Classification

Scientists use classification systems to separate organisms into groups based on similarities or differences in their structural and genetic characteristics. These systems allow scientists to easily identify different kinds of organisms.

Linnaean System

One major way that scientists classify organisms into groups is by using a hierarchical system, known as the Linnaean system, that moves from Kingdom, a general type of group that contains many types of organisms, to species, which is a much smaller, more specific type of group that contains only organisms similar enough to interbreed.
The flow chart below indicates the hierarchical levels of the Linnaean classification system.

Kingdom → Phylum → Class → Order → Family → Genus → Species

In the flow chart, the classification levels become more specific from left to right. An individual Kingdom, for example, is a more general rank, and it contains more organisms than an individual Phylum contained within that Kingdom.
Organisms have more similar structural characteristics to organisms with which they share more classification levels. For example, organisms that are from the same species will be similar enough to interbreed. Organisms within the same Kingdom, however, can be very different. In fact, Kingdom Animalia contains mammals, insects, and sponges, all of which look very different from one another.

The broadest and most general category of the Linnaean classification system is called a Kingdom.


Kingdoms group organisms together based upon their past relationships to each other, their types and numbers of cells, their ways of moving, their methods of nutrition, and their modes of reproduction.
In the 18th century, there were only two kingdoms (Plantae and Animalia), but as knowledge about organisms increased, new kingdoms were developed. As new organisms and new features of old organisms are found, the kingdom system can change to meet the changing needs for classification.
Important Definitions:

Nucleus - / the area in a cell that contains its genetic information
Prokaryotic - / a cell that does not have a membrane-bound nucleus
Eukaryotic - / a cell that has a membrane-bound nucleus
Multicellular - / composed of more than one cell
Unicellular - / composed of only one cell
Heterotrophic - / organisms that must consume, ingest, absorb, or eat organic compounds (food) from the environment for nutrition; humans are heterotrophic
Autotrophic - / organisms with the ability to manufacture complex organic compounds from simple inorganic molecules obtained from the environment; green plants are autotrophs that use sunlight to make their own food
Sexual - / type of reproduction that involves an exchange of genetic material (meiosis)
Asexual - / type of reproduction that does not involve an exchange of genetic material (cloning, binary fission, budding, fragmentation)


The six-kingdom classification system contains the following kingdoms: Eubacteria, Archaebacteria, Protista, Fungi, Plantae, and Animalia.

The Six Kingdom Classification System
Kingdom / Eubacteria / Archaebacteria / Protista / Fungi / Plantae / Animalia
Cell Type / prokaryotic / prokaryotic / eukaryotic / eukaryotic / eukaryotic / eukaryotic
Cell Wall / nearly all / all / some / all / all / none
Cell Number / unicellular / unicellular / unicellular
and
multicellular / unicellular
and
multicellular / multicellular / multicellular
Nutrition / autotrophic
and
heterotrophic / autotrophic
and
heterotrophic / autotrophic
and
heterotrophic / heterotrophic / autotrophic
and
(rarely)
heterotrophic / heterotrophic
Photo-
synthetic / some / some
(rarely) / some / none / all / none
Movement / some move with flagella / some move with flagella / cilia, flagella, or amoeboid / do not move / do not move / muscle organs, cilia, flagella, etc
Reproduction / asexually by binary fission / asexually by fission, budding, or fragmentation / varied methods of sexual and asexual reproduction, including binary fission, mitosis, and some meiosis / asexually by mitosis (budding, fragmentation, spores) and sexually by meiosis / predomenantly sexual reproduction by meiosis (cones, seeds, and spores) with some asexual reproduction by mitosis / predomenantly sexual reproduction by meiosis with some asexual reproduction by mitosis (parthenogenesis)
Example / Spirilla / Methanococci / Amoeba / Mushroom / Sunflower / Lion

Domains

Domains are classification levels that are not generally part of the Linnaean classification system. Domains are, however, the highest and most general classification rank of organisms. In fact, domains are even more broad than Kingdoms.
There are only three Domains: Archaea, Bacteria, and Eukarya. All of the organisms in both Domain Archaea and Domain Bacteria are unicellular. Plants and animals are both included in Domain Eukarya.