The basic unit of classification is the:

species. A species — organisms that can breed to give fertile offspring — is the basic unit.

In the name Escherichia coli, 'Escherichia' is the:

genus. The first, capitalised word of a binomial name is the genus.

Which kingdom has cell walls of chitin and absorbs nutrients?

Fungi. Fungi absorb their nutrients and have chitin cell walls.

Which orders the hierarchy from largest to smallest?

kingdom → phylum → species. Kingdom is largest and species smallest.

Bacteria are placed in the kingdom:

Prokaryotae. Bacteria are prokaryotic single cells, placed in Prokaryotae.

Cross-breeding to fertile offspring defines members of one:

species. A species is defined by the ability to interbreed and produce fertile offspring.

A dichotomous key identifies an organism using:

paired either/or questions about features. A dichotomous key uses a series of two-way questions about observable features.

A polar bear's thick fur and fat are examples of:

adaptation to its environment. They are adaptations that help it survive the cold.

Biodiversity & Classification

The variety of life, and the system used to name and group it

This topic appears in:

Practise the full question bank:

The variety of life

Biodiversity is the enormous variety of living organisms — millions of species, each adapted to its way of life. To make sense of it, biologists classify organisms into groups based on shared features, especially features that reveal shared ancestry.

The classification hierarchy

Living things are sorted into a nested hierarchy of ever-smaller groups: kingdom, phylum, class, order, family, genus, species. Members of a lower group share more features than members of a higher one. The species is the basic unit — a group of similar organisms that can breed together to produce fertile offspring.

Rank	Example (human)
Kingdom	Animalia
Phylum	Chordata
Class	Mammalia
Order	Primates
Genus	Homo
Species	sapiens

The five kingdoms

A widely used scheme divides life into five kingdoms. Prokaryotae (bacteria) are single prokaryotic cells with no nucleus. Protista are mostly single-celled eukaryotes. Fungi feed by absorbing nutrients and have cell walls of chitin. Plantae make their own food by photosynthesis. Animalia are multicellular and feed on other organisms.

Binomial nomenclature

Every species has a two-part Latin name — the binomial system. The first word is the genus (capitalised), the second the species (lower case), written in italics: Homo sapiens. This gives every organism one unique, universal name, avoiding the confusion of local common names.

Why Latin names?

A common name like "robin" means different birds in different countries. The binomial name is unique and understood worldwide — one species, one scientific name.

A worked example

Worked example

Two organisms are named Panthera leo (lion) and Panthera tigris (tiger). What does sharing the first name tell you, and are they the same species?

The shared first word, Panthera, is the genus.The first word of a binomial name is the genus.
So both belong to the same genus and are closely related.Same genus = a close relationship.
The second words differ (leo vs tigris), so they are different species.The species name distinguishes them within the genus.

They share the genus Panthera (closely related) but are different species, so not the same species.

Identifying organisms with keys

To name an unfamiliar organism, biologists use a dichotomous key — a series of paired questions, each with two options, that narrows down the possibilities step by step until only one organism fits. Each question is about an observable feature (Does it have wings? Are the leaves needle-shaped?), and following the answers leads to the organism's name. Keys turn classification into a practical identification tool anyone can use.

How a dichotomous key works

'Di-chotomous' means 'splitting in two'. At each step you choose between two statements; your choice sends you to the next pair, and so on until you reach a name. Good keys use clear, observable features.

Adaptation to the environment

The variety of life exists because organisms are adapted to their surroundings — they have features that help them survive and reproduce where they live. A cactus has a thick waxy skin and spines to survive the desert; a polar bear has thick fur and fat for the cold; a fish has gills and a streamlined body for water. These adaptations are the product of evolution, and they explain why different habitats hold such different species.

Practice questions

6 questions · 20 marks · full working on every one

Try each one on paper first, then open the working. The marks are shown where they are actually awarded, because that is where they are actually lost.

Short questions

4 · 9 marks

Two marks each, in the style of the short-question section of the paper. Answer in two or three lines.

SQ1[2 marks]

State the rules for writing an organism's scientific (binomial) name.

Model answer

Two Latin words: the genus first (capital letter) and the species second (lower case); the whole name is italicised (or underlined), e.g. Homo sapiens.

Examiner tip. Genus capitalised, species lower case, both italicised.

SQ2[3 marks]

Name three of the five kingdoms and give one distinguishing feature of each.

Model answer

Any three, e.g. Prokaryotae — no nucleus; Plantae — photosynthesise; Animalia — multicellular, feed on other organisms; Fungi — absorb nutrients, chitin walls.

Examiner tip. Give a defining feature, not just the name.

SQ3[2 marks]

Explain what a dichotomous key is and how it is used.

Model answer

A dichotomous key is a set of paired statements or questions, each offering two choices about an observable feature. At each step you pick the option that matches the organism, which directs you to the next pair, until you reach the organism's name.

Examiner tip. Two choices at each step, narrowing down to one name.

SQ4[2 marks]

State two adaptations of a named organism to its habitat.

Model answer

For example a cactus: a thick waxy cuticle to reduce water loss, and spines (reduced leaves) that lower water loss and deter herbivores. (Any organism with two valid habitat adaptations is acceptable.)

Examiner tip. Give two features and link each to survival in the habitat.

Long questions

2 · 11 marks

Theory and numerical together, as they appear in the long-question section.

LQ1[6 marks]

Classification organises the diversity of life.

Define the term species. [2]
List the classification hierarchy from kingdom to species. [2]
Explain one advantage of the binomial naming system. [2]

Mark scheme

A group of similar organisms[1]
that can breed together to produce fertile offspring[1]
Kingdom, phylum, class, order,[1]
family, genus, species[1]
Each species has one unique, universal name[1]
understood worldwide, avoiding the ambiguity of common names[1]

Species = organisms that interbreed to give fertile offspring; hierarchy kingdom→…→species; binomial names are unique and universal.

Examiner tip. Define species by fertile offspring; give the full 7-rank hierarchy.

LQ2[5 marks]

Classification and identification help biologists study biodiversity.

State two reasons biologists classify organisms. [2]
Explain why a scientific (binomial) name is more useful than a common name. [2]
Name the smallest group in the classification hierarchy. [1]

Mark scheme

To identify organisms and organise the diversity of life[1]
and to study their relationships / shared ancestry[1]
A binomial name is unique to one species[1]
and is understood worldwide, avoiding the ambiguity of common names[1]
Species[1]

Classify to identify/organise and show relationships; binomial names are unique and universal; the smallest group is the species.

Examiner tip. Species is the base unit; binomial names avoid the confusion of common names.