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In biology, a species (abbreviated sp., with the plural form species abbreviated spp) is the basic unit of biological classification and a taxonomic rank.

A species is often defined as the largest group of organisms in which two individuals can produce fertile offspring, typically by sexual reproduction.

While this definition is often adequate, looked at more closely it is problematic.

For example, with hybridisation, in a species complex of hundreds of similar microspecies, or in a ring species, the boundaries between closely related species become unclear.

Other ways of defining species include similarity of DNA, morphology, or ecological niche.

A fish is any member of a group of animals that consist of all gill-bearing aquatic craniate animals that lack limbs with digits.

They form a sister group to the tunicates, together forming the olfactores.

Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups.

Tetrapods emerged within lobe-finned fishes, so cladistically they are fish as well.

However, traditionally fish are rendered obsolete or paraphyletic by excluding the tetrapods (i.e., the amphibians, reptiles, birds and mammals which all descended from within the same ancestry).

Because in this manner the term "fish" is defined negatively as a paraphyletic group, it is not considered a formal taxonomic grouping in systematic biology.

The traditional term pisces (also ichthyes) is considered a , but not a phylogenetic classification.

Mammals are any vertebrates within the class Mammalia (from Latin "breast"), a clade of endothermic amniotes distinguished from reptiles (including birds) by the possession of a neocortex (a region of the brain), hair, three middle ear bones and mammary glands.

All female mammals nurse their young with milk, secreted from the mammary glands.

Reptiles are tetrapod (four-limbed vertebrate) animals in the class Reptilia, comprising today's turtles, crocodilians, snakes, amphisbaenians, lizards, tuatara, and their extinct relatives.

The study of these traditional reptile orders, historically combined with that of modern amphibians, is called herpetology.

Birds (Aves), also known as avian dinosaurs, are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton.

Birds live worldwide and range in size from the bee hummingbird to the ostrich.

They rank as the class of tetrapods with the most living species, at approximately ten thousand, with more than half of these being passerines, sometimes known as perching birds.

As a subgroup of Reptilia, birds are the closest living relatives of crocodilians, while birds and crocodilians together are the last living representatives of archosaurs.