In biology, a species 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 (and in botany other taxa are used, such asvarieties, subvarieties, and formae).
This page specifically lists the species of animals (plants aren't included in this category) found throughout the series. Click here to see the complete list.