Classification

• The science of naming and classifying organisms is called taxonomy.

• A simpler system for naming organisms was developed by the Swedish biologist Carl Linnaeus.
• Linnaeus’s two-word system for naming organisms is called binomial nomenclature.
• The unique two-part name for a species is now referred to as its scientific name.
• The first word is the genus to which the organism belongs. A genus is a taxonomic category containing similar species.
• The second word in a scientific name identifies one particular kind of organism within the genus, called a species. A species is the basic biological unit in the Linnaean system of classification.

• Each level of classification is based on characteristics shared by all the organisms it contains.

• Linnaeus worked out a broad system of classification for plants and animals in which an organism’s form and structure are the basis for arranging specimens in a collection.
• The genera and species that he described were later organized into a ranked system of groups that increase in inclusiveness.

• In 1942, the biologist Ernst Mayr of Harvard University proposed a biologically based definition of species, which is called the biological species concept.

• Mayr defined a biological species as a group of natural populations that are interbreeding or that could interbreed, and that are reproductively isolated from other such groups.

• Sometimes individuals of different species interbreed and produce offspring called hybrids.

• Only about 1.5 million species have been described to date.
• Scientists estimate that 5 million to 10 million more species may live in the tropics alone.

• Classification based on similarities should reflect an organism’s phylogeny, that is, its evolutionary history.

• Cladistics is a method of analysis that reconstructs phylogenies by inferring relationships based on shared characters.
• With respect to two different groups, a character is defined as an ancestral character if it evolved in a common ancestor of both groups.
• A derived character evolved in an ancestor of one group but not of the other.

• Cladistics is based on the principle that shared derived characters provide evidence that two groups are relatively closely related.
• A biologist using cladistics constructs a branching diagram called a cladogram, which shows the evolutionary relationships among groups of organisms.
• Organisms that share derived characters, are grouped together on the cladogram.
• The great strength of cladistics is objectivity. If a computer is fed the same set of data repeatedly, it will make exactly the same cladogram every time.
• The disadvantage of cladistics is that the degree of difference between organisms is not considered.
• Cladistic analysis does not take into account variations in the “strength” of a character, such as the size or location of a fin or the effectiveness of a lung. Each character is treated equally.

• In evolutionary systematics, taxonomists give varying degrees of importance to characters and thus produce a subjective analysis of evolutionary relationships.
• In this type of analysis, evolutionary relationships are displayed in a branching diagram called a phylogenic tree.
• Evolutionary systematics involves the full observational power of the biologist, along with any biases he or she may have.

6 Organized Kingdoms

1. Noncellular organisms – viruses

2. Monera – bacteria, blue-green algae

3. Protista – amoeba, other single-celled organisms

4. Fungi

5. * Animals

6. * Plants

* In descending order, organisms are named by:

(Domain) – not always used

Kingdom

* Phylum

* Class

* Order

* Family

* Genus

*Species

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Binomial Nomenclature

An organism is named using the two (2) smallest Taxa (genus, species) to which it belongs.