Each species is universally identified by a binomial consisting of the…

The idea

Every species has a two-word Latin name, the binomial: the first word is the genus and begins with a capital letter, the second is the species and does not, as in Homo sapiens or Luscinia svecica. In print the binomial is italicised; in handwriting it is underlined. After the first use the genus can be shortened to its initial, which is why exam papers write C. difficile and E. coli. The system is universal, so a species carries the same name in every country and every language, which removes the confusion caused by common names: one common name may be used for several species, and one species may have several common names in different regions. Because the first word is the genus, the binomial also carries classification information, so two species sharing the first word sit in the same genus and are closely related.

What earns the marks

• Give the binomial as two words in the order genus then species, since the species word on its own is not the binomial.
• When completing a classification table, put both words of the binomial in the species row.
• Explain that the name is universal, so the same species is identified by the same name everywhere.
• Use a shared genus name as evidence that two species are closely related.
• Distinguish between common names and scientific names, explaining why the binomial system is more useful to scientists.

Common mistakes and how to put them right

• Capitalising the species word of the name, or capitalising neither word.

  Put it right: The genus begins with a capital letter; the species word does not.

• Giving only the species word when the question asks for the binomial.

  Put it right: The binomial requires both genus and species words.

• Giving the common name when the question asks for the scientific name.

  Put it right: Use the binomial, not a common name.

• Assuming two organisms that share the species word, such as two unrelated species called officinalis, must be related.

  Put it right: Only a shared genus shows close relatedness.

• Treating the abbreviation in E. coli as part of the species name rather than as the shortened genus.

  Put it right: E. stands for the genus Escherichia.

Exam tips

• The capital letter and the underlining are conventions of scientific writing; what is credited is both words, in the order genus then species.
• If a table gives a genus in one column and a species in another, join them in that order to build the binomial.
• Papers may shorten the genus to its initial once a species has been named in full, as in C. difficile and E. coli, so read those as binomials and write both words out whenever a question asks you to name the species.