Taxonomy: Ranks, Species and Binomial Nomenclature

What's in a name?

In Canada, a robin is a large thrush with a brick-red breast that pulls worms out of lawns in spring. In Britain, a robin is a much smaller bird from a different family. In Europe, the animal Canadians call a moose is called an elk, while the North American elk (also called wapiti) is a different deer altogether. Imagine two biologists trying to compare research on "elk"!

To avoid this confusion, every species has one scientific name used by scientists in every language. The American robin is Turdus migratorius. The moose is Alces alces. Today you will learn how these names work and how organisms are arranged into a hierarchy of groups.

Why classify?

There are millions of species. Taxonomy is the science of describing, naming and classifying them. A good classification system:

• gives every species one unique, universal name;
• puts organisms in groups that show relationships (organisms in the same group share an ancestor);
• lets you predict features. If you learn that a new animal is a mammal, you can predict it has hair and feeds its young milk.

A bit of history. The Greek philosopher Aristotle (4th century BCE) sorted animals by features such as whether they had red blood and where they lived. In the 1700s the Swedish botanist Carl Linnaeus built a system of nested groups based on shared physical features, and made the two-part binomial name standard (for plants in 1753, for animals in 1758). We still use his naming system. What has changed is how we decide groups: today they are based on evolutionary relationships, using both morphology and DNA.

The taxonomic ranks

Every species belongs to a series of nested groups, called taxa (singular: taxon). From most general to most specific, the eight main ranks are:

Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species

A memory aid: Dear King Philip Came Over For Good Soup.

Each rank is inside the one above it. As you move down, each group contains fewer kinds of organisms, and the members share more characteristics. (Botanists sometimes use the word division instead of phylum.)

Binomial nomenclature: the rules

A species name has two parts:

1. The genus name, which always starts with a capital letter.
2. The specific epithet, which is always all lower case.

The whole name is written in italics when typed, or underlined when handwritten. After the first full use, the genus can be shortened to its first letter: Acer saccharum, then A. saccharum.

Names are in Latin or Latinized words, and they often describe the organism or honour a person or place. Saccharum means sugar; canadensis means "of Canada"; borealis means northern.

The epithet alone is not a name. canadensis is used for the lynx (Lynx canadensis), the beaver (Castor canadensis), the Canada goose (Branta canadensis) and many plants. Only the pair is unique.

Ranks above genus (such as family Canidae) are capitalized but not italicized.

What is a species?

The most widely used definition is the biological species concept: a species is a group of populations whose members can interbreed in nature and produce fertile offspring.

A horse and a donkey can mate, but their offspring, a mule, is almost always sterile. So horses and donkeys are separate species. Things that keep species apart, such as different mating seasons, different courtship behaviours or sterile hybrids, are called reproductive isolation.

The biological species concept has limits:

• It cannot be tested on fossils.
• It does not work for organisms that reproduce asexually, such as bacteria.
• Some separate species occasionally produce fertile hybrids (wolves and coyotes in eastern Canada sometimes interbreed).

So biologists also use morphology and DNA to separate species. You will look at how new species form in the Evolution unit.

Canadian taxonomy today

Taxonomy is an active science. At the University of Guelph, Canadian biologist Paul Hebert proposed DNA barcoding in 2003: identifying species from a short, standard stretch of DNA, much like a grocery barcode. His team founded the Biodiversity Institute of Ontario, and barcoding is now used around the world to identify species and find new ones.

Scientific classification is also not the only knowledge system for living things. First Nations, Métis and Inuit languages contain detailed names for local plants, animals and fungi, often grouped by relationships, behaviour, seasons and uses. Many researchers now work with Indigenous knowledge holders, an approach that Mi'kmaw Elder Albert Marshall calls Two-Eyed Seeing: using the strengths of Indigenous knowledge and Western science together.

Careers: taxonomists, museum curators, molecular biologists and conservation biologists all use classification. Most have a university degree in biology, and many have a master's degree or PhD.