I wanted to write about this one because there are a lot of misconceptions about how this is actually done. It is important to understand the process, by understanding a process we can understand why we would do something. So in this case by understanding the connection between nomenclature and taxonomy we can understand why I will recommend certain names be used.
When a species is described the actual published description is attached, or based, on a single specimen called the holotype. This specimen is a representative of a wild population. As a taxonomist I see types as so important I go to great lengths to ensure that any species I name is actually represented correctly and with useable material. There is nothing worse than have holotypes of different species that are effectively not comparable, for example, one species being represented by an adult and another by a juvenile. There are many changes in the morphology between juvenile and adult due to growth and maturity, likewise there can also be differences between the sexes.
Although not required I have come to the decision, over time, to only set adult females as Holotypes. I also set a Paratype which is an adult male and declare it the Allotype. A Paratype is one or more individual specimens examined at the same time as the holotype and declared as such in the original description, an Allotype is a specimen of opposite sex to the holotype, also declared at the same time as the Holotype. As I said the way I do this is not required, I can in fact set any specimen that is representative of the species I am describing as the Holotype and I do not have to set a Paratype or an Allotype. I do not think taxonomists working on other species should follow my view on this. My reasons for adult females are two fold, first by choosing one or the other and following it I have taken sexual dimorphism out of the issue when comparing types, second I choose females because they are somewhat easier to catch in turtles, they bask more, come out of the water more, go into traps easier, hence it is a little more likely that anyone comparing freshwater turtle species will be looking at females. An equally valid argument can be made in snakes, for example, to set males as holotypes since hemipene morphology is an important diagnostic character. But this is all preference, entirely up to the person working on the group. The only compulsory part under the ICZN Code is that a holotype is set.
The name for the taxon is attached to this holotype so whatever that holotype turns out to be that is where the name goes. As I discussed in the Chelodina oblonga article a misidentified holotype is generally a difficult issue, often requiring the intervention of the ICZN. It will often mean that a name, well used and understood, is going to be moved to another species. All the previously published literature on that species will now be using the wrong name. This can create a lot of confusion, and believe it or not anger. It also in vertebrates such as reptiles creates legal issues, as many are protected by various local and international laws and agreements.
The part of all this that attaches the name to the holotype and determines if that name is available for use is nomenclature, and is governed by the ICZN Code. The term available has a very specific meaning in Nomenclature. It means that the name has been published in accordance with the ICZN Code and has sufficient details in the description to meet Articles 10 to 20 of the Code. So put simply this would mean the original description identifies a type specimen, provides diagnostic characters that claim to differentiate the taxon and is not an identical name to another species in the same genus. There are several other minor rules also and readers can look at Articles 10 to 20 of the code for themselves to see what a description needs to have. Feel free to ask me questions on this.
So if the name is available the next part is the taxonomy. The population represented by the holotype is demonstrated by acceptable means to be diagnosable different to other populations. This can be done with morphology, molecular work and statistics. I personally prefer to see all three. Now that is taxonomy, this is where the two connect through that holotype which is a part of the taxonomic analysis, and of course is attached to the name. Taxonomy and Nomenclature are separate fields, but are also intrinsically entwined in their practice. The taxonomy will produce, among other things, a synonymy and this is a list of the available names in order of Priority, that is, in order of usage by date of publication. The oldest name, is the one to be used for the taxon. Or a new one is proposed if there was none, and that leads back to nomenclature.
The Holotype is, therefore, the most important specimen of any species. If you go to a museum to examine specimens of a species you are working on you will soon see this. The way Holotypes are stored, and who has access to them, and the fact that they generally cannot be loaned out shows the higher level of importance placed on them. Many older names have problematic types. Many do not have one at all, or they may have a type series, called syntypes. This is because the concept of types came into zoology some 100 years after Linnaeus so early descriptions do not have them actually set. This is fine under the Code; it’s not fair to hold someone’s work to a set of rules that came into being over 100 years after they wrote the paper. However, it is important to try and identify which specimens were used in the description and this is one of the causes of confusion in nomenclature with older names.
If there is a series of syntypes and there is potential for confusion, eg more than one species represented by the syntypes (eg Chelodina expansa, Emydura victoriae are both in this position) a lectotype is often set. A lectotype is a specimen from the type series that has been determined to be the name-bearing type, that is, the one the name is actually attached to. This is done by a later author who is reviewing the issues. If a holotype is absent altogether and there is good reason to expect confusion, a neotype can be set. This is again a name-bearing type that is equivalent to a holotype and will now represent the name. Again this must be justified and done by a later author who is reviewing the problem at hand.
The one thing I will emphasize above all else, examination of the types. If you want to publish a work naming a new species, or rearranging them significantly, you must examine all the relevant types. Many specimens in museums are misidentified, this is not their fault they have the label that the museum was given when they got them, this could be 50 or 100 years ago, even older. If you do not examine the types you cannot guarantee that a name is attached to the correct species. The two examples I mentioned before; Chelodina expansa has two syntypes, one is a C. expansa, one is a C. longicollis; Emydura victoriae has two syntypes, one is an E. victoriae, one is an E. macquarii. In the end seriously, examine the types, after all do you want to be remembered for the species you named, or the mess you made?