Variation and Classification: KS3 Biology Explained

Variation describes the differences between organisms of the same species, while classification is the system scientists use to group all living things based on shared characteristics. Together, these ideas explain both why no two robins look identical and how a dolphin ends up grouped with humans rather than fish.

What causes variation between organisms?

Variation has two main causes: genetic (inherited) and environmental.

Genetic variation arises from differences in DNA. Your genome is a unique combination of your parents' genes, shuffled by the process of sexual reproduction. This is why siblings share some features but are not identical.

Environmental variation arises from conditions during an organism's lifetime. A plant grown in poor light will be taller and paler than one grown in bright sunlight, even if both are genetically identical. Identical twins have the same DNA yet can differ in weight, fitness, and even some disease risk — driven by environment and lifestyle.

Most variation in living organisms is a mix of both. Human height, for example, depends on genes you inherited (genetic) and how well-nourished you were during childhood (environmental).

What is the difference between continuous and discontinuous variation?

Type | Description | Example
Continuous | Ranges across a smooth spectrum of values | Height, mass, hand-span, skin tone
Discontinuous | Falls into distinct, separate categories | Blood group (A, B, AB, O), tongue-rolling ability, sex

Continuous variation produces a bell-shaped (normal distribution) curve when graphed. Most people cluster around the average, with very few at the extremes.

Discontinuous variation produces a bar chart with clear, separate bars. There is no height in between blood group A and blood group B — you are one or the other.

A common exam question asks you to identify which type of variation is shown by a given graph — if the x-axis shows categories with gaps between them, it is discontinuous; if it shows a continuous scale, it is continuous.

How do scientists classify living things?

Classification organises organisms into groups based on shared characteristics. The modern system, developed from Carl Linnaeus's 18th-century work, arranges life into a hierarchy of nested groups called taxa:

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

A useful mnemonic: King Philip Came Over For Good Spaghetti.

There are five (sometimes six or eight, depending on the scheme) kingdoms: Animals, Plants, Fungi, Protists, and Prokaryotes (Bacteria). KS3 focuses mainly on the first three.

What is a species?

A species is a group of organisms that share similar characteristics and can interbreed to produce fertile offspring. This is the most important level of classification — and the hardest to define precisely.

Wolves and domestic dogs are the same species (Canis lupus familiaris is a subspecies of Canis lupus) and can interbreed. Horses and donkeys are different species (Equus ferus caballus and Equus africanus asinus): they can mate, but their offspring (mules) are sterile — so they fail the fertile-offspring test.

The scientific name of a species is always written in binomial nomenclature: Genus first (capital letter), species second (lower case), both in italics. Humans are Homo sapiens.

What are the main animal groups and how are they classified?

Animals are divided into vertebrates (with a backbone) and invertebrates (without a backbone). The five main vertebrate classes are:

Class | Features | Example
Fish | Scales, gills, cold-blooded | Salmon
Amphibians | Moist skin, lay eggs in water, cold-blooded | Frog
Reptiles | Dry scaly skin, lay eggs on land, cold-blooded | Lizard
Birds | Feathers, beak, warm-blooded, lay hard-shelled eggs | Robin
Mammals | Hair/fur, give birth to live young, warm-blooded, suckle young | Human

Invertebrates include insects, arachnids, molluscs, worms, and many others — they make up over 95% of all animal species.

How does natural variation link to evolution?

Variation is the raw material for natural selection and evolution (covered separately in its own guide). When individuals in a species vary, some variants will be better suited to their environment than others. Better-suited individuals survive longer, reproduce more, and pass their advantageous characteristics to offspring. Over many generations, this shifts the average characteristics of the whole population — which is evolution.

Classification and variation are therefore not just a naming exercise; they are the lens through which biologists read the history of life on Earth.

Frequently asked questions

What is variation in biology KS3?

Variation means the differences in characteristics between organisms — even within the same species. It can be caused by differences in genes (inherited variation), differences in environment, or a combination of both. No two organisms, other than identical twins at the moment of conception, are genetically identical.

What is the difference between continuous and discontinuous variation?

Continuous variation produces a range of values with no clear gaps, such as height or mass, and gives a bell-shaped curve on a graph. Discontinuous variation falls into clear, separate categories, such as blood group or eye colour, and gives a bar chart with distinct bars.

How do scientists name organisms?

Scientists use binomial nomenclature, a two-part Latin name invented by Carl Linnaeus. The first part is the genus (e.g. Homo) and the second is the species (e.g. sapiens). The full name is italicised, with the genus capitalised. This system is used internationally so scientists worldwide can communicate without confusion.

What is the difference between vertebrates and invertebrates?

Vertebrates have a backbone (vertebral column) and include fish, amphibians, reptiles, birds, and mammals. Invertebrates have no backbone and include insects, spiders, worms, and molluscs. Invertebrates make up the vast majority of animal species on Earth.

Does the environment affect variation?

Yes. Environmental factors such as diet, sunlight, exercise, and disease can change how genes are expressed, affecting characteristics such as height, skin colour, and mass. These environmentally caused changes are not passed on to offspring, which distinguishes them from genetic variation.

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