We can understand the evolutionary and ecological relationships between organisms if we can classify them.


Organisms can be classified into groups according to characteristics that they share. The first big division of living things in the classification system is to put them into one of five kingdoms. These are based on what an organism's cells are like. The table shows the names of the kingdoms and examples of the sort of organisms they contain.

Kingdoms and organisms

Animalia- goat

Cells do not have a cell wall. Multicellular and feed on other organisms.All multicellular animals, including: jellyfish, worms, arthropods, molluscs, echinoderms, fish, amphibia, reptiles, birds and mammals

Cells have a cellulose cell wall. They use light energy to produce food by photosynthesis.All green plants, including: algae, ferns and mosses (plants that do not produce seeds); conifers and flowering plants (plants that do produce seeds)

Cells have chitin cell walls. Reproduce using spores rather than seeds.Moulds, mushrooms, yeast

Have a cell wall but not made from cellulose. Cells have no nucleus.Bacteria, blue-green algae
An ameoba, a simple unicellular organism

An ameoba, a simple unicellular organism

Exist as single cells or colonies of single cells.Amoeba, Paramecium


The first rank in the classification system is called a kingdom. There are five kingdoms, based upon what an organism's cells are like:

  • animals (all multicellular animals)
  • plants (all green plants)
  • fungi (moulds, mushrooms, yeast)
  • prokaryotes (bacteria, blue-green algae)
  • protoctists (Amoeba, Paramecium).

Further divisions

There are several further ranks before we reach a particular species. In order, these are:

  • kingdom
  • phylum
  • class
  • order
  • family
  • genus
  • species.

For example, lions have the following classification:

  • kingdom - animal
  • phylum - vertebrate
  • class - mammal
  • order - carnivorous
  • family - cat
  • genus - big cat
  • species - lion.

One way to remember this is by using a daft sentence like this one:

'Kevin plays clarinet or flute - grotty sound!'


Invertebrates are animals without backbones. Arthropods are an important group of invertebrates. There are four smaller groups of arthropods, based on how many legs they have:

Arthropods and their characteristics

Close up of ant. Credit: Henning K. v. Vogelsang

An ant

Insects (6 legs)


Arachnids (8 legs)
Crustacean - a crayfish Credit: Mark Gillow

Crustacean - a crayfish

Crustaceans (10 to 14 legs)
Tropical Millipede. Credit: Stuart Wilson

Myriapods - A millipede

Myriapods (centipedes and millipedes – more than 20 legs).

You should be able to use these characteristics to classify a given arthropod.

Higher tier

Classification systems can be artificial. They use obvious differences in features so that the organism can be identified. Biologists often use ‘field guides’ so that they can work out what an animal or plant is. However, these classification systems may not accurately reflect the evolutionary relationships between organisms. For this, natural classification systems based on DNA are needed.

Today, scientists can sequence the DNA of many organisms relatively easily. Computer programs compare the DNA sequences of different species to look for similarities and differences. This allows evolutionary trees to be made.


A species is a group of organisms that can interbreed to produce fertile offspring. Individuals of the same species have more characteristics in common than they do with organisms of a different species.

Sometimes a species may have different kinds or breeds that show great variation, but the individuals still belong to the same species. Different breeds of pedigree dog are like this.

Dogs of varying breed showing different physical characteristics.

From left to right - Beagle, Wheaten Terrier, Rhodesian Ridgeback, Red Siberian Husky, Irish Setter, Golden Retriever, Boxer, Sheltie

Species and habitats

Similar species tend to live in similar habitats. Closely related species share a relatively recent ancestor- a ‘common ancestor’. If they live in different type of habitat, closely related species may have different features.

Binomial system

In the 18th century, Carl Linnaeus started the modern system of putting species of organism into certain groups and giving them scientific names. Each species is given a name using Latin words, so that the same name can be used all over the world. For example, the scientific name for human beings is Homo sapiens. The first part of the name tells you the genus, while the second part of the name tells you the particular species in that genus.

Higher tier

It can be difficult to classify some organisms into species. For example, evolution is a continuing process. Some organisms can only reproduce asexually, while some species can interbreed to produce hybrids. For example the liger is a hybrid cross between a male lion and a female tiger.

Evolutionary trees

Evolutionary trees are used to represent the relationships between organisms. The diagram shows an evolutionary tree.

A cladograph

An evolutionary tree

In this evolutionary tree, species A and B share a common ancestor. Species F and G share a common ancestor, which itself shared a common ancestor with species E. All seven species share a common ancestor, probably from the distant past.

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