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  • The total number of species on Earth vary from 10 million to 100 million.
  • The figure is likely to be around 14 million; these represent only the species that exist today.
  • Scientists have estimated that 99% of species that exiseted on Earth are now extinct and almost all have left no fossil record.
  • Classification is the organisation of living organisms into groups.

The concepts of a species:

A species is the basic unit of classification. A single species have certain things in common:

  • They are similar to one another by different from members of other species. They have very similar genes and therefore closely resemble one another physically and biochemically. They have similar patterns in development and similar immunological feature and they occupy the same ecological niche.
  • They are capable of breeding to produce living, fertile offspring. Therefore able to successfully produce more offspring and so belong to the same gene pool.

Naming species - the binomial system:

Organisms are identified by two names and hence the system called the binominal system. Its features are as follows:

  • It is a universal system based upon Greek and Latin names.
  • The generic name (first name), denotes the genus to which the organism belongs, i.e. the surname used to identify people and shared by their close relatives.
  • The specific name (second name), denotes the species to which the organism belongs. This name is never shared by other species within a genus.

There are a number of rules that are applies to the use of the binominal 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 upper case, but the specific name is in lower case.
  • If the specific name is not known, it can be written as 'sp'.

Grouping species together - the principles of classification:

  • Classification = the grouping of organisms.
  • Taxonomy = the theory and practise of biological classification.

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

  • Artificial classification: 
  • divides organisms 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 do not have the same evolutionary origins, e.g. the wings of butterflies and birds are both used for flight, but originated in different ways.
  • Natural classification:
  • is based upon the evolutionary relationships between organisms and their ancestors.
  • classifies species into groups using shared features derived from their ancestors.
  • arranges the groups into a hierarchy, in which the groups are containted within larger conposite groups with no overlap.
  • Relationships in a natural classification are based upon homologous characteristics. 
  • Homologous characteristics have similar evolutionary origins regardless of their functions in the adult of species. 
  • All systems of classificationa re human invetions and are developed for out convenience. The natural world does not follow any system of classification,


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