Most scientists recognize 36 or 37 species of cats, depending upon how they define a species and the latest knowledge of cat evolution. Some experts recognize as many as 41 species. Although numerous domestic cat breeds exist, common household cats derive from a single species, called Felis catus.
Scientists agree that the common house cat is a single species despite being highly variable in appearance. Most modern science on the classification of species focuses upon biochemical or genetic data, rather than outward appearance or number of similarities between creatures.
All cats, including the big cats, such as lions and tigers, belong to a single scientific grouping, called Felidae. Within that scientific grouping, scientists determine what is a species; varying scientific opinions lead to disagreements over the exact number of species in a group.