Because life is so diverse, it is helpful to have classification system to group organisms into categories. Taxonomy As more is learned about living things, including the evolutionary relationship between species, taxonomy changes.[Greek: tasso = arrange, classify and nomos = usage, law], is the discipline of identifying and classifting organisms according to certain rules.
Taxonomy makes sense out of the bewildering variety of life on Earth and provides valuable insight into evolution. As more is learned about living things, including the evolutionary relationship between species, taxonomy changes.
Taxonomist are constantly making observations and performing experiments that will one day bring about changes in the classification system adopted by this article.
CATEGORIES OF CLASSIFICATION
Several of the basic classification categories, or taxa, going from least inclusive to most inclusive:
Each successive classification category above species contains more types of organism than the preceding one. Species placed within one genus share many specific characteristics and are the most closely related, while species placed in the same kingdom share only general characteristics with one another.
For example, all species in the genus Pisum look pretty much the same- that is like pea plants. However, species in the plant kingdom can be quite varied, as is evident when we compare grasses to tress, you can really tell the differences.
By the same token, only modern humans are in the genus Homo, but many types of species, from tiny hydras to huge whale, are members of the animal kingdom.
Species placed in different domains are the most distantly related.
Biochemical evidence suggests that there are only 3 domains namely:
Bacteria, Archaea and Eukarya. Both domain Bacteria and domain Archaea contain unicellular prokaryotes, which lack the membrane-bounded nucleus which can be found in the eukaryotes of domain Eukarya.
Methanosarcina mazei - an archaeon
Escherichia coli - a bacterium
Prokaryotes are structurally simple but metabolically complex (see the 2 photos above).
Archaea can live in aquatic environments that lack oxygen or are too salty, too hot or too acidic for other organisms. Perhaps these environments are similar to those of the primitive Earth, and archaea are representative of the first cells that evolved.
Bacteria are variously adapted to living almost everywhere- in the water, soil, atmosphere or even in and entire of our body! Although some bacteria cause dreaded diseases, others perform many valuable services both environmentally and commercially. They are used to conduct genetic research in our laboratories, to produce innumerable products in our factories, and to purify water in our sewage treatment plants, for example.
If you refer to the succession diagram under the categories of classification, you will see that Domain will give rise to Kingdom. This is the part where I should be explaining about Eukarya, another member in the Domain taxon. Keep the story about Eukarya in mind first, because we will be looking about Six Kingdoms first.
For hundreds of years, biologist viewed organisms as belonging to 2 broad groups, the plant and animal kingdoms. As organisms underwent closer study, it became increasingly obvious that many did not fit very well into either the plant or animal kingdom.
For example, bacteria, single-celled organisms that lack cell nuclei were originally considered plants! The fact that all bacteria lack nuclei is now considered a fundamental difference between them and other organism - this difference is more significant than the differences between plants and animals. With our present knowledge, it is difficult to consider bacteria as plants.
In addition, certain organisms like Euglena seem to possess characteristics of plants and animals. This single-celled organism is plantlike because it can photosynthesize and is animal-like because it can move about from place to place. In the past, Euglena was classified in the plant kingdom (when algae were considered plants) and in animal kingdom (when protozoa were considered animals)
Today, most biologist place Euglena in the Protista Kingdom.
These and other considerations have led to the Six Kingdoms system of classification used by most biologist today.The kingdoms currently recognized are:
Archaeabacteria, Eubacteria, Protista, Fungi, Plantae and Animalia.
Ok, I know that this story seems to be complicated now. Just understand this way, Domain has 3 members namely Archaea, Bacteria and Eukarya. Kingdom under domain Archaea is Archaeabacteria and kingdom under domain Bacteria is Eubacteria. Eukarya is the one which has 4 kingdoms under, namely Plantae, Animalia, Protista and Fungi.
P/s: This seems too much, but I really want you guys to understand this taxonomy and biological classification. Once you've known where plants lie in the hierarchy, you can have better understanding and will appreciate plants even more.