# Binomial System ## What is a species? - A **species** is a group of organisms that are able to reproduce to produce **fertile offspring** - Members of the same species can successfully mate and their offspring can also reproduce - This is the key criterion that defines a species ## Binomials - Scientific names - **Binomials** are the scientific names given to individual species - Each binomial consists of two parts: the organism's **genus** and **species** name in modern Latin - Examples of binomials: - The binomial for humans is *Homo sapiens* - The binomial for domestic cats is *Felis catus* - The binomial for the European robin is *Erithacus rubecula*

A binomial is a two-part Latin name used to identify a species, consisting of the genus name followed by the species name

## Why use binomials? - Binomials are extremely useful for scientists as they allow species to be **universally identified** - The binomial for a species is the **same across the entire globe**, regardless of language or location - This prevents confusion when scientists from different countries communicate about organisms ## The problem with common names - Species are often given **common names**, but these names frequently differ between countries - Common names do not always translate directly between different languages - Different regions may use completely different common names for the same species - For example, the bird *Prunella modularis* is called a "dunnock" in England, a "hedge sparrow" in some regions, and has different names in other languages ## History of the binomial system - To avoid confusion about what group of organisms scientists are discussing, all species are given a **two-part scientific name** using the binomial system - This naming convention was **developed and established** by the Swedish scientist **Carl Linnaeus** in the **18th Century** - Linnaeus created a standardized system that is still used today across all of biology ## Rules for writing binomials - The binomial name is always **italicized** in writing (or **underlined** if it is not possible to italicise) - For example: *Quercus robur* or Quercus robur

The genus name must have a capital letter but the species name must not

### Abbreviating binomials - It is common to **abbreviate the genus name** after the first mention - Only the first letter of the genus is used, followed by a period - The species name is always written in full ### Examples of binomial usage - The most commonly studied single-celled fungus is *Saccharomyces cerevisiae* (baker's yeast) - After first mention, this can be abbreviated to *S. cerevisiae* - The genus *Saccharomyces* contains many species of yeast - *Saccharomyces pastorianus* is another species of yeast that belongs to the same genus as *S. cerevisiae* - Both species share the genus name *Saccharomyces* because they are closely related - The different species names (*cerevisiae* and *pastorianus*) distinguish them as separate species ## Benefits of binomial nomenclature | Common names | Binomial names | |--------------|----------------| | Vary between countries and languages | Universal across all countries | | Can be ambiguous or misleading | Precise and unambiguous | | May change over time | Stable and consistent | | Don't show relationships between organisms | Show evolutionary relationships (genus groups related species) | | Multiple names for one organism | One unique name per species |

When writing a binomial by hand in an exam, remember to underline both parts of the name if you cannot italicize

## Reading information from binomials - The first part (genus name) tells you about the group of closely related species - All species in the genus *Canis* are closely related dog-like animals - *Canis lupus* (wolf), *Canis latrans* (coyote), and *Canis familiaris* (domestic dog) all share the same genus - The second part (species name) identifies the specific organism within that genus - The species name is unique within the genus - Different genera can have the same species name (e.g., *Quercus alba* and *Abies alba* both use "alba" meaning white)

The binomial automatically provides information about the last two taxonomic ranks: genus and species