14. Classification

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The Concept of a Species

CLASSIFICATION - organisation of living organisms into groups, based on a number of accepted principles.

The concept of a species - members of a species have certain things in common:

  • Similar to one another, different to others - similar genes, they physically and biochemically resemble each other, and they have similar patterns of development and immunological features (occupy the same ecological niche).
  • Capable of breeding and producing living, fertile offspring - successfully able to produce more offspring. When species sexually reproduce, any genes can be combined (belong to the same gene pool).
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The Binomial System

Binomial System (naming species) - Organisms are identified by 2 names. Its features are:

  • Its a universal system based on Latin/Greek names.
  • Its first name, Generic name, tells us which genus the organism belongs in.
  • Its second name, Specific name, tells us which species the organism belongs in.


  • The names are printed in italics. Or if handwritten, they are underlined to show they are scientific names.
  • The first letter of the Generic name is UPPERCASE, the rest is lowercase.
  • If the specific name isnt known, it is written as 'SP'.

Classification of species is constantly changing as our knowledge is increasing.

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Grouping a Species

Why? It allows better communication between scientists and avoids confusion.

TAXONOMY - the theory and practise of biological classification.

There are two main forms:

  • ARTIFICIAL CLASSIFICATION - divides organisms according to their differences. E.g. colour, size, number of legs (ANALOGUS characteristics). It is only temporary.
  • NATURAL CLASSIFICATION - based on evolutionary relationships between organisms and their ancestors.
    • Classifies species into groups based on their shared features derived from their ancestors.
    • Arranges the group into a heirarchy, groups are contained within larger composite groups with no overlap.

Relationships in natural classification are based on HOMOLOGUS CHARACTERISTICS - similar evolutionary origins regardless of their functions as an adult of a species.

All systems of classificationa are human interventions, for our convenience.

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TAXON - each group within a natural biological classification.

Taxonomy is the study of these groups, and their positions in the heirarchy, based on their evolutionary line of decent of the group members.

Taxonomic ranks - heirarchy.

  • KINGDOM - largest group, and every organism is placed in one.
  • PHYLUM - different bodyplan from organisms in other phylums.
  • CLASS - due to diversity in phyla.
  • ORDER - have additional features in common.
  • FAMILY - the differences are less obvious.
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Difficulty in Defining a Species

  • Species are always changing/evolving - some may develop into a new species.
  • Within a species, there can be incredible variation.
  • Many species are extinct and have left no fossils.
  • Some species rarely/never reproduce.
  • Members of different groups in a species may have become isolated - may be classed as a different species.
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