Classifying living things
What is a species?
Classification
Living organisms can be classified according to their characteristics. The binomial system names an organism using its genus and species. Keys can be used to identify different species.
The binomial system
There are millions of species on our planet. Although species can be very different from each other, many have similar features that allow us to put them into groups. This is called classification.
Kingdoms
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. This is becoming an old version of classification.
The five kingdoms are:
- animals (all multicellular animals)
- plants (all green plants)
- fungi (molds, mushrooms, yeast)
- prokaryotes (bacteria, blue-green algae)
- protoctist (e.g. Amoeba)
Newer version of classification is the three domain version.
Further divisions
Living things can then be ranked according to:
- phylum
- class
- order
- family
- genus
- species
The binomial system of naming species uses Latin words. Each name has two parts, the genus and the species. For example, human beings belong to the genus Homo, and our species is sapiens - so the scientific name is Homo sapiens.
The binomial system is important because it allows scientists to accurately identify individual species. For example, the European robin is Erithacus rubecula. It is much smaller than the American robin, Turdus migratorius, which belongs to a different genus.
The new way of classification
Classification of vertebrates
Vertebrates are animals with backbones. They can be classified according to their features, and include bony fish, amphibians, reptiles, birds and mammals.
Bony fish
Bony fish absorb oxygen through their gills and they have wet scales. Their body temperature varies according to their surroundings (organisms that do this are called ectothermic animals). Bony fish reproduce by external fertilization and lay eggs (they are oviparous).
Amphibians
Amphibians usually have lungs but they can also absorb oxygen through their moist skin. Their body temperature also varies according to their surroundings (ectothermic animals). Amphibians also reproduce by external fertilization and lay eggs.
Reptiles
Reptiles have lungs and dry scales. As with bony fish and amphibians, their body temperature varies according to their surroundings. However, reptiles reproduce by internal fertilization and lay eggs.
Birds
Birds have lungs, feathers and beaks. They can maintain their body temperature (organisms that do this are called endothermic animals). Like reptiles, birds reproduce by internal fertilization and lay eggs.
Mammals
Mammals have lungs and fur. Like birds, they can maintain their body temperature. Mammals reproduce by internal fertilization and they give birth to live young (they are viviparous).
Classification of invertebrates
Invertebrates are animals without backbones. They include annelids, nematodes, mollusks and arthropods.
Arthropods are an important phylum of invertebrates. They have:
- a hard exoskeleton (their skeleton is on the outside rather than on the inside)
- a segmented body (their body has different sections)
- jointed legs
Arthropods can be divided into different groups depending on how many legs they have.
Other invertebrate groups include:
- annelids
- nematodes
- mollusks
Unlike arthropods, these invertebrates do not have jointed legs.
Annelids
Annelids are segmented worms with soft bodies. Most of the worm’s body consists of identical segments, giving it a ‘ringed’ appearance.
Nematodes
Nematodes are tiny, thin worms without segments. Many species are free-living but others are parasites that cause disease. For example, hookworms can infect humans.
Molluscs
Mollusks usually have a broad muscular foot and may also have a shell. Examples of mollusks include snails, slugs, squids and octopuses.