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All organisms (apart from viruses) have a two‐part name, or binomen – hence, the term binomial nomenclature. The two parts consist of the generic (or genus) name and the trivial name (also called the specific epithet or specific name).

The trivial name may be followed by the naming authority, i.e., the name of the person who first described the organism, along with the date the description was published – this is placed in brackets.

Italics

‘normal’ script

Ascaris lumbricoides (Linnaeus, 1758)

Genus

specific epithet

naming authority

Genus + specific epithet = the binomen, also called the ‘species name’.

Surprisingly, there is no universally accepted definition of what is meant by the term ‘species. Indeed, there are currently over 20 different definitions. Furthermore, over the course of thousands of years, there is never a single point at which one species becomes two: it is like attempting to identify the day one ceases being a child and becomes an adult. To further complicate matters, some species have distinct forms that are called sub‐species, and these are distinguished through the use of trinomens. For example, the human body louse Pediculus humanus humanus and the head louse Pediculus humanus capitis are usually distinguished as separate sub‐species. However, for many years there has been a debate about whether the reported differences in their morphology and behaviour are consistent enough to justify them being considered closely related sub‐species or separate species in their own right. Current molecular evidence suggests that they are morphotypes of a single species (Light et al. 2008). Similarly, there are two physiological variants of the mosquito Culex pipiens: Culex pipiens pipiens, which bites only birds, and Culex pipiens molestus, which only bites humans. The two variants of Culex pipiens cannot be differentiated morphologically. They can be crossed in the laboratory, but, in the United Kingdom, the populations remain genetically isolated in the wild. Distinguishing between the variants is important because this mosquito can act as a vector for the potentially fatal West Nile Virus, and therefore its biting behaviour has a major impact on whether the disease spreads from birds to humans.

The difficulty of differentiating between species and sub‐species can give rise to ‘taxonomic inflation’ in those groups that are particularly well studied. For example, ant taxonomists tend not to recognize sub‐species, so everything is separated at the species level. By contrast, butterfly taxonomists are enthusiastic users of trinomens. Not surprisingly, this often results in ecological surveys revealing a greater species diversity of ants than butterflies.

Some scientists state that the increasing use of the phylogenetic concept of species rather than the older biological species concept is driving taxonomic inflation. The traditional biological species concept operates on the premise that two organisms should be considered different species if either they are incapable of mating or, if they do mate, then their progeny are infertile. Allowances have to be made for the likelihood of gene flow between populations. For example, the fact that tigers and lions can hybridise does not mean that they are the same species, as this would never happen in the wild. By contrast, the phylogenetic species concept is based on the observation that separate populations of organisms often have distinct inheritable differences (for example, a colour pattern or the size of a body part). What constitutes an inheritable difference sufficient to qualify a population as a ‘species would depend upon the views of the taxonomist. The increasing use of DNA analysis by taxonomists has undoubtedly contributed to the popularity of the phylogenetic species concept, because it often identifies differences in gene sequences between populations. Some workers have suggested that the adoption of a phylogenetic species concept can result in up to 48% more species than the biological approach for the same group of organisms (Marris 2007). It also causes complications by suggesting some unlikely taxonomic relationships. For example, for many years, microsporidian parasites were classified as protozoa until molecular studies indicated they have a closer relationship to fungi. However, molecular evidence is not always conclusive, and some scientists now consider that the microsporidians should be moved back to the kingdom Protista.