BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. THE DIVERSITY OF LIFE ON EARTH

2.1. Classification

2.1.5. Artificial and Natural Classifications

There are two types of Classification: artificial and natural. An artificial classification is based on one or a few easily distinguishable features. It is developed and used to solve practical problems where ease of use and simplicity are paramount. The classification system adopted in ancient China, mentioned earlier, was also an artificial one. Linnaeus grouped all worm-like organisms into a single category called Vermes. This group brought together extremely diverse animals, ranging from simple Roundworms (nematodes) and earthworms to snakes. Linnaeus's classification is likewise considered artificial because it failed to account for important natural relationships—specifically the fact that snakes have a backbone, whereas earthworms do not. In reality, snakes have far more in common with other vertebrates than with worms. An example of an artificial classification of fish is their division into freshwater, marine, and brackish-Water species. This classification is based on the animals' preferred environmental conditions. Such a division is convenient for studying the mechanisms of osmoregulation. Similarly, all organisms visible under a Microscope are referred to as microorganisms (sec. 2.2), thus grouping them into a single category that is convenient for study but does not reflect natural relationships.

A natural classification is an attempt to reflect the natural relationships among organisms. It utilizes a broader range of data than an artificial classification, taking into account both external and internal features. Similarities in Embryology, Morphology, anatomy, physiology, biochemistry, cellular Structure, and behavior are all considered. Today, natural and phylogenetic classifications are the most widely used. A phylogenetic classification is based on evolutionary relationships. In this system, organisms believed to share a common ancestor are grouped together. The phylogeny (evolutionary history) of a given group can be represented as a family tree, such as the one shown in Fig. 2.3.

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Fig. 2.3. The evolutionary tree of life encompassing the five kingdoms According to the classification of Margulis and Schwartz (sec. 2.2). Line lengths do not reflect the duration of the corresponding periods.

Along with the classifications already discussed, there is also phenotypic classification. This approach represents an attempt to bypass The Challenge of establishing evolutionary kinship, which can often prove extremely difficult and controversial, especially when the necessary fossil evidence is sparse or entirely absent. The word "phenotypic" comes from the Greek phainomenon, meaning "that which is seen." This classification is based exclusively on external, or visible, traits (phenotypic similarity), with all evaluated traits considered to be of equal importance. A wide variety of an Organism's characteristics can be taken into account following the principle that the more, the better. It is not strictly necessary for these traits to reflect evolutionary relationships. Once a sufficient amount of data has been gathered, the degree of similarity between various organisms is calculated; this is typically done using a computer due to the extreme complexity of the computations. The Use of computers for this purpose is known as numerical Taxonomy. Phenotypic classifications often closely resemble phylogenetic ones, even though achieving that resemblance is not their primary goal.