AQA AS Level Biology Unit 2 - classification

topic of classification in unit 2 as aqa biology

HideShow resource information


The organisation of living things into groups based on a number of accepted principles.

1 of 9

The concept of a species

A species is the basic unit of classification. A definistion of species is not easy, but members of a single species have certain things in common:

  • They are similar to one another but different from members of other species - they have similar genes and therefore closely resemble one another physically and biochemically. They have similar patterns of development and similar immunological features and they occupy the same ecological niche
  • They are capable of breeding to produce living, fertile offspring. - they are therefore able to successfully produce more offspring. This means that when a species reproduces sexually, any of the genes of its individuals can in theory be combined with any other. i.e they belong to the same gene pool
2 of 9

Naming species - the binomial system

At pme time scientise often gave new organisms a name that described their featurs e.g blackbird, rainbow trout. This practice resulted in the same names being used in different parts of the world for very different species. Therefor it was difficult for scientists to be sure they were referring to the same organism. Therefore a system was created in order to solve the problem.

Organisms are identified by two names and hence the binomial system:

  • it is a universal system based upon Latin or Greek names
  • The first name, the generic name, denotes the genus to which the organism belongs. This is equivalent to the surname used to identify people and shared by their close relatives.
  • The second name, the specific name, denotes the species to which the organism belongs. This is equivalent to the first (or given) name used to identify people. However, unlike in humans, it is never shared by other species within the genus
3 of 9

The Binomial system (2)

There are a number of rules that are applied to the use of the binomial system in scientific writing:

  • The names are printed in italics or if handwritten they are underlined to indicate that they are scientific names
  • The first letter of the generic name is in uper case but the specific name is in lower case
  • if the specific name is not known, it can be written as 'sp' e.g felix sp

The naming of organisms is in a constant state of change. Current names reflect the present state of scientific knowledge and understanding. In the same wat the classification of species is regularly chaning as our knowledge of evolution, physical features, biochemistry and behaviour increases.

4 of 9

Grouping species together - the principle of class

The grouping of organisms is knows as classification, while the theory and practice of biological classification is called taxonomy.

There are two main forms of biological classification, each used for a different purpose.

  • Artificial classification - divides organisms according to differences that are useful at the time. Such features may include colour, size, number of legs, leaf shape etc. These are described as analogous characteristics where they have the same function but fo not have the same evolutionary origins. e.g wings of birds and butterflies are both used for flight but they originates in different ways
  • Natural classification: (a) is based upon the evolutionary relationships between organisms and their ancestors, (b) classifies species into groups using shared features derived from their ancestors
  • (c) arranges the groups into hierarchy in which the groups are contained within larger compositie groups with no overlap

Relationships in a natural classification are based upon homologous characteristics. Homologous characteristics have similar evolutionary origins. e.g the wings of butterflies and birds are both used for flight but they originated in different ways

It must be remembered that all systems of classification are human inventions. They are developed for our convenience. The natural world does not follow any system of classification, nor is it bound by our ideas

5 of 9

Organising the groups of species - Taxonomy

  • Each group within a natural biological classification is called a teaxon (plural taxa).
  • Taxonomy is the study of these groups and their positions in a hierarchal order, where they are known as taxonomic ranks.
  • These are based upon the evolutionary line of descent of the group members.
  • The largest group is a kingdom and each organism is placed into one of these.
  • Within each kingdom the largest groups are known as phyla.
  •  Organisms in each phylum have a body plan radically different from organisms in other phylum.
  • Diversity within each phylum allows it to be divided into classes.
  • Each class is divided into orders of organisms that have additional features in common.
  • Each order is divided into families and at this level the differences are less obvious.
  • Each family is divided into genera and each genus (singular) into species.
6 of 9

A way of remembering the order in taxonomy

Kingdom      -       King

Phylum         -      Prawns

Class           -      Can

Order           -      Only

Family          -       Fill

Genus          -      Giant

Species        -      Sandwiches

King Prawns Can Only Fill Giant Sandwiches

Kingdom, Phylum, Class, Order, Family, Genus, Species

7 of 9


  • The hierachical order of taxonomic ranks is based upon the evolutionary line of descent of the group members.
  • This evolutionary relationship between organisms is known as phylogeny.
  • The term is derived from the work 'ohylum' which, in classification, is a group of related or similar organisms.
  • The phylogeny of an organism reflects the evolutionary branch that led to it.
  • The phylogenetic relationships of different species are usueally represented by a tree-like diagram called phylogenetic tree.
  • In these diagrams, the oldest species is at the base of the tree while the most recent ones are represented by the ends of the branches
8 of 9

The difficulties of defining species

A species may be defined in terms of observable similarities and the ability to produce fertile offspring. There are however, certain difficulties with this definition. These incule:

  • Species are not fixed forevem but change and evolve over time. In time, some individuals many develop into a new species
  • Within a species there can be considerable variation among individuals. All dogs, for example, belong to the same species but artificial selection has lef to a variety of different breeds
  • Many species are extinct and most of these have left no fossil record
  • Some species rarely, if ever, reproduce sexually
  • Members of different groups of the same species may be isolated e.g by oceans so never meet and therefore never interbreed
  • Groups of organisms that are isolated from one another may be classifies as different species. These groups may turn out to be of the same species when their ability to interbreed is tested
  • Some species are sterile
9 of 9


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all DNA, genetics and evolution resources »