As soon as you start getting interested in animals (or plants, for that matter) you will hear words like species, genus and family banded about, especially you want to buy anything other than a dog, a cat or a rabbit. This is especially true if you fancy keeping reptiles, amphibians, birds or fish, or unusual rodents. While dog- and cat lovers or fanciers talk about "breeds", this is really not the same thing. Different breeds of dog can mate, as any glance at the neighbourhood mutts will confirm. So can different breeds of cat. The idea of "species" is a lot more fundamental and is rooted in the concepts of biology and zoology.
This way of talking about living organisms, or classifying them (a process or method known as taxonomy), started with a Swedish naturalist called Charles Linne, better known as Linnaeus, in the eighteenth century. Up until then although people (from Solomon and Aristotle onwards) had been investigating and exploring plants and animals for centuries, nobody had tried to draw up a way of organising them in their relationship towards one another. There was also no common way of describing them, so that what might be known as a spotted water lizard in one country could equally well be called a slimy water newt in another. To make matters worse, one common name might end up being applied to several creatures, eg "hairy rat" might actually describe several completely different types of rat around the world. Linnaeus wanted a universal system, ie one where each organism would have a unique name that would be recognised internationally, regardless of whatever popular name it was called by. The name would also help to describe how that particular organism related to other organisms.
The most basic grouping in biology according to the Linnean classification system is the species. The word species in Latin simply means semblance or appearance: thus a species means a group of organisms that resemble one another closely. A species can be roughly defined as a group of individual organisms sharing a common genetic identity and who are capable of reproduction among themselves. Thus all dogs, no matter whether alsatian, poodle, pit bull or dachshund, belong to the species Canis domesticus. Similarly all breeds of domestic cat, whether Persian, Siamese, Russian Blue or various long-haired varieties, are all members of the species Felis catus.
According to the Linnean system, a member of one species should not be capable of reproducing offspring by mating with a member of another species. In fact this is not a watertight law, since some species can and do interbreed, but it is not frequent in nature and seems to happen where there is still some dispute over the exact status of the species in question. In other cases, such as the horse and the donkey which together can produce the mule, viable offspring are produced but the young are infertile.
Above the species level, any group of species that has a reasonably common set of shared characteristics is placed into a genus (plural genera). For example, the domestic cat Felis catus belongs to the genus Felis, which it shares with other cat species such as the cougar Felis concolor, the lynx Felis caracal and the ocelot Felis pardalis, all of which have fairly obvious similarities. Similarly the dog Canis is placed into the genus Canis together with the wolf, Canis lupus, the coyote Canis latrans and other species. Some species are so unique or unusual that they have a very small genus: for example the poisonous Gila Monster of the Arizona desert, Heloderma suspectum, is one of just two members of the genus Heloderma, the only other member being the Mexican beaded lizard, Heloderma horridum. These are the only poisonous lizards in the world.
Most species have just two Latin or Greek style names, eg Felis catus, Gekko gecko, Canis lupus, etc. The first part of the name (always written with an initial capital) is the genus. The second part of the name is the species within that genus. Thus Felis catus refers to the genus Felix and the species therein called catus. Often you will see a species name where the genus has been abbreviated to just a capital, eg F. catus. This is usually where it is assumed the reader or listener knows which genus or group of creatures is being talked about. For example, in a book on cats it might be easier to talk about F. catus rather than writing out Felis catus every time. Similarly books on rat snakes, members of the Elaphe genus, usually abbreviate the Elaphe to just an E., eg E. situla instead of Elaphe situla. Finally, species names are usually written in italics (if printed or word-processed) or underlined (if by hand).
No two species can share the same name, but a species may have a number of subspecies. Not everybody accepts the idea of subspecies, but as it has been around for quite a while and is still in use, especially for describing wild populations, it is worth taking on board.
A subspecies can be described as a subgroup of a species which has its own distinctive genetic characteristics, eg different colouring or temperament, shorter length, etc, but which can still interbreed with other members of the subspecies. In my experience this is a particularly common concept with snakes. Often snakes which have different common names are actually just subspecies of the same snake, and can interbreed (such matches and their offspring are called intergrades). Subspecies normally have a distinct part of the area (distribution) occupied by a species: for example you often hear talk of "the western subspecies", "the eastern subspecies", etc. It seems that a certain amount of isolation or separation is necessary to produce subspecies: according to evolutionary theory, over a period of time this separation or isolation may in fact lead to subspecies becoming two entirely different species.
The naming convention for subspecies is quite simple: a third name, the subspecies name, is tacked on to the end of the species name. For example the Asian Water Monitor Varanus salvator has a number of subspecies scattered throughout SE Asia, including Varanus salvator cumingi, Varanus salvator marmoratus, Varanus salvator nuchalis and Varanus salvator salvator. Where a subspecies name is the same as that of the species name (eg Varanus salvator salvator), this subspecies is known as the nominate subspecies. Needless to say, writing all this out leads to abbreviation, so with subspecies the first and second part of the subspecies name are abbreviated to initials, eg V. s. salvator, V. s. marmoratus, V. s. cumingi and so on.
It should finally be noted that at subspecies level everything is in a state of flux nowadays, especially with certain groups of animals. Every year some subspecies gets promoted to being a full species after research, deliberation or observation, and similarly some species are demoted to being simply a subspecies of a species they were considered closely related to. But as a way of describing variations within a species, the subspecies concept is a useful one.
Above the level of the genus, general are organised into a family. Once again this is done on the basis of common characteristics, but more basic ones than those of the species. For example one of the big differences between agamid lizards such as water dragons, bearded dragons and dabtails, and iguanid lizards such as green iguanas, marine iguanas, collared lizards and green anoles is the positioning of the teeth. Agamid lizards (Family Agamidae) all have acrodont teeth, ie teeth fused together and set along the upper front of the jaw, whereas iguanid lizards (Family Iguanidae) all have pleurodont teeth, ie teeth set separately, each in its own socket, along the edge of the jaw . This is one of the things that sets them apart as families.
Families in turn are grouped into an order, or sometimes a suborder. For example, the Order Squamata contains the three suborders Sauria (all lizards), Serpentes (all snakes) and Amphisbaenidae (all amphisbaenians).
Orders are grouped into classes: eg, the orders Chelonidae (turtles and tortoises), Squamata (lizards, snakes, tuatura and amphisbaenians) and Crocodilia are placed together in the class Reptilia. Note that a class can also be expanded to include its extinct members: this can be an interesting exercise since the extinct orders may outnumber the living ones considerably! Similarly, Class Avia contains all orders of birds, and Class Amphibia, all orders of amphibians. There are two classes of fish: Class Chondroichthys (fish with cartilage skeletons, such as sharks and rays) and Class Osteichthys, bony fish.
The next level of division is the phylum (called division in the plant kingdom). Phyla (plural of phylum) tend to divide animals into very large groups, eg all molluscs (Phylum Mollusca) or all vertebrates (Phylum Notochordata, which strictly speaking also includes a strange group of animals, the tunicates, which are not strictly vertebrates but which have a similar "backbone"). "True" vertebrates are considered to be the Subphylum Chordata, which comprises Class Chondroichthys (Cartiliginous Fishes) Class Osteichthys (Bony Fishes), Class Amphibia (Amphibians), Class Reptilia (Reptiles), Class Aves (Birds) and Class Mammalia (Mammals).
The most fundamental level of life is the kingdom. There are actually two or three different kingdom models, depending on whose point of view you accept: animal (Kingdom Animalia), plant (Kingdom Plantia), and again depending on which viewpoint you take, Fungus and/or Protists, single-celled organisms that seem to act in some ways like animals, in others like plants, and in a few cases like fungus, plus some bacterial kingdoms. From now on we are just concerned for the purposes of this discussion with the Kingdom Animalia.
It is remarkable that although Linnaeus knew nothing about genetics or evolutionary theory, and believed that all species were fixed and could not change, his system of binomial classification has endured up to the present day. Although there are now other ways of defining the relationships of organisms to one another, such as cladistics or DNA analysis, none of these has seriously threatened to replace the old system, although obviously research and new scientific methods have led to a revision of much of the original species names and classifications. Although it is realised that to a certain degree the system is artificial, it is convenient, does seem to express basic relationships reasonably well, and provides a universal system whereby human beings the world over can know which animal they are referring to.