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Classification of organisms

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Species and their classification


Taxonomy refers to the classification of living things by giving unique names to each species, and creating a hierarchy based on evolutionary descent. This is a challenging task, as most species that have ever lived on this planet are now extinct, and many more alive today have yet to be discovered and classified.

In order to achieve the above, though, we need a definition for both species and hierarchy. In the old days, a species was known as a collection of individuals similar enough in resemblance to be put in the same box. This was purely based on physical features. Today we know that similar physical characteristics on their own aren't enough to define a species.

species is defined in terms of observable physical features as well as the ability to produce fertile offspring.


This is Hercules, the liger. Hercules has a lion father and a tiger mother. Does that mean tigers and lions are really one species? This is one example of the issues surrounding both the definition of species, and taxonomy generally.

What is a hierarchy? A hierarchy, put simply, is a system of classification comprised of small groups contained within larger groups contained within larger groups, and so forth, where there is no overlap.


The above diagram is a phylogenetic tree. It is a representation of various species in terms of their genetic relatedness. Each "crossroads" is a different ancestor. From this diagram it is easy to see that humans are more closely related to whales than to birds, or indeed any other species represented.

The species with a red circle beneath are extinct. If a phylogenetic tree was made with all species that have ever lived up to today, the vast majority would be extinct. 

Aside from simple observable features and their similarity, advances in immunology and genome sequencing can add to the information required to create, maintain and update the tree of life according to new findings. Different organisms' genes, proteins and physiologies can be compared to see how closely related they are.

The names above are used for convenience, yet the scientifically correct way of classifying organisms is by giving them a two-word (binomial) name. These names are in Latin or Greek.

Let's take Homo sapiens for example (us!):

1. It's written in italics as all species names should be, by convention.

2. It's made up of two words: Homo and sapiens.

3. Homo denotes the genus to which the species belongs to. A genus is the group higher than species. For example, Homo erectus and Homo neanderthalensis are part of the same genus as Homo sapiens (both now extinct). That genus is called Homo... getting the hang of it?

4. Sapiens denotes the species itself, and is the smallest group in the hierarchy.


What does the rest of the hierarchy look like?

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species (fearing you can't possibly remember this sequence?)

Dazzling, Kinky, Policemen, Can, Often, Find, Gay, Sex. You're more than welcome.


Three domains or five kingdoms?

An example of the role of the scientific community in validating evidence is the classification of life into 3 domains versus 5 kingdoms. ArchaeaBacteria and Eukarya or perhaps MoneraProtistaPlantaeFungi and Animalia



Domains are higher up than kingdoms, so refer to the very earliest differences between living things. Originally, the kingdoms were established based on superficial analysis of different organisms, and an even earlier classification called Linnean classification simply split everything into animals and plants, based on whether they moved!

Monera (or Prokaryotae) kingdom was split into bacteria and archaea because very fundamental differences between these organisms were found as a result of better comparisons such as molecular biology techniques such as those outlined above. The remaining 4 kingdoms could then be grouped under Eukarya.

The response of scientists to this evidence follows that older evidence was treated differently in light of newer, more insightful evidence. Evidence can always build up, break down or change in other ways to change our understanding of life. At first, archaea and bacteria looked similar, so they were treated as closely related. As their inner workings were investigated with the use of more sophisticated tools, it became apparent that their characteristics beyond appearance were very different.

Ok byeeeeee

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