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Classification of Organisms

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Classification of Organisms

Why is classification important?

  • Biologists use a universal language just as groups of people communicate with a universal language.

  • Scientists constantly exchange information at the international level.

  • Organizing the more than 2 million known species is important.

Taxonomy: The science of naming and classifying organisms.
Linnaeus invents a simpler system for classification – binomial nomenclature.
Binomial nomenclature uses a two – word Latin name for every organism. Every species has its own, unique Latin name.

The Latin name consists of a Genus name and a species name.

This two – part name is called specie’s

Scientific Name.

The Genus name is always capitalized and the species name is always in lower – case. The entire scientific name is always in italics.

All species are classified in a broad classification scheme by placing them in smaller and smaller groups based on similar, shared characteristics.

Each classification group contains species that all share common characteristics. This can help you quickly associate characteristics with a species that you are not familiar with.

The modern classification system uses this hierarchy of groupings.

Note that Kingdom is the most inclusive group. As you move through the classification groupings each group becomes progressively smaller and more specific.
This diagram, for example, shows the classification of the grizzly bear, Ursus horribilis.

Ursus americanus Ursus maritimus




















Scientific classification

Check out this website on classification

Plants, like all living things, are also classified. This is the classification of the Moss rose, Rosa gallica.

Even bacteria are classified according to this classification system.

Staphylococcus aureus
















Staphylococcus aureus

The pyogenic cocci are spherical bacteria, which cause various suppurative (pus-producing) infections in animals. Included are the Gram-positive cocci Staphylococcus aureus, Streptococcus pyogenes and Streptococcus pneumoniae, and the Gram-negative cocci, Neisseria gonorrhoeae and N. meningitidis. These bacteria are leading pathogens of humans. It is estimated that they produce at least a third of all the bacterial infections of humans, including strep throat, pneumonia, food poisoning, various skin diseases and severe types of septic shock, gonorrhea and meningitis. Staphylococcus aureus is arguably the most successful of all bacterial pathogens because it has a very wide range of virulence determinants (so it can produce a wide range of infections) and it often occurs as normal flora of humans (on skin, nasal membranes and the GI tract), which ensures that it is readily transmitted from one individual to another. In terms of their phylogeny, physiology and genetics, these genera of bacteria are quite unrelated to one another. They share a common ecology, however, as parasites of humans.

Figure 15. Gallery of pyogenic cocci, Gram stains of clinical specimens (pus), L to R: Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumoniae, Neisseria gonorrhoeae, and Neisseria meningitidis. The large cells with lobed nuclei are neutrophils. Pus is the outcome of the battle between phagocytes (neutrophils) and the invading cocci. As the bacteria are ingested and killed by the neutrophils, the neutrophils eventually lyse (rupture) and release their own components, plus the digested products of bacterial cells, which are the make-up of pus. As a defense against phagocytes the staphylococci and streptococci produce toxins that kill the neutrophils before they are able to ingest the bacteria. This contributes to the pus, and therefore these bacteria are "pyogenic" during their pathogenic invasions.

What defines a species?

Do these scorpions

belong to the same


Do these primates

belong to the same species?

Biological concept of species: A group of naturally occurring organisms that interbreeds, or could potentially interbreed and is reproductively isolated from other groups.

Historically, classification based on similarities between species should reflect an organism’s phylogeny. Phylogeny is the evolutionary history of an organism.
However, using these similarities to make evolutionary connections can be misleading.

For example, if we look at bats, butterflies and birds we could reach the conclusion that they are closely related because they all have wings.

What do you think? Are these three organisms closely related on the evolutionary tree of life?

These three “wings” are called analogous structures. Analogous structures serve the same function but are not found on closely related species.

This is an example of convergent evolution – species evolve similar characteristics to adapt to similar habitats.

If we grouped organisms by analogous structures, we would have some very confusing classification systems. For instance, bats would be placed in the same group with butterflies, but bats are much more closely related to us than they are to butterflies. (They are mammals like us.)

Evolutionary Trees

Evolutionary or phylogenetic trees are the traditional method by which taxonomists classify and organize species. This system does take into account the evolutionary relationships between organisms.


Cladistics, a new approach to classification, allows scientists to reconstruct evolutionary relationships through the use of shared characters.
Ancestral characters: ancestral characters are characters that evolved in a common ancestor of all groups “above” on the cladogram.

A derived character is a characteristic that evolved in some groups but not others.

According to the cladogram below, which characters are ancestral characters of salamanders, turtles and leopards?

Which character is a derived character of tuna that separates tunas from lampreys?

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