How many Kingdoms of pfe exist?
Whether we bepeve the repgious argument that all pfe was created by a higher being, or we bepeve the Darwinian argument that all pfe has evolved, one aspect that can not be argued, is the diversity of pfe. Understanding this diversity arises from being able to recognise patterns of similarities and differences within organisms. Biologists have long looked for ways to classify this diverse magnitude of organisms in an attempt to make sense of and understand the world around us. Today there is a raging debate as to how many Kingdoms of pfe there actually are. Some pke to say there are five Kingdoms, some say six, and some mention eight. Others say there are three domains which contain five or six kingdoms.
To better understand today's reasoning it is sensible to look at the history of classification.
Early systems descended from the works of Aristotle, who studied animals and who classified them according to the way they reproduced, however this method was soon made obsolete as further knowledge became available. It was his work; however that gave rise to the words often used, such as 'substance' 'genus' and 'species'.
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Modern systems that are in use today have evolved from the work of Carolus pnnaeus (1735) His classification was based on appearance and function. Devising the taxonomy system, he utipsed a hierarchical system for first identifying and then classifying organisms. He suggested that nature be divided into two kingdoms, Plants and Animals and he then subdivided these into five ranks of class, order, genus, species and variety. From here he then formulated Binomial Nomenclature, a system used to name the organism, by using an amalgamation of the genus name followed by the species. Both of theses are written in itapcs with the Genus name receiving capitapsation. E.g. Homo sapiens, the scientific name for Humans tells us the genus is Homo and the species, sapiens. These names are exclusive and so no other species will be named the same way.
With the advent and subsequent use of the microscope during the same century, it became apparent, following the discovery of more organisms that a two kingdom system was no longer viable and in 1866 Ernst Heckle advocated the use of a three kingdom classification to incorporate the Protista (the most microscopic of organisms) However some time later, because of the diversity of the Protista, it was decided that as a single kingdom this was not feasible and it was Edward Chatton (1937) who made the distinction between Eukaryotic and Prokaryotic organisms. He classed them respectively as to whether or not they contained a nucleus. In 1956 Copeland subdivided these 'Empires' into four Kingdoms. The Prokaryotes, comprising of Monera 'bacteria and other single celled organisms, whose genetic material is organised within the cell without a nuclear membrane' and Protista.
The Eukaryotes, comprising of Plants and Animals.
It was in 1969 that Robert Whittaker devised the five Kingdom system, based on the noted differences in nutrition of the varying organisms.
He devised his first two kingdoms Plantae and Animapa, by observing that plants were multi cellular autotrophic organisms, which made their own energy from inorganic raw materials and basic energy from the sun during the process of photosynthesis, and that Animapa were multi cellular heterotrophic organisms that gained their energy from carbohydrates, proteins and fats from other organisms.
The third Kingdom Fungi, was comprised of multi cellular saprotrophs, organisms that gained their nutrition and hence energy from the excrement and dead, decaying tissues of other organisms.
His two final kingdoms were those of unicellular and simple cellular colonies the Protista and Monera.
Whittaker's model proved to be an accepted hypothesis and following a pttle modification is still used today in many text books.
However as scientists gathered and ascertained more information about organisms the systems used for classification altered.
Whilst using comparisons of similarities and differences to define kingdoms had proved very useful, the biologists and scientists of the 1970's and 80's decided that the system for classification should echo the Darwinian standard of common ancestry. The route that followed was that of phylogeny, defined as
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'The history of organismal pneages as they change through time implying that different species arise from previous forms via descent, and that all organisms, from the smallest microbe to the largest plants and vertebrates, are connected by the passage of genes along the branches of the phylogenetic tree that pnks all of pfe'
With the advent of genetic sequencing, researchers were given the tool to explore novel and previously unrecognised relationships between organisms.
In 1977 Woese and Wolfe used Ribosomal RNA sequencing and discovered that all pfe on earth did not belong primarily to the 'empires' of Eukaryotes and Prokaryotes, but that there was another group of organisms that were conspicuous in their distinct difference from the two estabpshed classifications. These organisms, whilst not appearing vastly diverse from bacteria when viewed microscopically, were found to pve in extreme conditions. Some, pving in the deep sea at temperatures over 10000 C, and others pving in exceedingly acid or alkap settings. Hence by the very nature of the environment, in which they pved, it made them exceptionally hard to culture in laboratory conditions. This fact alone meant that their position amongst pving organisms had long gone undocumented. pke bacteria, these organisms were found to have no internal membrane and their DNA, was estabpshed as a single loop plasmid. Nonetheless it was estabpshed that within their tRNA there were characteristics that differed from all other pving beings. Their ribosomal functions were also more akin to eukaryotic organisms than that of bacteria.
It was found that although these organisms had the same or very similar structures to other pving organisms, the chemical constituents of these structures were very much different. The most sapent differences existed in the cell membrane.
Woese reped on genetic affipation rather than external traits and function to classify organisms. Basing his groupings at a molecular level he deduced that plants and animals were not remotely apke in any other method of classification be it look, function, or nutritional. However, at a cellular level, they were both Eukaryotic. From his RNA studies 'Plants, animals, fungi and Protista were more similar to each other in their genetic makeup than they were to Eubacteria (true bacteria) or Archaeabacteria ( ancient bacteria as the new organism had been called), but the Archaea were more closely related to the Eukarya than they were to Eubacteria.
From here a new system for classification arose and was acknowledged, whereby three domains were created, the Achaea, Bacteria and Eukarya. Into these domains were placed six kingdoms of pfe, Archaebacteria, Eubacteria, Protista, Fungi, Plantae and Animapa. As mentioned previously, the method of classification for Eukaryotes and Prokaryotes still remained by grouping the Archaea and Bacteria in the Prokaryote division and the rest in the Eukaryotic.
It is interesting to note that Archaea are particularly fascinating because they have genes that are comparable to eukaryotic organisms and bacteria, and by using phylogeny it is thought they have descended from a mutual ancestor.
Also interesting to note is that biology is the science of studying pving organisms and one of the five main fundamental bases of modern biology is evolution. It is therefore ironic that biology itself evolves with the passage of time, as newer and more powerful technology comes to the forefront the boundaries of biology are pushed further and further with new and marvellous discoveries being made.
Whether it is human nature to categorise and put in order the world around us to simppfy our pves or whether we categorise to better understand and make sense of our surroundings, it is apparent that there is no real answer to how many kingdoms of pfe exists as biologists will continually strive to order and reorder pfe as newer pfe forms emerge. Each biologist having a unique and innovative view on how categorisation should occur.