• Scientists group organisms by looking at all the characteristics of each organism. Organisms with many characteristics in common are grouped together as a species.
  • Different species that share many characteristics are grouped as a genus (plural genera). Genera that share many characteristics are grouped as a family, and so on up to the level of kingdom.
  • The order of classification:

The five kingdoms of organisms

  • Organisms may be classified into one of the five kingdoms, based on basic characteristics. For example, some organisms are unicellular while other multicellular. Other organisms have cells which contain a nucleus, but bacteria do not.
  • Another characteristic is the way the organism gets its food. Plants, for example, get their food autotrophically using photosynthesis, while other animals feed heterotrophically by digesting other organisms. Fungi also digest organisms but this is outside their body so they are said to feed saprophytically.
  • There is no kingdom for viruses because scientists do not believe they are alive; it may enter a cell and change how it works to make copies of it cells, it does not show other life processes such as growth or feeding which is present in other organisms.

Vertebrates and invertebrates

  • Vertebrates are animals that have a backbone (a series of small bones called vertebrae).
  • All vertebrates belong to the phylum Chordata because they have a supporting rod that runs the length of their body.
  • Animals that do not have a backbone are called invertebrates. The vertebrate and invertebrate groups are then divided into smaller groups.

How to group organisms

Oxygen consumption

  • Organisms can be characterised in the way they absorb oxygen.
  • Fish – have gill to gain oxygen from water.
  • Young amphibians – also have gills
  • Adult amphibians – normally have lungs but can absorb oxygen through their moist skin.
  • Mammals, birds and reptiles – have lungs


  • Organisms can be characterised based on how they reproduce.
  • External fertilisation – adult female releases her eggs in water and are fertilised by sperm released by an adult male.
  • Internal fertilisation – takes place inside the female’s body.
  • Oviparous – lays eggs (many vertebrate do this)
  • Viviparous – give birth to live young (mammals)


  • Organisms can be characterised on the way they regulate body temperature.
  • Homeotherms – keep their body temperature constant and often warmer than their surroundings from reactions all over body.
  • Poikilotherms – their body temperature varies based on their surroundings.
  • Within a group species have different characteristics to the rest of the group; a few amphibians have gills even as adults and sharks have internal fertilisation and give birth to live young, even though they are fish. As a result, we have to look at many characteristics to see where to put an organism.


  • A species can be defined as a group of organisms that can interbreed to reproduce offspring that are fertile. However this definition isn’t always clear cut because two closely related species can interbreed but produce hybrids which are infertile most of the time. This means they are unlikely to have offspring of their own and pass on their mix of characteristics.
  • Another problem is that not all organisms need to interbreed to produce offspring. For example, many plants and fungi can produce new individuals from parts of adult organisms. Bacteria and many protoctists reproduce by splitting in half.
  • If organisms never interbreed we cannot test whether or not two individuals are the same species.

Difficulties with classification

  • Mallard ducks can hybridise with other closely related species to produce fertile hybrid offspring. These offspring then breed with other hybrids or mallard ducks or closely related ducks. As a result, this produces ducks with a continuous range of characteristics, rather than separate species.
  • Neighbouring populations of the same species may have slightly different characteristics but still interbreed.
  • Sometimes there is a chain of different populations that can all breed with their neighbouring populations but the two populations at either end of the chain cannot interbreed. The chain often forms a ring shape and so these organisms are called a ring species.
  • It is hard to divide a ring species into separate species, however, the gulls named below are traditionally considered to be two species – Larus fuscus (the lesser black-backed gull) and Larus argentatus (The European herring gull).
  • Classifying the gulls between these two is difficult because there is a gradual change of characteristics between the two species.
  • The platypus was difficult to classify as it had a mixture of features characteristic of different types of animal. Eventually it was classified as a mammal as even though it laid eggs, most of characteristics were like that of a mammals e.g. homeotherm, suckles its young etc.




Naming a species

  • An organism’s scientific name has two Latin words, genus and species. This is called the binomial system. Organisms that share the first word in their name are closely related. Scientists can tell from the name whether two organisms are closely related.
  • The binomial system is useful in cases where organisms with common names are different species. A robin in America is not the same bird as a robin the UK. The system is agreed by scientists all over the world to allow them to communicate clearly without misunderstandings.


  • To identify different species you can use a key. This is a key for rats:

The Importance of classification

  • Accurate classification using the binomial system allows biologists to:
    • easily identify existing species and new species
    • see how organisms are related
    • Identify areas of greater and lesser biodiversity.
  • Biodiversity is a measure of the total number of different species in an area. To count the species you need to be able to identify them, which can be tricky if the species are very similar to one another.
  • The more accurate the classification system, the easier identification becomes.
  • Biodiversity is important because we obtain many products from living things. The more species there are the more choice we will have, both now and in the future.
  • Biodiverse areas also much better at recovering from natural disaster (e.g. floods) than less diverse areas.

  • All plants and animals have characteristics that are in some ways similar to their parents’. This is because an organism’s characteristics are determined by the genes inherited from their parents.
  • Most animals, and significant numbers of plants, get some genes from the mother and some from the father.
  • The combining of these genes from the two parents causes genetic variation. No two of the species are genetically identical.
  • Genetic variation also occurs due to mutations – changes in an organism’s genes. These changes can cause differences in an organism’s characteristics.
  • Some characteristics are determined only by genes.

Environmentally influenced variation

  • The environment that organisms live and grow in also causes differences between members of the same species – this is called environmental variation.
  • Environmental variation covers a wider range of differences, such as losing a limb or dying your hair.
  • Environmental factors that can cause variation include diet, exercise, temperature, light level, amount of water etc.

Both are influential

  • Most characteristics that we have are determined by a mixture of genetic and environmental factors.
  • For example, the maximum height that an animal or plant can grow to is determined by its genes. But whether it actually grows that tall depends on its environment.


  • Deep under the surface of the sea, conditions are hard to live in. Food is scarce due to lack of sun, thus lack of photosynthesis, and thus organisms survive on scraps that sink down from above.
  • Some animals have adapted to living in the deep ocean:
    • Some deep-sea fish are able to emit light from parts of their body. For example, the angler fish has a rod-shaped spine sticking out of its face which gives out light. The light attracts prey which the angler fish then eats.
    • Deep-sea fish often have huge mouths e.g. the rat-tail fish which moves along the seabed scoops up particles of food.
    • Many deep-sea fish have huge eyes adapted to the dark and long feelers to help their way around.

Volcanic vents

  • There are volcanic/hydrothermal vents in the seabed that send out hot water and minerals into the cold ocean. Some organisms have adapted to living around them.
    • The chemicals from the vents support bacteria that are able to make their own food, using chemical energy. This is called chemosynthesis. It’s a bit like photosynthesis, but it uses chemical energy instead of light energy.
    • The bacteria are at the bottom of the food web and are the producers. Bacteria which live near the vents have to be specially adapted to cope with the high temperature and pressure.
    • Deep sea Pompeii worms are covered with a thick layer of bacteria which helps to protect it from the heat. As it lives at depths of over 2km its body is adapted to pressures over 200 times those at sea level.


Polar regions

  • Polar bears and penguins have adapted to living in the cold, freezing conditions of the Arctic and Antarctic. Polar bears have:
    • A compact shape which gives them a small surface area compared to volume which reduces heat loss.
    • A thick layer of blubber for insulation which also acts as an energy store for when food is scarce.
    • Thick hairy coats to trap a layer of warm air next to the skin, and their greasy fur shed water (this prevents cooling due to evaporation).
    • Big feet which help by spreading their weight. This stops them from sinking into the snow or breaking thin ice.
    • Polar bears have white fur to match their surroundings which acts a camouflage.
  • Penguins:
    • Have similar adaptions to polar bears such as the thick layer of insulating fat and oily feathers.
    • Huddle together in groups to conserve heat.
    • Have a streamlined body to reduce water resistance – so they can swim fast and catch fish.