The Critical Zone

Classification of Life


The myriad variety of life is classified in a number of ways, dependent on scientific questions and need. Scientists utilize taxonomy to name and classify organisms, and they use systematics to create phylogenies—the evolutionary history of a species or group of species. Perhaps the most commonly known classification scheme utilizes evolutionary relatedness and a hierarchy of increasingly exclusive categories: domain, kingdom, phylum, class, order, family, genus, and species.

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Generalized trophic diagram.

Reading assignment

  • To learn more about topics of classification, visit General Biology/Classification of Living Things/Classification and Domains of Life.
  • Organisms can also be classified according to the manner in which they derive their life energy, or nutritional mode—chemoheterotrophs, chemoautotrophs, photoheterotrophs, and photoautotrophs, for example. Autotrophs ("self feeders") are organism dependent upon CO2 as their sole source of carbon, whereas heterotrophs ("feeders on others") cannot grow solely with CO2 and therefore must use organic compounds derived from other organisms. To learn more about this topic, visit Autotroph -v- Heterotroph.
  • Scientists assign organisms to trophic levels based on their source of nutrition and energy. The trophic level that supports all others contains the primary producers and consists of autotrophs. Organisms in higher trophic levels are heterotrophs, subdivided further into categories like primary, secondary, and tertiary consumers. Follow these hyperlinks to learn more about trophic levels and dynamics.
  • For information on classifying life in soils, visit Soil Organisms.

Check it out . . .

The following hyperlink provides a nice classroom Powerpoint slide show: Naming and Classifying Organisms.