How to … Choose a type specimen for a name of a new species (or infraspecific taxon)
A type specimen is the specimen to which a name is permanently attached. Choosing a good type specimen is important to ensure that future workers can properly understand and assess named taxa.
A specimen is defined as a gathering, or part of a gathering, of a single species or infraspecific taxon, disregarding admixtures (Art. 8.2).
A gathering is defined as a collection presumed to be of a single taxon made by the same collector(s) at the same time from a single locality (Art. 8,2 footnote).
To be eligible as a type, the specimen must be preserved permanently and may not be a living organism or a culture (Art. 8.4), although fungi and algae may be preserved in a metabolically inactive state.
A specimen can consist of a single organism, parts of one or more organisms, or of multiple small organisms; the specimen is usually mounted on a single herbarium sheet or in an equivalent preparation, such as a box, packet, jar, or microscope slide; it may be mounted on more than one sheet, slide, etc. so long as the parts are clearly labelled as being part of that same specimen, or bear a single, original label in common; otherwise the parts are duplicates, i.e. separate specimens that belong to the same gathering (Art. 8.2, 8.3, and 8.3 footnote).
When choosing a type, ensure that:
it is reasonably typical for the species or infraspecific taxon you are naming
it adequately shows the diagnostic characters mentioned in the protologue
it is deposited at a registered herbarium where it will be available for study by other workers
if possible, it has duplicates in other herbaria
While only one specimen can serve as the holotype (Art. 9.1), in some circumstances further specimens can be designated as paratypes (Art. 9.7). For example, if both flowers and fruits are diagnostic and the holotype is flowering, a fruiting specimen could be designated as a paratype. Similarly, for unisexual organisms a specimen of one sex may be designated as the holotype while a specimen of the other sex may be designated as a paratype. However, there is no rule against both flowering and fruiting (or both male and female) material being represented in a single gathering or indeed in a single specimen (see definitions above), including the holotype.
Note that, for non-fossil microscopic algae or non-fossil microfungi, a suitably diagnostic illustration (usually a photomicrograph) can serve as a holotype if it is not practical to preserve a specimen (Art. 40.5). Otherwise, an illustration cannot be a holotype of a name published from 2007 onward (Art. 40.4).
For more on other types of types, see How to understand the types of types.
For more information, see Chapter 7 How to designate a type in The Code Decoded.