Whittaker's Classification

School text books and even syllabuses have been slow in adopting the advances in understanding of the living world, so you may find no mention of new classification even in books that have been published in recent years.

Let us take a look at the traditional classification system, though, as this helps to show how living things are classified.

  • Animals
  • Plants
  • Fungi
  • Bacteria
    Protists
It is probably best to describe the classification system by illustration.  Using microbiological terms might be confusing, because most people will not be familiar with the unseen world of the microorganism.  Instead we shall use the animal kingdom and specifically the domestic cat,  to illustrate how the classification system works.

Phylum - Vertebrate including fish, amphibians, reptiles, birds and mammals.

Class - Placental mammal, including primates, rodents, ungulates and others.  We may also break the class into subclasses, for instance there are also marsupials and monotremes in the mammal category.

Order - Carnivore - Including dogs, bears, seals and others.

Family - Cat, including lions, tigers, leopards, lynx, wildcats and domestic cats.

Genus - Felis, including lynx, wildcats and domestic cats.

Species - Felis domesticus including all breeds of domestic cat, such as Persian, Siamese, etc.

In 1735 the Swedish naturalist Carl Linné described in his book, Systema Naturae, a method for the classification of all living organisms.  This has been universally adopted across the world and enables life-scientists to be precise about their study subjects.  Linné's system uses two words to describe each living organism.  The first describes the Genus - the main group - to which the organism belongs and the second word describes the Species.  The former always starts with a capital letter, while the latter is always lower-case. The system is also known as the binomial system - ie two names.

The generally accepted definition for a species is that two individuals who can sexually reproduce successfully together are of the same species.  However, it is possible to produce hybrid offspring from two organisms that are not strictly of the same species.  Lions and tigers can produce hybrid cubs - ligers and tigons for example - but hybrid animal offspring are almost always infertile and therefore cannot reproduce themselves.

Plants, however, are a different matter.  Hybrid plants can be fertile, and plants that are clearly not of the same species can reproduce successfully.  One case occurred in England in 1998, when a strawberry and a tomato crossbred to produce a hybrid fruit which became known as a tomberry.  Another problem, particularly for microbiologists is that many microbes do not reproduce sexually, so how can they be classified by species?  The question "What is a Species"  is therefore something of an open one, to which there is no definitive answer.

 

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