Topic 1.2 · Core
The Binomial System
Scientists need one consistent way to name a species, and common names fail at that job in three separate ways: one species can have different common names in different places, the same common name can refer to different organisms depending on where you are, and scientists working in different languages still need a name they all recognise.
The binomial system is an internationally agreed system in which the scientific name of an organism is made up of two parts showing its genus and species. Human beings, for example, are Homo sapiens. The first word, Homo, gives the genus. The second word, sapiens, gives the species.
There’s a fixed writing convention Cambridge expects you to follow exactly: the genus begins with a capital letter, the species begins with a lower-case letter, and both words are normally written in italics when typed. Getting the capitalisation backwards, or writing the species name with a capital, is the kind of small slip that costs a mark despite the biology being completely correct.
Organisms placed in the same genus share enough important characteristics to be classified more closely together than organisms placed in different genera — the genus name itself is already carrying classification information, before you even reach the species name. The whole point of the system is that it lets scientists anywhere in the world use the exact same recognised name for a species, regardless of what that organism happens to be called locally.
Naming a species this way only works once you’re confident about what a species actually is — the binomial name is a label for the group defined by the ability to produce fertile offspring, not a separate concept of its own.