Classification of Organisms

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The classification of living organisms has been controversial throughout time, and these schemes are among those in use today. Firstly, Aristotle’s system distinguished only between plants and animals on the basis of movement, feeding mechanism, and growth patterns. This system groups prokaryotes, algae, and fungi with the plants, and moving, feeding protozoa with the animals. Then, the increasing sophistication of laboratory methods and equipment, however, revealed the differences between prokaryotic and eukaryotic cells, prompting a classification system that reflects them; then most recently, five kingdoms have emerged to take both cellular organization and mode of nutrition into account.

Greek philosopher Aristotle (384-322 bc) grouped life forms as either plant or animal. Microscopic organisms were unknown.

  • Plants - Plants and Fungi
  • Animals - Animals

In 1735 Swedish naturalist Carolus Linnaeus formalized the use of two Latin names to identify each organism, a system called binomial nomenclature. He grouped closely related organisms and introduced the modern classification groups: kingdom, phylum, class, order, family, genus, and species. Single-celled organisms were observed but not classified.

  • Kingdom Plantae - includes Plants and Fungi organisms
  • Kingdom Animalia - Animals

In 1866 German biologist Ernst Haeckel proposed a third kingdom, Protista, to include all single-celled organisms. Some taxonomists also placed simple multicellular organisms, such as seaweeds, in Kingdom Protista. Bacteria, which lack nuclei, were placed in a separate group within Protista called Monera.

  • Kingdom Protista - includes all single-celled organisms, such as amoebas and diatoms, and sometimes simple multicellular organisms such as seaweeds.
  • Kingdom Plantae - Plants
  • Kingdom Animalia - Animals

In 1938 American biologist Herbert Copeland proposed a fourth kingdom, Monera, to include only bacteria. This was the first classification proposal to separate organisms without nuclei, called prokaryotes, from organisms with nuclei, called eukaryotes, at the kingdom level.

PROKARYOTES

  • Kingdom Monera (Prokaryote) - Bacteria

EUKARYOTES

  • Kingdom Protista - includes Bacteria Amoebas, diatoms, and other single-celled eukaryotes, and sometimes simple multicellular organisms, such as seaweeds.
  • Kingdom Plantae - includes Plants and Fungi.
  • Kingdom Animalia - Animals

In 1957 American biologist Robert H. Whittaker proposed a fifth kingdom, Fungi, based on fungi’s unique structure and method of obtaining food. Fungi do not ingest food as animals do, nor do they make their own food, as plants do; rather, they secrete digestive enzymes around their food and then absorb it into their cells.

  • Kingdom Monera (Prokaryote) - Bacteria
  • Kingdom Protista - includes amoebas, diatoms, and other single-celled eukaryotes, and sometimes simple multicellular organisms, such as seaweeds.
  • Kingdom Fungi - includes multicellular, filamentous organisms that absorb food.
  • Kingdom Plantae - includes multicellular organisms that obtain food through photosynthesis.
  • Kingdom Animalia - includes multicellular organisms that ingest food.

In 1990 American molecular biologist Carl Woese proposed a new category, called a Domain, to reflect evidence from nucleic acid studies that more precisely reveal evolutionary, or family, relationships. He suggested three domains, Archaea, Bacteria, and Eucarya, based largely on the type of ribonucleic acid (RNA) in cells.

PROKARYOTES
Domain: Archaea
Kigdoms:

  • Crenarchaeota - includes ancient bacteria that produce methane.
  • Euryachaeota - includes ancient bacteria that grow in high temperatures.

Domain: Bacteria

EUKARYOTES
Domain: Eucarya
Kingdoms:

  • Protista
  • Fungi
  • Plantae
  • Animalia


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