There is not just one species of bearded dragon.
Although in the UK, the majority of dragons are the Inland bearded dragon (Pogona vitticeps), there are other species around in captivity, as well as several different wild ones.
This is a quick look at these other species, as well as explaining the broader groups that bearded dragons fall into and who they are most related to.
"Bearded Dragon" can be used to describe any lizards in the genus Pogona; it's not unique to a single species, although in practice it almost always refers to the inland bearded dragon (see below).
There are currently seven or eight different species of beardie recognised - seven are widely recognised as being separate species, but there is some debate as to whether Pogona mitichelli is a separate species, or just a sub-species of Pogona minor. By far the most common in captivity is Pogona vitticeps, the Inland (or Central) Bearded Dragon. Also widely available is the Rankins Dragon (Pogona henrylawsoni). The other six species are almost non-existant in the UK, although some may be kept in small numbers in Europe or the US. The reason for this is the Australian ban on the export of Fauna - this means that no Bearded Dragons can be caught from the wild and exported to the rest of the world for breeding, so in effect we're stuck with the species that we captured, exported and bred before the export ban was put into place.
There is some speculation that the original "German Giant" morph was created by hybridising P.vitticeps and P.barbata, although it may have originated from a localised population of larger P.vitticeps.
P. vitticeps and P. henrylawsoni are known to be compatible and produce fertile hybrids known in the UK as "Vittikins Dragons". Due to the scarcity of other species, it's not known whether or not they are able to hybridise or not.
There are also suggestions that the old lineage of "German Giants" (a now very diluted line of dragons, which - in the "pure" genetic form - can apparently get to a larger size than regular dragons) started as a hybridisation of P. vitticeps and P. barbata.
Pogona vitticeps - Inland / Central Bearded Dragon
Pogona barbata - Eastern / Coastal Bearded Dragon
Pogona henrylawsoni - Rankins Dragon
Pogona nullarbor - Nullarbor Bearded Dragon
Pogona minor - Dwarf Bearded Dragon
Pogona minima - Western Bearded Dragon
Pogona microlepidota - Kimberly Bearded Dragon
( Pogona mitchelli ) - Northwest Bearded Dragon (possibly a P. minor subspecies)
Under the standard classification system, bearded dragons (all 7/8 species of them) are in the Genus Pogona, which is within the Family "Agamidae" (the agamid lizards), who represent the closest living relatives of Bearded Dragons. Also in this group are lizards such as the Dab-Tails (genus Uromastyx), the Sail-Finned lizard (Hydrosaurus), the Flying Lizards (Draco), Water Dragons (Physignathus) and the Frilled Dragons (Chlamydosaurus), along with several other groups of lizards which are much less common in captivity, such as the Thorny Devil (Moloch horridus). These are the lizards which Bearded Dragons are most closely related to.
The Agamidae family is placed within the sub-order "Iguania", which also includes Chameleons, Iguanas, Chuckwallas, Collared Lizards, Anoles and a few other groups. This means that bearded dragons are more closely related to Chameleons, Anoles etc than they are to other lizard sub-orders, such as Geckos, Skinks etc. Iguania is within the order "Squamata", which is composed of all Lizards, all Snakes, and all Amphisbaenia.
Squamates and Sphenodontids form the group "Lepidosauria". Today Sphenodontids are only represented by the Tuataras (Sphenodon), although in the past they were very widespread, with a large number of species, including some aquatic forms and some very large forms.
Lepidosauria is part of the Class "Reptilia", which also includes groups such as the crocodiles, tortoises, the extinct dinosaurs, and various other groups such as the giant, extinct marine reptiles such as Ichthyosaurs. Technically birds should also be included within Reptilia in order to make it "monophyletic".
Reptilia is within the Phylum "Chordata", which includes all vertebrates - birds, reptiles, amphibians, fish, mammals etc. Humans are chordates. It's believed that the Chordate group as a whole is most closely related to the "Echinoderms", which include Starfish, Urchins and Sand Dollars.
One of the main reasons for the popularity of dragons is the variety of different colours they come in, as discussed in our Beardie Genetics section.
Virtually all "morphs" of bearded dragon are the same species - Pogona vitticeps. The exceptions to this are the "Vittikins" dragon (a hybrid of P. vitticeps and P. henrylawsoni), and possibly the "German Giant" dragons (a possible hybrid of P. vitticeps and P. barbata). This means that when someone is talking about a "morph" of Bearded Dragon, they are just referring to one species.
Hopefully that gives a quick overview of who the bearded dragons are and who they are most closely related to!