are often unclear. Differentiating measures include similarity of DNA, morphology, or ecological niche.
Presence of specific locally adapted traits may further subdivide species into "infraspecific taxa" such as subspecies.
Species hypothesized to have the same ancestors are placed in one genus, based on similarities. The similarity of species is judged based on comparison of physical attributes, and where available, their DNA sequences. All species are given a two-part nam, a "binomial name", or just "binomial". The first part of a binomial is the genus to which the species belongs. For example, Boa constrictor is one of four species of the Boa genus. While the genus gets capitalized, the specific epithet does not. The binomial is written in italics when printed and underlined when handwritten.